Nasty alternator/generator control voltage problem

Vehicle:
-95 GM Lumina APV Van with 3800 (3.8L VIN L)
General Symptoms:
-Intermittent loss of alternator/generator charging (particularly when hot right after startup or unplugging and then plugging in
the generator control connector). -Rare erratic shifting and PCM undervoltage code.
Specific symptoms all the time (even when generator working):
-Voltage at "L" terminal, when disconnected from generator, ~30mV when ignition in 'RUN' position and ~10.5V when engine running. Should be B+ (~12.5V) in both cases.
-Voltage at "L" terminal, when connected to generator, ~30mV when ignition 'ON' and ~11V when engine running. Should be ~1V and B+ (~13-14V) respectively.
Specific symptoms during failure (engine running, generator not working because control voltage to low or erratic):
-Voltage at "L" terminal, when connected to generator, highly erratic on scope (smear from 0 to 10.5V for 50ms and 10V for 10ms or constantly 0 to 8V) averaging 3-6VDC. Sometimes (very rarely) it is stable at 9-10V that still does not allow the regulator to turn on the alternator.
-Curiously; revving the engine usually kicks it out of this failure mode into the ~11V generator control output.
-If driving and generator failing consistently this is sometimes accompanied by erratic shifting or PCM going into limp mode or PCM undervoltage code. This is probably when, due to loads and the alternator not working, the system voltage drops to low.
Diagnostic Details:
-Generator/Alternator good; replaced unnecessarily :-( -C110 OK -Generator connector (PLF/IS) OK -CKT225 OK (Good impedance, no short to ground, no voltage drop from PCM to generator; ie. if it is 10.5V at the generator it is 10.5V at PCM). -PCM voltage good, fusible links OK, etc. -PCM ground good at G105 (checked as per TSB 23-81-06) -Many other PCM outputs that should be at B+ checked and are good. -PCM swapped with loaner from dealer with no change :-(
Discussion:
I have been over the TSBs and large portions of the service manual and am rather at a loss here. Something is pulling or allowing the generator control voltage to go down. The PCM power and ground is good and I have disconnected the blue connector (red and white have to stay in since they have power and ground) and as many sensors/outputs as I can get at to see if one of them is dragging the output down; but to no avail. A few remaining, long shot, possibilities include:
-Second, test, PCM also bad. -PROM screwy -Some sensor/output that I missed is dragging it down
This is similar to this problem that I found on the net:
http://www.flatrater.com/Free/News/Alternator.html http://www.flatrater.com/Free/News/Alternator2.html http://www.flatrater.com/Free/News/Alternator3.html
but replacing the PCM did nothing. The dealer expert and I are rather stumped. I am probably going to see about trying a 3rd PCM. Any other ideas?
--
Daryle A. Tilroe


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BillW wrote:

Forgot to mention among all the other stuff that the battery has been thoroughly tested in and out of the vehicle and all connections triple checked. It and the grounds and main power connections are A-OK. Heck with much of the diagnosing and faulting the vehicle has run off battery alone for 10+ minutes. Thanks for the reminder though.
--
Daryle A. Tilroe


