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Greetings,

Another newcomer to the forum here. I just picked up a 99 Chevy Tracker (60,000 miles and very clean) and have the common "4wd won't engage" issue. When shifted into 4wd, the 4wd light does not turn on and the front wheels are not getting any power.

I've done the search, but still have a question.

I've groped the actuator and in turns on and off when the 4wd lever is engaged, and does eventually shut off.

So here, finally, is my question: When I disconnect the air line from the pump to the diff, shouldn't the pump turn back on (ie is there a pressure sensor of some sort that should turn in back on when the pressure goes below a certain level?)?
 

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I suggest 1st determining if the actuator pump is actually working. You CAN hear it run momentarily when you shift to 4WD, with the key on (engine off) if you listen carefully, or have a 2nd person listen. It is located on a bracket bolted to the back side of the passenger side of the front bumper. If you hear nothing, then pull the power lead to the pump and check for 12V to it when you shift the lever. If no power to the pump, start running through fuses, wiring and the transfer case switch connections to see where the "open" circuit is before you put down hard earned $'s on a new pump. ;)

You can access the pump by just crawling under the front passenger side. Pull the air lines, the electrical connector, and remove two mounting bolts. That's it...5 min at most, and NO need to remove the front grill. You don't even need to jack up the front end! Just be sure to get the air lines on the right nipples when reinstalling. See actual pump pic in follow on posts.

When I disconnect the air line from the pump to the diff, shouldn't the pump turn back on (ie is there a pressure sensor of some sort that should turn in back on when the pressure goes below a certain level?
Yes! If key-on and 4WD is selected.

4WD CONTROL SYSTEM (Service Manual quoted.)

When the 4WD switch is turned ON by shifting the transfer shift lever, ECM (PCM) (G16 and J20 engine models) or 4WD controller (H25 engine model) actuates (energizes) the air pump assembly. Then positive pressure is sent from the air pump assembly to the actuator installed to the differential case and causes the axle lock clutch and the free axle hub in the differential case to be engaged, resulting in the 4WD mode. Only when running in the 4WD mode, front axles are joined to differential case, and they become free when 2WD mode is used (or when transfer shift lever is shifted to N position), thereby reducing the power loss."

The free axle hub, axle lock clutch and actuator are installed in the differential left case. The positive pressure produced in the air pump assembly is applied to the actuator which then pushes the axle lock clutch to be engaged with the free axle hub. In this state, the drive force is transmitted to the front axle, resulting in the 4WD mode.
When the actuator is free from the positive pressure [when it is under the atmospheric pressure], the axle lock clutch is pushed back by the return spring force and cannot be engaged with the free axle hub, resulting in the 2WD mode."


**


Air Pump tear down and repair!
4WD Freewheel air pump teardown

Shifting on the fly!...
2004 GV 2.5l not able to shift from 2h to 4h on the fly

Diagnosing a suspected front differential internal actuator air leak...
Either attach an independent air source (bike pump or shop air with a gauge) at the diff inlet fitting, Max 10 PSI and then isolate and observe for a drop in pressure (see pic below), OR with a known proper stock air pump, connections intact and leak free and diff still assembled... listen / observe the differential VENT hose for signs of escaping air.


Wiring Diagram, Circuit Operation:
GV_4WD
http://www.mediafire.com/convkey/6a95/255tqa5a2eb66kfzg.jpg

Drive line WIND UP and GEAR BINDING explained:
GV2002 4WD Problem

Later all-wheel drive (AWD) and four wheel drive (4WD) drive system differences explained:
Suzuki Auto

Service Manual, Transmission and axles
Transmission and axle manual

Bulletin No:TS 3-02 02096[R]
Model(s):all suzuki models
Section : drivetrain
CONDITION:transfer case bind-up
CAUSE: operating the vehicle 4WD with automatic or manual hubs engaged on dry pavement or on hard surfaced roads.
CORRECTION:follow procedure as listed
REVISION:Transfer case bind-up is a phenomenon that can occur if a 4WD vehicle is driven on dry, hard surfaced roads
Should the vehicle continue to be driven in this condition,the transfer case gear shift lever will be come stuck,the vehicle may become immobile and serious transfer case damage may occur.
To relieve this situation,place the transmission in neutral,and jack up each end of the vehicle,one at a time,and allow the wheels to spin freely.This procedure will allow all driveline pressures to be relieved and the vehicle can be operated normally.

