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Discussion Starter #1
I am hoping to get some assistance in troubleshooting the charging system on my 1994 Geo Tracker.

I have a brand new battery and just replaced the alternator (new/rebuilt).

I am using a Fluke multimeter and get a voltage reading across the battery terminals of 12.6 volts (engine off).
I turn the key to on (engine still off) and the dashboard charging light (small battery) illuminates. When I start the car the charging light goes off.

Voltage across the battery terminals (vehicle running) is 14.5 volts. This is also the reading at the B+ alternator terminal.

When I switch on the headlights and/or the blower motor the voltage (measured across battery terminals) drops (falls to 12.5 and contuies to go slowly go down).

Belt tension is set correctly and the wiring between alternator and battery is good (checked impedance with meter - .1 amps resistance).

I also removed the two wire connector from rear of alternator and checked voltage at black/white wire and white/red wire with ignition on. Both voltage readings are 12.6 volts (battery voltage).

Again I am SURE the battery and alternator are good. Any suggestions on what to check next?

Thanks for any/all help.
 

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Try raising the RPMs a little and look for a change in output? Also, at idle, do the RPMs change automatically when you put a draw on the electrical by turning things on? They should. Controlled by the ISC or Idle Speed Controller, I believe.
 

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A quick test to see if you have some kind of electrical drain is to get a voltmeter or 12vdc test lamp. Disconnect the negative battery clamp. With the car off, connect the test lamp or DVM to the negative terminal of the battery, and the disconnected negative clamp. If the lamp glows, or the DVM measures volgate, it means that something is on, drawing current from your battery, even with the key off.
 

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The HL and the blower are the 2 most power hungry accessories and the standard alternator will not maintain them at idle.

Since you are using a Fluke meter my guess is you know how to use it. Raise the RPM to >1200rpm and check charge voltage and most importantly charge current (+Amps, Use AMP Clamp)... Philip
 

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Discussion Starter #5
Increasing the RPM's does not have any effect on the charging voltage. I did try testing for a current draw with ignition key off and did not locate any draw on the battery. My best guess so far, and maybe you guys can confirm, is that the alternator does not get a "signal" to increase voltage / current output when there is an increased load (ie: blower motor or headlights). I appreciate the suggestions......any other thoughts? It is strange I can get 14.5 at idle (no load) but as soon as I place a load on the battery the voltage drops. Outside of the test I did on the 2-wire alternator connector wires, I am not sure what to check next. Thanks again.
 

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The sensing circuit is via the cluster battery light, since it illuminates when engine is off and extinguishes when engine running indicates that the system has continuity...

Have you any way of inducing a load (carbon pile) and read direct amp/volt values?? You may have an open rectifier diode in the alternator or a lazy regulator.....

To facilitate appropriate responses, please post your earthly location.......Philip
 

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Discussion Starter #9
Alternator / Rectifier / diodes are all good........
My next step is to checked each and every wiring path related to the alternator. Very frustrating. Thanks
 

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Did you check for corrosion underneath the fuse box that is in the engine compartment?
 

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I chased down a charging problem for a year before I changed out the alternator. Boy did I feel like an idiot, when I saw that it failed ALL THREE tests at Autozonk! Problem was, it worked intermittently which lead me in the wrong direction.

So, are you totally absolutely completely certain beyond the shadow of a doubt that it's NOT a bad alternator/regulator?? :blink:

(I lump them together because the regulator costs the same separately as a rebuilt alt, so no point in replacing just the reg)
 

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Discussion Starter #12
Completely,totally, absolutely sure the alternator/IC regulator is good. When the problem first appeared the alternator was less than a year old..(installed early December of 2010). I removed it and obtained a replacement from NAPA (free, under warranty). I installed the new alternator and had the same issue. I took this one out and had two seperate NAPA stores test the alternator----both said that it "passed" the bench tests. The manager at the second store was kind enough to exchange the alternator for another new one--"just in case". This "3rd" alternator was also confirmed as good and I installed it and still the problem remains.

Battery is brand new (less than 2 weeks old). New batteries terminals have been installed, wiring between alternator and battery is good (good, clean connections and continuity/impedance checked with DMM.

I have the factory manual and wiring diagram and tracing most of the related wiring, I have not found any loose connections,breaks or high impedance readings. I know it will turn out to be something "simple" but so far no luck.......as I said, quite frustrating. Again I appreciate all the suggestions and advice.....must be missing something and hopefully somebody can help me locate the problem.

Thanks again.
 

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Repeat the voltage tests in your first post with the meter's positive lead connected to the alternator output post - make sure that you're getting 14.5 volts there - if not the problem is that the alternator is not supplying the current that you're pulling from the system - at that point you need to determine, using an ampere meter or a DC amp clamp, what current you're pulling, and what the alternator is delivering.
 

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Discussion Starter #14
"Voltage across the battery terminals (vehicle running) is 14.5 volts. This is also the reading at the B+ alternator terminal." (From 1st post)

Readings at Positive + battery terminal are same as at alternator B+ terminal.
I will measure current draw and let you know.

Thanks.
 

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I'm not certain if we're both on the same page - so let me make it clear - I read your first post, and noted that you measured 14.5 volts at both the battery & alternator terminals - what your first post does not specifically say is that you measured the voltage under load at the alternator terminals - which is what I am asking you to do.
 

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Discussion Starter #16
Under load it drops to 12.6 and continues to slowly go down......measured at battery termnal and at alternator B+ terminal.
 

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Under load it drops to 12.6 and continues to slowly go down......measured at battery termnal and at alternator B+ terminal.
The Alternator is not producing enough to carry the load.

Use an AmpClamp, short the regulator and see what the alternator is capable of.(unregulated).. if it is low, you are missing one of the rectifiers or its winding..Philip
 

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As Philip has pointed out the alternator is not outputting enough current to feed the load - now it doesn't matter if a rectifier diode has failed or a winding has failed - they're both internal to the alternator.
 

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As Philip has pointed out the alternator is not outputting enough current to feed the load - now it doesn't matter if a rectifier diode has failed or a winding has failed - they're both internal to the alternator.
Do replace the alternator?
 
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