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Discussion Starter · #1 ·
Hi all, so I'm a long time jeep guy and frequent the jeep forums often. Knowing the value of the combined knowledge of people on these forums I thought I'd use the suz forum to get some help.

My dad bought a x90 thinking it would be a fun project that has turned into a nightmare.

USA 1996 X90 1.6L I4 4x4

He's done a ton of work to it so far. Most recently was the timing, plugs and distributor /wires, and fuel nozzles. Timing and compression are within spec now, they were off before. Once he got it reassembled P1600 popped up where it was not present before the above mentioned work. I can clear it with obd but it comes back immediately and appears to be stuck in "limp mode" when driven.

I've found that this relates to a serial communication issue between the ecu and tcu. Through this forum I've found common issues and checked everything I can think of or have found. Grounds are good, wires are good, voltage is good at pins, removed and inspected both ecu and tcu, no obvious indications of being burned up. Just for kicks I did a smoke/leak test today and didn't uncover anything.

I'm basically at the point of recommending he buy a refurbished ecu and go from there but wanted to check here first. Maybe someone can relate the recent work done to a potential cause for the 1600 code. Or maybe someone can confirm that the ecu is probably just toast or recommended additional checks.

Sorry for the novel, just wanted to provide as much information as possible.

Thanks in advance for any insights or troubleshooting guidance.
 

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99 Tracker, 5 door, 2L, 4x4
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You need to go back and verify the power to the ECU, there are more than one, you are missing power to the KAM..
 

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Discussion Starter · #3 ·
You need to go back and verify the power to the ECU, there are more than one, you are missing power to the KAM..
Hi Aquanaut, thanks for the reply, I understand KAM to mean keep alive memory. If that is incorrect please let me know.

I also wonder why if that was the case I wouldn't also be getting a P0603. I have what I think is a fairly good wiring diagram and a voltmeter but don't see KAM obviously listed. If you can elaborate any I would really appreciate it.
 

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Look for a full time power to the ECU, best to test as a loaded circuit (test light). The other power circuit is the power on key/power relay.

Dont forget the ground side, the ECU has a chassis and a sensor ground..again load them with a test light, a MM can show voltage even when the circuit cannot support any amperage...
 

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Discussion Starter · #5 ·
I'll break out the test light tomorrow, hadn't considered amperage, was just happy to see voltage.

Thanks again for the guidance, I'll post back with any results or additional questions. 🙏
 

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I believe that the code 1600 is a code for the serial communication between the ECU and the tranny computer specifically…..
 

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Discussion Starter · #7 ·
That was my understanding of the code as well, from the wiring diagram I found 2 pins that directly communicate with the "trans control system".

I should note that the tcu appears to be refurbished. With a sticker that says rebuilt 5/2021 which is right before he bought it. Maybe someone was having the same issue before. Just strange that it wasn't coding until now.
 

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I believe that the code 1600 is a code for the serial communication between the ECU and the tranny computer specifically…..
Quiet so, but before chasing the LAN network all power and grounds MUST be in working condition, as the digital signal relies on good clean power...
Once ALL power and grounds are verified you will need a scope to confirm the comms circuits..
 

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This wiring diagram may be helpful, showing the connections from the ECU to the TCM:
If you look at the diagram, it appears that the blue wire from the ECU to the TCM is the serial data line, so I’d probably start there. The connector schematic shows that this connects to cavity #7 of the connector.
The entirety of Acks FAQS is located here, which might be helpful if you need to access the auto tranny section:
 

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Discussion Starter · #10 ·
Really great source of information there thanks!

So I tested what both of you have suggested.

  • Serial data line has continuity
  • TCM has good power in all locations on and off
  • ECU has good power in all locations on and off
  • All grounds cleaned, reseated, and ohm checked

Code just pops right back up. I'm at a loss and open to recommendations.
 

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Did you check and verify the connector pins, as well?? The issue is not that the ECU or TCM individually have good power or grounds, but that the communication between them is incorrect. I believe that the basic idea of this code is that the ECU is unable to send any signal to the tranny computer.
What happens if you disconnect the tranny computer? Normally, this should allow you to mechanically go through the gears and drive the car ‘manually’. You would lose the overdrive. If you do this, does the engine come out of ‘limp home’???
Is your O/D light on? If you disconnect the connector to the tranny computer, and you jump the connector (which should be near the steering column) do you get any code? A code 12 would be a code that the TCM sees no faults. If so, then potentially this confirms that the issue is not with the TCM, but with the connection between the ECU and TCM. Photo of the connector is here:
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Discussion Starter · #13 ·
Great troubleshooting tips, thanks. Out of curiosity yesterday I did remove the TCM, code reappears. I assume that was logical since there was no tcm to communicate to. No other codes presented.