GM Tech Service Bulletin, Bulletin No.: 01-04-19-003, Date: April, 2001
Subject:

Harsh Clunk Noise From Front Axle While in 4WD and During Turning Maneuvers (Replace Front Differential Case, Actuator and Side Bearings)
Models:
1999 Chevrolet Tracker with Four-Wheel Drive
Condition
Some customers may comment on a harsh clunk or popping noise coming from the front axle. This condition is most apparent while driving in 4WD at low speeds on loose traction surfaces such as gravel or snow and turning right or left.
Cause
Inside the front differential case, the teeth of the free wheel axle hub may jump a tooth resulting in the noise.
Correction
A modification was made to the design of the front differential case, which should reduce the likelihood of the free wheel hub teeth from jumping.
Replace the front axle differential case, actuator and side bearings using the service procedure and part numbers as shown.
Important: With the new design differential case, the teeth may still jump under extreme torque load conditions, such as making tight turn maneuvers in 4WD on hard dry surfaces resulting in a similar noise. This noise is now considered to be a normal vehicle operating characteristic. Driving on high traction surfaces in 4WD is not necessary and contributes to excessive torque load on the front axle. Customers should be advised to shift into 2WD when driving on such surfaces.

Related info and Alternative repairs:

Diff oil seal / nylon bushing mod:
Front end bearing, 00 Vitara

Adding selectable front hubs
Vitara/Tracker Drivetrain - AlteredEgo Motorsports**

Internal actuator repair, front Diff
4x4 Issues


You WILL have to join / sign up to another site to view the following TWO references:
Shift On The Fly [SOTF] Elimination for Vitara and XL.7
TECHNICAL SUZUKI BLURB – Shift On The Fly [SOTF] Elimination for Vitara and XL.7 | ZUKIWORLD Online - Everything Suzuki

Front diff air actuator "tie wrap" locking mod:
Login

Transfer case mounted shift switch
From the FAQ sourced wiring diagram there are two wires, color noted and the switch is on the top of the X-Fer case.
4WD System Circuit and Operation

#6 here...

More switch info here...
2001 xl7 4x4 not working!

System component layout:
4WD System Circuit and Operation

UPDATED 01/12/20 Additional important scattered info and links as they pertain to the 4WD system, assembled and merged for ease of use:
 

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Discussion Starter #3
I am starting to understand this 4wd system now.

After some testing this evening, the actuator pump appears to be functioning properly. I suspect it is the mechanism inside the front diff that is gummed up. I will flush it this weekend and see if that helps. It sure would be nice if I could find a parts diagram for the front diff. Now to test my Google-fu.

Thanks for the welcome. I love these little trucks. I had a Geo Tracker that I did a fair amount of work on and regretted selling it. The '99 Tracker is a much nicer ride. Plus it get 29 MPG!!!

I am starting to understand this 4wd system now.

Tips for testing the actuator pump:
1. If it runs/pumps when the 4wd lever is shifted (key in ignition) and runs for ten seconds and then stops, there is a leak in the system.
2. If it runs/pumps for a second or two, it is functioning properly.

The 4wd light is activated when the pressure sensor in the pump reaches the proper level. I tested that out by plugging the hose, shifting it into 4wd, hearing the pump turn on for a second, hearing the pump shut off by iteself, and watching the 4wd indicator light turn on (for the first time since I've owned the vehicle). Therefore, it seems to indicated that a flickering 4wd light tells you the pump is turning off and on to keep the system at the proper pressure.

Conclusions to my troubleshooting:

My pump is working just fine and there is an air leak in my front diff.
Now I have an excuse to get manual hubs and permanently engage the actuator. :)

It's not just a leaking hose or hose CONNECTION is it? :)
I have a history of replacing perfectly good parts when troubleshooting mechanical issues, so it's always good for me to double-check my reasoning.

