Alternator question

[C man]

I have a question. Lately when I turn my car on the battery light doesn't go out quickly like it usaully does. I happed once about 2 months ago. The battery light just stayed on until I cut the car off and turned it back on. Now it is happening more frequently. I just restart the car when the light stays on and it usually goes away. Is this an early sign of alternator faliure?

[HotZ28]

We need to know the year! On some year Buicks, the PCM controls the alternator/generator regulator and on other years, the BCM controls it. In addition, in some years, neither one controls it! First, have the alternator tested at your local auto parts store; they will do it free!

[C man]

it's a 95 and I know either the BCM or PCM controlls it. I'll go to advance auto or see if I can do it at school. What should I look for? Does it need to be load tested. The problem only occurs on start up.

[HotZ28]

It’s possible that you could have a failing diode in the Rectifier Bridge, or the regulator is getting lazy. It would be better to test it off the car and on their machine. BTW, here is the schematic for the 94-95 Buick PA alternator circuit that shows PCM control of the regulator.

[C man]

I finally went ahead and changed my alternator. I bought a re-manufactured one from napa. The new one only bolts on in two places and does not bolt behind the belt tensioner. Also the fan for the alternator is made of plastic and the alternator is lighter than the old one. I hope it last (made in China). But i did notice that my car starts up almost instantly now. I didn't realize how much slower the start up had gotten.

[C man]

Got another question. Notice some things about my new alternator. When I turn on my turn signal the voltage fluctuates back in forth between the 13v mark. Is this normal or am just paying to much attention to my voltage reading. Also the light has come back on a couple of times but not like it was at first. also it seem to be charging more than usual today but it went down. Am I looking at early signs of PCM failure

[HotZ28]

A slight fluctuation of battery voltage is normal with the turn signal flashing. It should stay close to 13.0-13.2v. If this is all you have, you are OK. The amount of charging depends on the amount of drain created with different accessories on. Were you using the A/C today?

[C man]

You're right Hotz28 i did notice that. But I'm getting frustrated now . It seems I did need a new alternator but I'm still having trouble. Today I had to tun my car on about 3 times before the light went of. First I drove about a mile expecting it to cut of but it didn't. Then I cut in on and off a couple of times in the light finally went off. I got my battery tested and it was good. so what could be the problem because I've eliminated the battery and alternator. Could it be the PCM

[HotZ28]

Alternator, battery and the PCM is all I see in the schematic! You have replaced one, and tested the other, so what remains?

[C man]

The most expensive one,that I rather not replace.

[HotZ28]

Try O'Reilly Auto Parts 100 bucks for a remanufactured unit with a lifetime warranty! You will need to use your existing prom if you elect to go this route.

[C man]

I have a question. I know for sure now that my pcm is bad. Yesterday I unhooked the wire that grounded to the frame from the negative battery cable. when i was sliding it back on it would constantly make and break contact before I got it on all the way. So when I turned my car on it the charging light stayed on and didn't go off after driving a mile or to as it was doing at first. so I used a voltmeter and I got a 14.24v reading which is the normal charging voltage. To make sure that the alternator was supplying the power i disconnected the battery cable(I know i shouldn't do this) while the cars was still running. Well the car kept running but the light was bright as ever. So my question is how long can I run the care in this situation. I have to wait at least another month before I can change it out because I had to get a paint job (i'll post in another thread). I rather get it new than go to the junkyard for that.

[HotZ28]

Well, let's hope you did not damage the new alternator by disconnecting the battery while the engine was running! That little maneuver can fry a regulator. Otherwise, you can keep driving the car until the battery runs low. It might be best to carry a portable jump-start, or a spare battery in the trunk if you have one. Check the alternator output again to see if it is still charging.

[C man]

Ok I finally got around to the PCM. I change it with the O'Reilly PCM. The one i pulled was a remanufactured PCM itself.The only problem is that the charging light is still on. I checked the battery voltage and its was 12.76 I checked the voltage from the alternator to the battery there was no voltage drop and then I checked the voltage with the car on and it was charging at 14.27 volts and it was 14.14 volts across the battery terminal. I know that unless the voltage drop is above .5v its not considered excessive. Also i found a link where someone had the same problem as mine but there question never got answered http://www.automotivehelper.com/topic209054.htm. So what could be my problem?

[C man]

Looks like nobody wants to answer my question. Today I did some more investigating. I checked the cells in my batterry and the water had got kinda low. That explained the longer starts. now it starts instantly again. I also probed the wire conection that leads from the PCM to the altenator and got a reading of 12.2volts. My question now is what should the voltage be from the PCM to the altenator, should it be the same as the charging voltage? Could someone make some reply. This is driving me crazy