stumped - erratic charging circuit - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


1992-1999 Series I L27 (1992-1994 SE,SLE, SSE) & Series II L36 (1995-1999 SE, SSE, SLE) and common problems for the Series I and II L67 (all supercharged models 92-99) Including Olds 88's, Olds LSS's and Buick Lesabres Please use General Chat for non-mechanical issues, and Performance and Brainstorming for improvements.

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Old 03-17-2003, 11:02 AM   #1
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Default stumped - erratic charging circuit

I have a '95 SE with supercharged 3.8. My ammeter guage is erratic and the alternator intermittently charges. The problem occurs when there is a lack of, or low and erratic field voltage applied to small red wire (the only wire) of the alternator plug. When the alternator is working and outputting 14V, I measure 10.5V on this lead. Over a few days the problem first was mostly happening at lower speeds driving in town, but then also showed up while on the highway and more frequently. I have replaced the alternator, battery, cleaned connections, and checked the field wire where it enters the harness for problems. The belt is not slipping and it is near new. My Chilton'* manual doesn't have a full wiring diagram for this year - it just shows the field wire going to the PCM. Can anyone tell me how this circuit (wire locations, ammeter, fuses, PCM, alternator field voltage) is configured?

I am wondering if I have a wiring problem, but I don't know where to look. Or do I have a computer problem? Other electrical problems - 3 times the headlights got stuck on regardless of swith and automatic light control settings, also I can tell that someone has been in the steering colum for some reason since the dimmer switch is broken and the plastic cover is screwed up. I bought the car with 28k and it now has 82k miles.

Any help is appreciated. This site has helped me find a water leak in the O-ring in the water pump housing tube. Thanks
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Old 03-17-2003, 02:57 PM   #2
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By alternator plug, what do you mean?

There is a red cable that goes directly to the battery that screws on to the alternator. There is also another set of wires that plugs into the alternator. Is this the plug that you are referring to?
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Old 03-17-2003, 03:37 PM   #3
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I am referring to the plastic plug-in connector (supplies field voltage to the alternator) on the alternator. Chiltons says it goes to the PCM. If I pull this plug and apply field voltage to the alternator manually via a jumper, the alternator charges fine.

It is not the connection / mounting stud with wire going to the battery.
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Old 03-17-2003, 09:04 PM   #4
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Here is what it says in the 94 service manual:

When the ignition switch is placed in "RUN," voltage is supplied to generator terminal L (red wire) by the PCM, turning on the regulator. The generator'* regulator uses digital techniques to supply current to the rotor, thereby controlling the output voltage....

I would guess that the red wire connection is bad at the alternator plug, or somewhere between the alternator and PCM, or at the PCM. Or the PCM could be bad, but I would start with the wire.

Apparently the PCM signals the low voltage light thru a brown wire from the PCM. This appears to be separate in 94. (In 93 the alternator signals the low voltage light directly).

95 is likely the same as 94.
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Old 03-19-2003, 02:25 PM   #5
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Thanks for the info. Yesterday I signed up for Alldata.com and I got all of the wiring diagrams and trouble charts - I am not ready to fork out $120 for the Helm manual, yet.

I first ohmed out the red wire from alternator to firewall connection and it was fine - even when doing a shake down test. Everything was solid and clean. I then reassembled the connector, and started the car and the charging voltage was intermittent as before. By now, I am fearing a computer problem. I shut the car off and decided to take a look (did not remove) at the connectors and wiring between the computer and firewall. After finding nothing wrong here, I started the car, drove it for about an hour, and have yet to see the problem appear.

Now my fear is that it will show itself again, but probably at a great inconvenience. For now, I bought a replacement alternator plug for $2.30 and attached a length of wire and alligator clip and I will keep this handy should I need to get field voltage applied to the alternator in an emergency.