1. When I plugged the airline right at the diff, the pump would cycle for a second, shut off, the 4wd light would turn on, and the pump would not re-cycle (ie it holds pressure all the way to the diff).
2. I used a hand vac pump (connected directly to the diff nipple) to test if the actuator engages and holds pressure. It would not hold any pressure at all.
3. I removed the diff oil filler plug and looked in at the actuator while applying pressure with the hand pump and turning the tire. It would wiggle a bit, but not slide over. I was able to manually slide over the actuator through the hole, which was kind of exciting. When applying vac pressure, I could hear a sucking sound from within the diff.
4. Therefore, if there is an air leak, it is within the diff. If I were to try and speculate the source of the air leak, I would guess there is an o-ring issue if there is one in the actuator . . . .

Does that reasoning make sense, Max?

I will be ordering the axle seals, so will not be pulling the front diff for a couple weeks at the earliest. I'm not in a big hurry, but do need to make sure the 4wd is working before winter . . .
 

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Before I tear into this thing, can someone tell me where I can get a parts list for the front diff? It would be nice if I could order all the parts (seals etc) ahead of time. And if someone could tell me the OEM part number for the actuator pump, that would be greatly appreciated.
Edit 01/23/19:
PUMP ASSY, FREE AXLE, 27940-65D03

They ARE a little pricey new (currently in the $140+ range).



Rebuilt units and overhaul services too...
http://www.actuatorpumprepair.com/home.html

I'd look for a "good / used" replacement pump, after attempting to inspect / rebuild yours. ;)
Car-Part.com--Used Auto Parts Market


ACTUATOR ASSY, 2775067D01 (located within the differential)
 

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Discussion Starter #5
I have a GV1600, the 4wd light blinks off about every minute for just a split second. Is this correct? The vehicle runs OK and stays in 4wd.
My light did the flashing thing when I test drove the Tracker. Soon after that the 4wd stopped working. As note earlier, my leak is in the diff.

Unfortunately I didn't get around to pulling the front end out this fall, so it may be a while before I get at it. I'll probably park it for most of the winter and drive my 4runner since fuel prices dropped back to a reasonable level.

I did put in manual hubs, which look nice but didn't improve the fuel economy as much as I was hoping.
 

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I don't think this is quite right! My pump won't come on at all - but there is a click in the actuator, and the light turns on! So if the pump isn't running, the light is not turned on by the pressure!
That is a paraphrasing of the factory service manual - and it's quite right.

On the other hand, I'd say you're either mistaken or confused - it is the air pressure from the pump that moves the actuator - so - if the pump doesn't turn on there will be no air pressure to "actuate" the actuator.

Either your pump IS working or the click you're hearing is not the actuator, and seeing as the 4WD light is coming on, I'd say your pump is working.

Of course - if you have a different air source ....

No tampering! It was working fine last winter! Also, there is no switch on the transfer box - this is actuated by pump pressure from an air pump under the back of the front bumber. Pressure/vacuum hoses run from the pump to the hubs.
There is one switch on the transfer case, if you have a manual transmission, and two if you have an automatic.

One switch is used to tell the 4WD controller that you have engaged 4WD, and this is what causes the pump to be turned on, the second switch (on the automatic) is used to tell the ECU it's in 4LO.

I notice you mention pressure/vacuum hoses running from the pump to the hubs - I think the discussion is specific to the '99~'05 Vitara/GrandVitara/Tracker - and on those vehicles there are no hoses connected to the hubs - are you talking about the same vehicle as the other folks here?
 

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As to me being confused - that's why I'm here!

I think we may be confusing some terms. By hoses, I mean air lines!

So the pump then provides pressure to the switch on the front differential - correct? (I had thought that it was a vacuum pump to actuate the hubs, but was apparently incorrect)

I am certain beyond doubt that the pump is not running - and that I hear a click when I move the 4wd lever into 4wd. I also checked the three pronged electrical plug that this pump plugs into and see that there appears to be one power wire - and 2 grounds (returns). There is power to the plug. I also applied 12V across the terminals on the pump. If I put 12V accross the power and one of the terminals, I get the click - if I apply 12V accross the power and the other terminal, the pump runs! Armed with just a bit more knowledge now -it appears that whatever tells the pump that the pressure is adequate and to turn off - is stuck! I can only assume at this point that that is a relay inside the pump assembly.

One more piece of info is that I've also plugged this in - without the airlines attached. In this case, you would think the pump would run continuously as the pressure never builds!

Thoughts?