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I've seen a number of batterys test the "load tester" at Autozone (etc) while still being obviously bad. Take it in to be tested, they say it's fine, but it won't hold a charge for 48 hours. In another case I brought in a battery that had been boiled by a 140 amp alternator that lost its voltage regulator. They tested it and said it was fine. I laughed and bought a new one anyway.
So anyway, the point of all this is that if the battery is more than a few years old, it would be in your best interests to buy a new one just to rule it out.
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Batteries can be problem prone, due to vibration, age / heat / excessive charging. I wouldn't be suprized to see battery perform OK in older car, and cause weird symptons in modern / computer controlled cars. I had a delco freedom that defied logic, finally the parts man / as a dare / offered to put battery in one of his trucks, and give me the truck's battery for a loaner. My car had no trouble, his truck stranded the driver twice. The battery was sent to Delco, where it passed every bench test.
As for grounds [ here I go ! ] Bad grounds cause too much heartache for me to bear, Don't assume a ground is good, hook a VOM probe to positive battery terminal, and put negative VOM probe on Negative battery terminal . Read voltage, now probe around with negative probe, test alternator case, a wimpy alternator ground makes for all sorts of fun. If grounds prove good, you've got some weird problem that's best fixed with a replacement alternator, or battery.
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Well I have basically eliminated the alternator and battery from the equation. The consistent and simplest manifestation of this problem is simply the output of RB4 (CKT225) on the PCM, with the alternator unhooked, and with the key in the "run" position. It should be B+ but is only 30mV; basically zero.
I am convinced that this is the root of the problem (I even tried another battery just for the heck of it: still zero). However, I am still at a dead end; as are the two separate electronic experts at two different dealerships.
From the troubleshooting charts: once you eliminate any issue in CKT225 (as I am doing by pulling the pin at the PCM and measuring the voltage directly on RB4) then you replace the PCM.
I have now tried a third PCM from a wrecker and it behaves exactly the same as the other two. I used their PROM too so I eliminated any weird issue there. Over the last couple days I have also rechecked all the PCM power and grounds and all the sensor/actuator pins for voltages.
ARGGGGGHHH. What I am left with is the PCM again. Against million to one odds perhaps they are all flaky in the same way. It is true that none of them are brand new/rebuilt. However, other than the output on RB4 they are all behave just fine. I sucks to be me :-(.
--
Daryle A. Tilroe


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with the engine running try placing a clamp on ammeter on the output of the altenator. should read 20 amps or so, with no accessories on.
do you have any thing plugged into the system, cigarette plugs, or other add ons?
have you checked the resistance of your leads in the charging system?
This sounds like a wiring issue. sometimes the battery lead goes bad at a terminal connection.
If you have high resistance in the alternator output lead, no charge!
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Baboo wrote:

Nope.
Unfortunately the alternator is not the issue. A) It is fine, tested, and the second one. B) All high current connections are fine; when the alternator is turned on, either when the PCM output is adequate to activate the regulator or when you bypass the PCM completely and feed the alternator regulator with B+, everything is great. C) Finally, and most importantly, the alternator is not in the equation at this point; the PCM output is wrong when the engine is not even running. You could take the alternator and battery out, run the system off an external power supply, and still have the problem.
It may be time to do some bench top experiments with the PCMs I have.
--
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I would examine the PCM grounds. I am not sure how you are referencing the voltage at the PCM when you take your V measurements. Perhaps you have a large ground offset voltage.
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Baboo wrote:

Done. I did two things:
1. Measured the PCM input voltage and ground potential wrt a part of a metal dash structure. I did this while the engine run (reasonable PCM load) and they were basically B+ and zero respectively. Again note that with the exception of the generator/alternator control output all levels are fine, eg. other outputs that are supposed to be B+ are B+.
2. At the suggestion of a tech I took a reasonable load (65W high beam headlamp) and, hooking it up to the 2 battery feeds, 1 ignition feed, 2 PCM grounds, and 2 injector grounds in turn, confirmed that it lit up nice 'n bright and had basically all of the B+ drop across it.
Would you suggest any other tests?
PS. I went back to the wrecker and snipped off the three wiring harnesses so I can hookup the PCMs on the bench.
--
Daryle A. Tilroe


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Daryle A. Tilroe wrote:

Well for anyone still following this sad saga here is an update: I pulled the wiring harnesses out of the wreck that I got the third PCM from. On the bench RB4 only outputs a similar 15-30mV rather than the spec of B+. So either I have 3 PCMs all bad in the same way or GM's service manual, specs, and schematics are wrong.
At this point just about the only thing left is to buy a new $250+ PCM and see what it does on the bench and in the vehicle. The only problem here is potential issues with the parts department since all electrical equipment is none returnable. I suppose my fear is that the new one exhibits the same symptoms and I am screwed out of $250+.
I will talk with the service department guy and see if I can get him to confirm the results of the bench tests on my PCMs; them to test a new one on his bench and confirm it is good before I buy it.
--
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modern alternators have temperature (and internal, just like the V reg) compensation that reduce output as temp rises, so batteries do not overcharge. From what you have mentioned, your model must have external temp comp.
Perhaps you should monitor all alternator leads, under fault and normal conditions.
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