Any recommendations on where or what brand of PCM to get?
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Old 03-19-2003, 04:22 PM   #6
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Good bit of troubleshooting going on in here. Very good work. PCM'* can be purchased from the dealer or most auto parts stores. Jr has the good skinny on them. He'* bought a couple already.
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Old 01-21-2004, 11:59 PM   #7
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Hey BONNY95....What ever happened to this problem? I have encountered the same ordeal that you posted here and came up with the same conclusions that you have- going through my service manual and checking PCM voltages (WC7~10.5 v, WC8~10.5v, WC9~ 7.9v). My question is, did you replace the PCM, what brand, and did it FIX IT? This thing is driving me nuts, I've searched high and low for bad grounds, you name it. My car had the ignition switch replaced as well before I got it, and I've been questioning the quality of job done (ign power feed to PCM). I'm anxious to hear if you fixed the "real" problem, and what it was. I had already replaced the PCM with one from Autozone :( , and was pissed when it made no difference, I had to turn in my ACDelco as a core (which turned out to NOT be the problem) Help before I park it in front of the firing squad. Appreciate any insight you may have. I'll send you a beer
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Old 05-25-2004, 11:53 PM   #8
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Default updated to erratic charging

Well...I thought I had posted a reply but I am not sure where it went. Forgive me if this duplicates anything...I have not visited the site in quite some time.

First, I want to say that I have read many of the posts regarding the recommendation to buy Delco versus chain store alternators, and I can understand this. I will however point out that the original Delco unit had exactly the same symptoms as this unit so I don't know that I would place fault with the chain store alternator. If I had not spent the money on this lifetime unit I probably would go the Delco route next time.

Anyway...
I had my problem disappear for many months immediately after replacing a cracked coil pack that was found after a terrible engine miss developed. I thought this was odd but it seemed somehow to clear up the alternator charging problem.

Months later the problem showed up as before - intermittent and usually the problem lasted for a short time and the battery would never discharge much.

The problem became more frequent and finally I had the battery run down completely. I installed a new battery. OReilley Auto Parts tested the alternator on bench and it failed the initial diode check (anyone know what this check is?). They also said it had low output. I replaced the alternator and the symptoms were exactly the same.

I then brought it to the GM dealer. They saw the problem but later that afternoon when they pulled it in the shop it worked fine. They test drove it, washed the engine bay thinking it could be a moisture problem, and never saw the problem. They said they also tighted the battery cable a little. By the way, this cost me $60.

I had no problems for about a month. Last week it acted up twice in two consecutive days and has been fine.

This is really frustrating. I now have a spare battery in the trunk.
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Old 05-26-2004, 07:16 PM   #9
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Default Re: updated to erratic charging

Quote:
Originally Posted by bonny95
OReilley Auto Parts tested the alternator on bench and it failed the initial diode check (anyone know what this check is?). They also said it had low output. I replaced the alternator and the symptoms were exactly the same.
A diode check simply checks conduction of the diode forward & reverse. The diode should conduct current in one direction, and block current in the opposite direction.
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Old 08-19-2004, 12:04 AM   #10
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Default problem still remains

I still have not found a problem to my erratic charging problem. Sorry this is so long but this is driving me nuts.

It does not seem to be a loose battery cable, alternator, or battery.

In the last week the alternator has rarely been charging (I have been putting the battery charger on at night). I am almost ready to give up and bring it to an automotive electrical specialist. The GM dealer has had one chance at it. So far I have had two alternators and battery replaced. All speculations and outcomes of troubleshooting listed previously still hold true. Currently I have been driving around with my digital multimeter bridged to the PCM connection that directly (this does not pass through the alt dash gauge like some years) feeds the alternator.

The alternator charges if the voltage goes above about 11V. During problem conditions the DC voltmeter is all over the place from around 2V, maybe less, to around 9-10V. I am assuming that the PCM should deliver 11 or 12V DC, correct? Has anyone checked this on their car? Could I safely "hotwire" this alternator and feed the plug directly from the battery or a regulated source off of the battery? I had tried this with alternator #2 and it only seemed to work a few times. That alternator later was bench tested to be bad (although it charged ok intermittently just like the last three alternators).

The harness has less than one ohm of resistance between PCM connection and the alternator terminal and erratic voltage is measured at the PCM plug and the alternator plug no matter if alternator plug is in place.

I just cannot believe that the PCM is faulty and only this function is affected. I also cannot understand what could cause a PCM to screw up only this single function.
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