Update - I gave up for a while - then had some thoughts. Since the pump doesn't appear to be working - I thought I would check to see if the 4wd engages when air pressure is applied to the pressure lines.

So I jacked all 4 wheels off the ground. Then I had my son put the vehicle in 4wd - and shift into drive.

With the air lines disconnected from the pump - but the pump (actuator) plugged in, I applied about 15 psi to one of the air lines.

The front wheels then kicked in!

So the 4wd hubs/transfer case/ - and the entire drive train IS functional!

Now I know it's either the pump (actuator) or the Powertrain Control Unit (PCU).

My suspicion is that that the pressure switch in the actuator is stuck - telling the PCU that there is pressure in the line - and so the pump never runs!

Now - if I can figure out what electrical connection signals the PCU, I can simply override it, I shoudl be able to tell if it's the Actuator or the PCU!
 

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Electrical connections on 4wd air pump

I have my air pump on the bench and have the plastic tubing removed from the wires between the plug and assembly. I will try to explain the wiring to you. There are 3 wires that go into the pressure switch/dump valve and 2 wires that go to the motor. The center black wire from the plug is ground and goes to the pressure switch/dump valve with no splices. The red wire from the plug goes to the pressure switch/dump valve and is also spliced to the red wire to the motor. The pink wire from the plug goes to the pressure switch/dump valve and is spliced to the green wire to the motor.

This is how I think it works. When you shift into 4wd the control unit send 12volts on the red wire to both the motor and the pressure switch/dump valve. The black wire is ground and feeds into the pressure switch/dump valve. This closes the dump valve to seal the air line. If there is less than 6PSI the pressure switch is closed and provides ground on the pink wire out of the pressure switch/dump valve to the green wire going to the motor. The motor should pump until it builds up 6PSI when the pressure switch opens. If the system builds pressure in less than 10 seconds the pressure switch opens and removes the ground from the pink wire which stops the motor and tells the control unit you are up to pressure. The control unit turns on the 4wd light and keeps the 12v on the red wire to keep the dump valve closed. If the pressure drops too low the pressure switch closes and the motor will run again to 6PSI.

There have been anumber of posts that say their compressor runs for about 10 seconds and stops. This is what is happening. The control unit tells the air pump to run and waits for the ground to be removed from the pink wire saying you are at 6PSI. If this does not happen in 10 seconds the control unit shuts it down.

With the problem you are having I would guess your dump valve is stuck. In bench checking the pump my manual says not to run the pump more than 10 seconds at a time or you could damage the pump. If you want to check it on a bench put ground on the pink wire and 12v on the red wire. The pump should run. This tells you the motor is good. Put ground on the black wire and 12v on red wire. If the pump runs the pressure switch is good. If not, the pressure switch is bad or there could be a problem with the wires going into the pressure switch/dump valve. I have some picture on how to open this up but I can not figure out how to load the pictures.

Please help. Can anyone explain to me how to load pictures into a reply.
 

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I just went through the ringer on this - so understand it quite intimately now. Try this. Remove the rubber line that goes into the front differential - and plug it with a golf tee, bolt, or phillips screwdriver.

Then engage the 4wd. If your 4wd light comes on and everything looks normal - then you have a leak internally in the differential. You'll have to repair the leak in the differential. If you do, let me know how that goes - because I haven't done mine yet!
 

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Hello Guys,

I was searching the internet on 4wd grand vitara problems and found this forum and this topic.
I have a Grand Vitara 2003. I live in Holland and as it's snowing I want to use my 4wd but nothing. Now I checked and the pump is not working when shift into 4wd. So I take it out and did what amforeman wrote, test it with ground on the pink and 12v on the red and the pump worked. So I tested the wires from the car and when it's in 2wd and in 4wd I have,black ground and pink 12v so nothing changes.
And if I'm correct that's not how it suppose to be.
Hope someone can help.
Thanks,
Erik
That test checks only one thing - the pump itself - the dump valve or pressure switch could also be defective, along with several other aspects of the system.

visoundguy's test will allow you to determine if the problem lies in the differential or not - but only if the rest of the system is functional.

Here's one that should take you further.

When you select 4WD, a switch on the transfer case senses this and sends a signal to the 4WD controller (on a 2003 GV, this function is probably handled by the ECM/PCM), which then turns the pump on by putting +12V on the red wire.