I did not try driving without tcm. But my dad says he was able to manually shift from D,2,L when he attempted to drive it with TCM attached. Will toy with that some today.

No OD light.

I did not check pins, just plugs. To be completely honest I get overwhelmed by the diagrams and wires and kind of lose track of where I was, what I need to do next, etc. Also the confined area of the x90 is painful to work in for long.

BUT I am determined to get it sorted out for my dad and am grateful for your help so will dive in again.

The TCM jumper is interesting, I will definitely try that today as well.

I've driven jeeps for 25 years, 99% of electrical issues have been a bad ground. I wish this was so simple haha.

Thanks, will report back
 

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Discussion Starter · #14 · (Edited)
Just a follow up

It does drive "manually" without the tcm plugged in. With some well-timed shifts between L,2, And D it drives quite nice really...

No OD light visible

I cannot seem to locate the TCM DLC mentioned above, I've got the whole dash taken off, nothing.

The vehicle is equipped with OBD 2 DLC
 

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The DLC should look just like the photos, and, from what I understand, is in between your legs as you are sitting in the driver’s seat, somewhere close to the steering wheel column. I don’t have the 4 speed, so cannot tell you from experience…..maybe someone else will chime in, or else search through the forum to see if a better location has been posted for this…..That info should also be in the Acks FAQS link above.
 

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This post says DLC #2 is up by the right hand headlight, along with the other diagnostic connector for the ECU??
 

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Discussion Starter · #17 ·
Thank you Bex, I located them up by the HL. Different than the white box pictured, small black box. I confirmed it by checking that the blue/orange wire was in fact coming from the tcm as the diagnostic wire. It Is also accompanied by a black ground wire.

Jumped the blue/orange wire and supplied ground with no flash codes. I wasn't certain if the Service engine light or OD lights were supposed to blink. I confirmed that the OD light is not burned out by replacing it with a known working bulb, confirmed continuity of the DLC wire, and confirmed that the ground was good.

Regardless no blinking lights on dash.

I'm hoping you will say that the TCM is likey dead. Since I'm lacking OD light and cannot retrieve flash codes.

Currently have the tcm open on my bench... again, desperately searching for some sign of a burned up capacitor or something...anything

Interested in your thoughts
 

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This is unchartered territory for me, frankly. It would be interesting for you to post up photos of the TCM. What is the model number of the TCM? I checked over on Rhino’s site (he builds the ECU’s and has knowledge of the TCM), and found this posted:
Personally, I don’t know of any tests to run, to confirm if the TCM is good; perhaps someone else on the forum might. And presumably Rhino might be helpful, as well. He does have a forum:
He might be able to give some insight into this, or instructions that someone knowledgeable in computers (or you?) might be able to follow.
But just for the interest of this forum, I’d be interested to see the internals of your TCM.

In the meantime, at least your car is driveable as a 3 speed…somewhat better than it sitting in the driveway unable to be driven.
 

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Discussion Starter · #19 ·
I may reach out to Rhino, thank you for that and all your other help.

I think at this point I'm going to order him a TCM, if that doesn't fix it we will try a ECU. At the very least he will have "spares" of these critical components that I assume will be increasingly hard to find as time goes on. That's the only way I can justify the expense atleast haha.

PN 38880-79E1

Supposedly refurbished May 2021

Some internals, not going to lie, I have no idea what I'm looking at or where to even begin testing. Apart from looking for anything that might look "burned up"

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Thanks for the photos! If you post over at Rhino’s, with your photos, he may be able to tell you whether various caps, etc., have had problems, needed to be replaced, etc. Being old school from OBD1, I look at the capacitor in the top left corner with horror….lol. But up until now, the 96+ capacitors have not presented a problem. However all caps do eventually dry out. Maybe before you order one, post over at Rhino and perhaps all that is needed is capacitors to be changed out. He’ll be able to give you good info, and it appears that refurbished ones may not be so cheap:

In the meantime, at least the car is driveable without it. If you do post over with Rhino, perhaps keep us advised, or provide a link to your thread there, so we can follow. It’s good to always learn….;)
 
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