VERY IMPORTANT - the 4WD controller will remove the +12V from the red wire within 10 seconds if it does not detect a change on the pink wire.

The problem is we don't know where along this time line you are making your measurement - or - if the switch on the transfer case ever sends a signal to the controller to start the process rolling.

Disconnect the harness from the pump and connect a small 12v lamp between the red & black wires and place it where you can see it from the driver's seat - switch the ignition on (you don't need to start the engine), the lamp should be OFF, shift into 4HI and the lamp should come on and go off after 10 seconds.

If this does happen - you have a problem in the pump assembly - or an air leak - visoundguy's test will tell you which.

If this does not happen - your problem lies either with the switch on the transfer case or the ECM/PCM - to find out which you need to find the wiring to the switch on the transfer case and measure the voltage there - it should be 0V when in 4HI or 4LO and 6~12V when in 2HI or N.

Unfortunately my documentation is for '02 and earlier vehicles so I cannot be more specific - but at least, you have the basics.
 

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Thanks for your reply's.
As soon I got some time I check both of the answers.
First the 12v lamp between the red and black to see if it turns on when i shift from 2wd to 4wd and if that works I will check the pomp with closing one tube, if not I have to look further into the electric part.
But is it normal that when it's in 2wd there is 12v on the pink and black as ground?

My GV is an earlier model and does not use the same controller logic as yours - however my guess is NO.
Fordem
 

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For whomever this will help.

This setup has worked flawlessly for 3 seasons so far in the hills of northern West Virginia.

I understand if you would rather be stock, but if you are at witt's-end, or you like tweaking it, then this might be for you. If nothing else, it makes a nice test rig for troubleshooting purposes. I have this portion at the shift lever, and used 3/16 poly tubing to route the pressure to the differential. About 4 squeezes gives me 9-10 PSI, enough to give me good front traction.

I shove the shifter into 4H, then squeeze the bulb to engage. To disengage, I twist the pressure release valve on the bulb, then shift to 2H.

It's not as convenient as stock, but it is way more dependable, with less mechanical, electrical and moving parts to go bad. And you don't have to stop and get out to lock-up!


 

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Hello,
tryed a lamp on the red and black but nothing, so I have to go deeper.
Do you know were the ECM/PCM is situated maybe I can find the red-black-pink so I can test it on the ECM/PCM else I have to test the switch on the transfer case.
Let you know,

Erik

ps are you the same that answers my post at zukiworld? I did the two posts because this is about the electric part and at zukiworld it's more how to fix it when the diff is broke.
The ECM/PCM (engine control module/powertrain control module - just two names for the same part) is probably under the glove box, at least, that's where it is on the earlier years - I strongly suggest that you start at the 4WD switch, and yes, I did answer you over at ZukiWorld.
 

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Red Wire Always Hot

1999 Chevy Tracker 2.0L AT. My Haynes manual says that the PCM does all of the 4wd control, and I've verified this by tracing wire colors to the PCM. Not sure what that little black plastic box is by the fuses, but it's not a 4wd control module...I'm fairly sure of that.

I'm having a similar issue. Pump does not run while installed in vehicle. Red wire to pump is always hot.

With a voltmeter, the voltage on the red wire to the pump increases by .2V when a 4wd position is selected. I tested the 4wd switch on top of the case, and it is functioning properly - 0 resistance when in 4wd modes and infinite resistance when in 2wd or neutral.

My pump runs if 12V battery voltage is applied to red and black is grounded. Pump does not run with 12V to pink and black grounded or with 12V to red and pink grounded.

Doesn't seem right that the red wire is carrying over 10V all the time and only varying by .2V based on transfer case shifter position, but it would seem to indicate that the PCM is not brain dead.

Also interesting that when testing voltage on the wires to the 4wd switch on the transfer case, there is not enough amperage/voltage to light a test light, but the voltmeter shows over 10V.

I'm so confused! Thanks for all of the previous posts in this thread and for the other threads on this issue. I wouldn't be this far along with my Haynes manual alone! Appreciate any advice you might have.
 

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I've dealt with this issue so many times in so many threads that I no longer remember what I've posted in any particular thread, and I'm not going to read this one to see.

You seem to be bothered by a suggestion that the 4WD control is done by a black box over by the fuses - I've said that in some thread somewhere and on my GV it is - I've also pointed out in some thread somewhere that later models have this functionality in the PCM - so if you've traced the wires to the PCM then fine, yours is controlled by the PCM - ignore the black box near the fuses.

Let's move on and look at the specifics of your problem.

You say your pump does not run when installed in the vehicle.

You say it runs when you apply +12V battery power to the red wire and ground to the black wire, so we know the motor runs. You say it doesn't run with +12V power to the red wire and ground to the pink wire - it should, so that suggests a problem with the pump module.

You also say the red wire is hot all the time - it shouldn't be - it should only be hot when you have selected 4H or 4L - so that suggests a problem with the PCM.

Now - if the red wire is hot all the time and the pump runs when you connect +12V to the red wire and ground the black wire, then the pump should run all the time when it's in the vehicle, because the black wire is (or should be) grounded - so that suggests a problem with the wiring - OR - a problem with your methodology

Your symptoms & tests suggest multiple problem areas - let's try to eliminate them one by one.

Get a 12V test lamp - something like a park lamp or turn signal bulb - connect two wires to it and connect it to the red & black wires in place of the pump - also connect your meter at the same time.

Position the bulb & the meter where you can see them from the driver's seat, switch the ignition on and select 4H - the lamp should light and the meter indicate 12V (or close to it) - the lamp should go out and the meter indicate 0V when in 2H.

Let me know what happens & we'll take it from there.
 

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Well, wiring and pcm good now

Cleaned the ground near top of radiator and got the test lamp to work. Before I could only see voltage changes with a voltmeter. When connected between red and black, the red is hot for ten seconds then goes low. Good!

Pump was still not running in the vehicle, so I removed it and partially disassembled. All wires firmly connected to solder points on the board. Did not take apart far enough to check red and green to the motor. However, after seeing everything connected inside, I wanted to be thorough and cut off the loom on the pump pigtail. Red wire had been severed by a kink in the loom. Repaired wire and now pump runs with pink and red connected but only "clunks" when red and black are connected. I believe I saw some info on this in another post, so I'm off to search for that. Guessing I need to get farther into the pump, as the motor runs, and I'm assuming the "clunk" is the dump valve engaging/disengaging, so there must be something crossed up inside the pump wiring somewhere.

Wish me luck! Many thanks.

Interesting thing is that even though the pump doesn't run when connected into the vehicle, the 4wd light comes on about a second after shifting into 4hi and stays on past the 10 second point. Thinking the pump pressure switch is "stuck" on, AND there is a bum circuit inside the pump that isn't running the pump when red and black are hot and ground respectively. Good news is that both issues are inside this little bastard of a pump! Not sure whether I'll dig into the pump more or just try to find a replacement. Would rather try to repair, as I only have a grand US into this little red monster
 

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Combined info

Fordem:
A defective pressure switch can cause both of those systems - it may be possible to replace the pressure switch - it uses a standard microswitch operated by a diaphragm.

Here's my teardown pictorial - you can see the pressure switch in the 6th picture down, and although I don't show it, you can unsolder the PCB from the dump valve and remove it to get access to the switch.

If the circuit board has failed it's probably repairable - it serves two purposes - a physical mounting for the microswitch and a convenient interconnection board.

If you have to go the replacement route, new pumps are available from the dealers - just under the $400 mark, or from Hawk Suzuki - the last quote I saw was $125 + shipping.

If using a substitute pump to test the differential, don't go over 10~15 psi - you run the risk of blowing out the actuator.

Wvkartracer
Interesting...I tested the microswitch by connecting one side of an ohmmeter to the black wire on the pigtail that is connected to one side of the switch by the board and the other side to the solder point on the opposite side of the switch. The switch is working, but it is "closed" by default and "open" when I apply pressure to the switch (what would happen when the system reaches operating pressure).

I'm trying to figure out the board based on this, but I'm several Miller Lite's into this project. It will bother me if I don't figure it out and consequently why my pump doesn't run with red-black connected.

Also noticed that the board connection between the "hot" side of the microswitch and the pink wire connection is not good (copper exposed and "loose"). I'm guessing that is part of the equation.

Anyway, I've pretty much concluded that I need a new pump, but I'm gonna have to figure this one out. It's a sickness, I know.

Will be testing the differential parts with a pump prior to purchasing pump, as I may just dump this one if I have to get that far in...

If using a substitute pump to test the differential, don't go over 10~15 psi - you run the risk of blowing out the actuator.

Well, I finally got it all figured out, and I wanted to share what I've learned. I never would have figured it out without standing on all of your shoulders, so many thanks for the previous posts. I'll take it one component at a time. All of this applies for 1999 or newer Chevy Tracker with 2.0L and 4wd.

4wd Switch - This is the switch on the top of the transfer case, just fore of the transfer case shifter. 12V are constantly applied to one side of this switch when the ignition is in the ON position. Switch is open (infinite resistance) in 2wd or neutral and closed (0 resistance) when in 4wd hi or lo. When closed, the 12V signal is passed through the switch and on to the PCM to activate the actuator pump assembly. Switch function can be tested with an ohmmeter connected across the switch side of the connector. Voltage to the switch can be tested by connecting a test light across the terminals of the harness side of the connector with the ignition switch in the ON position.

Edit: The switch does not "pass +12V to the PCM". It applies a ground to a sense circuit in the PCM, telling the PCM that 4WD has been selected.

4wd Lo Switch - Located on the right side of the transfer case just fore of the transfer case shifter. Functions just like the 4wd switch above, and is only closed when the transfer case shifter is in 4wd lo position (all the way forward). Operates the 4wd lo instrument panel light/indicator only. Has no bearing on the function of the 4wd system whatsoever.

PCM - When the transfer case shifter is placed in 4wd hi or lo and the 4wd switch is functioning properly, the PCM responds by sending 12V on the red wire to the actuator pump assembly. Removes 12V on the red wire to the pump assembly if 10 seconds expire and the signal from the pump to the PCM on the pink wire is still high (12V). Holds 12V on the red wire if the pink wire from the pump goes low before 10 seconds expire. Illuminates the 4wd indicator lamp when the pink wire from the pump assembly goes low, signalling that the system is fully pressurized. To test that the 4wd logic in the PCM is not dead, connect a test light between the red and black terminals on the pump connector. Place the ignition switch in the ON position and shift the transfer case to either 4wd position. The test light should illuminate for 10 seconds and then extinguish.

Actuator Pump Assembly - Closes the dump valve and runs the pump compressor when 12V are applied to the red wire from the PCM in response to the transfer case being shifted into 4wd lo or hi. If/when the system is successfully pressurized, a microswitch inside of the pump opens and in response, the compressor is turned off and the pink wire to the PCM goes low (0V) to signal the PCM that the system is properly pressurized. The dump valve remains closed to keep the hubs locked in until the red wire goes low (0V) from the PCM (because of shifting out of 4wd or pressure not being achieved within 10 seconds). Note that the actuator pump assembly circuitry and switches (not the PCM directly) control the dump valve and compressor within the pump assembly. When bench testing, the pump should run when 12V is applied across the red and black or pink and red terminals...otherwise the circuitry inside of the pump is bad and the pump must be replaced. Do not run the pump for more than 10 seconds during bench testing.

All black and black spotted with gray wires are grounds. Best of luck!

Merged:

My previous post was more general in nature and was a compilation of other posts along with information from diagnostic testing and a Haynes manual wiring diagram. I'll pass along some more specific information on my troubles here, in the hopes that it will help someone else.

My 4wd failure was due to two issues:

Wire chafing in the harness - The red wire leading from the pump connector to the pump was chafed nearly to the point of failure from a kink in the wire loom rubbing against it. I had to cut the red wire at the point of chafing and insert a butt connector to fix the connection. This issue was discovered when the pump would not run during bench testing with 12V applied to the red wire and the black wire grounded.
Circuit Board failure within the actuator pump - The circuit board within my actuator pump had lost the connection to the pink wire. This resulted in the dump valve being closed and the 4wd indicator lamp illuminating, but the compressor would not run at all. Only the faint "clunk" of the dump valve closing would be heard upon shifting into 4wd hi or lo. No way to repair this, so I'm awaiting a pump from Hawk Suzuki now.
 

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