2002 Pontiac Grand Prix



February, 25, 2013 AT 10:14 PM

I have a 2002 Pontiac grand prix SE 3.1 (200,000 miles). I have just installed the THIRD new alternator in less than 30 days. The battery is about 60 days old and I've had it checked multiple times to make sure I didn't get a bad battery. Everything checks out fine on the battery. I've tested each alternator and I'm getting some crazy results at times. 1st new alternator: I installed it and everything was fine for about 3 days. Then the battery light came on. I put a volt meter on and it was outputting 5.4 volts. I took it back and got the. 2nd new alternator: I installed it and it was outputting 21 volts! I thought perhaps my tester was bad so I used a different tester. Still got the same results. I took the car to the parts store and had them test it. By the time I got there the alternator was putting out 14.7 volts. I also tested it again with my tester to make sure I didn't have two bad testers. I got the same results as the parts store so my testers are fine. About 14 days later the battery light came on. I tested it and was getting about 6.4 volts. Took the car back to the parts store and they tested it again. They got the same results I did so they gave me another new alternator. 3rd alternator: I installed it yesterday. Additionally, I installed a new harmonic balancer and the crankshaft sensor. Everything went off without a hitch. I tested the new alternator and it was putting out 14.4 volts. I took the car to the parts store to return the 2nd alternator and get my money back. I had them bench test it to make sure it was bad and it was. It was putting out about 6 volts. I drove back home retested everything again and the output was 13.5 to 13.8. Went out to dinner later that night and the battery light came on. Tested it again and sure enough it was putting out 6.2 volts. This time the light is on for a few minutes then back off, then back on, etc. I'm at a loss here! The first alternator I bought was NEW (not rebuilt). The other two were rebuilt. I've had the battery checked multiple times. I've cleaner the battery terminals and cables repeated (not that they needed it). There was/is no corrosion on either cable/terminal. I DID have a high-end audio system with separate amps and such. I disconnected it after the 2nd alternator thinking that maybe I had something going on there but that obviously didn't help.
UPDATE: Since I posted this question originally I did some further testing. I connected a 10 gauge wire directly from the alternator post to the positive battery post. The voltage is now pretty stable from 14.7 volts (when I first started the car) to 13.5 volts after it ran for about 10-15 minutes. Also, I checked both fusible links that are down near the starter. Both look good with no melting or corrosion or cracking.
I disconnected the 10 gauge wire from the battery to the alternator that I added and waited about an hour. I tested the car again WITHOUT the 10 gauge wire and got 6.4 volts. I reconnected the 10 gauge wire again and all the readings were normal.
However, I'm now getting a P0502 code which I have never gotten before. That is the only code in the system.
Any ideas on what could be wrong with the alternator/charging and now the P0502 code?
Thanks in advance for any insight! Jack


6 Answers



February, 26, 2013 AT 2:15 AM

The p0502 code is a vehicle speed sensor code, that can be set by low voltage. You have a serious problem on your hands, I would suggest that you take it to a competent shop atleast for diagnosis and then if you wanted maybe you could perform the repair. Diagnosing tis problem is going to require pariculair knowledge, training and equipment.



February, 26, 2013 AT 3:23 AM

I think you identified the problem without realizing it. But first of all, clarify where you're getting that 6 volts. That can't be at the battery because the engine and fuel pump wouldn't run. If you're getting that at the generator's output terminal, you can't have a different voltage there than at the battery, ... Unless there's break in that circuit, which is what you're fixing temporarily with your ten-gauge wire. You also got 21 volts once. I know THAT wasn't at the battery. It could only be at the generator.

This goes back to another reply I posted today about a fellow who removed the battery cable to see if his generator was working. (REAL bad idea). In effect, the same thing is happening to your car with the break in the wire going back to the battery. The break isolates the battery from the generator with its built-in voltage regulator. The battery is the key component in helping the voltage regulator maintain a safe system voltage. I did a demonstration every year for my students to show that without a battery connected, it is real easy for the generator to produce in excess of 30 volts. That will destroy computers and any light bulbs that are turned on, but in your case that high voltage isn't making it to the car's electrical system. You would have seen lights getting REAL bright. What WILL happen is that high voltage will still be seen by the generator and that will take out the internal voltage regulator and diodes. Once one of the six diodes shorts, the generator will only be able to develop exactly one third of its rated current. The most common ones are 90 amp generators, and 30 amps is not enough to run the entire electrical system under all conditions. The six volts indicates the voltage regulator was destroyed and it's not allowing any current to flow through the field winding to produce a magnetic field.

I suspect the six volts you're measuring is not coming out of the generator. I think that's what you're seeing from the battery. Regardless, leave the engine off, then measure the voltage at the generator's output terminal. If you find 0 volts, great; you've proven there's a break in the wire. If you find full battery voltage, all it takes is one tiny strand of wire to be all that's left in a break for enough current to get through that the voltmeter can pick it up. You need to cause a higher current to go through the break to make it have a greater affect. The first thing to try is a test light instead of a digital voltmeter. This is one case where the cheaper the test light, the more accurate it will be compared to a voltmeter. Clip the test light on so you don't have to hold onto it, then follow the output wire back to the battery. Wiggle and tug on it. If you see the test light go out, you found the suspect area.

GM likes to use the larger starter terminal as a convenient tie point so you'll probably end up down there. There will be short lengths of "fuse link" wire spliced in. You test those by tugging on them. If they're good, they'll act like a piece of wire. If they're burned open, they'll act like a rubber band.



February, 26, 2013 AT 3:32 AM

Very, very nice response cardiodoc!



February, 26, 2013 AT 4:07 AM

Hi homer1967. I couldn't resist butting into your conversation. I only know a few things, and that was one of them!



February, 26, 2013 AT 4:04 PM

Thanks for the replies!

Just to clarify. All my test readings were done at the post on the alternator and I would NEVER run the car when it's not disconnected to the battery.

While I was waiting for your reply I did some additional testing. I followed the hot wire from the alternator through the car. I found two fusible links down near the starter. Both looked really good. One thing I did discover was that the positive cable on the battery looked great and the post and terminal were sparkling (I've cleaned them about 10 times). However, when I took a closer look at the underside of the terminal where the wires go in, there was some corrosion. Not a lot of corrosion that I could see but some to make me suspicious. So I cleaned them as best as I could with battery terminal cleaner, shook and bent them, scraped around, etc. And blew it out with some air. I hooked everything back up and behold. The alternator was putting out 14.5 volts initially and after about 10-15 minutes it was putting out about 13.5 to 13.8. I'm NOT getting the battery light on and everything appears to be charging fine. At this point I'm thinking I might have this nailed and I'll get a new positive cable for a more permanent fix.

HOWEVER, I was still getting the P0502 code. So I cleared the code and took the car for a test drive. The code has not come back BUT the transmission is shifting pretty hard and is definitely not acting normally. Additionally, the speed odometer stopped working. Not sure what the heck that means since it's never been an issue in the past.

I'm guessing that I still need a speed sensor?

Just to clarify. The speed sensor in question is the one that mounts into the tranny (on the passenger side) with a little metal bracket at the top?

Any suggestions or insight is greatly appreciated.

Thanks again for your replies.




February, 27, 2013 AT 7:10 AM

If you can get hold of a scanner, that will display live data while you're driving, and you can see what the speed sensor is reporting.

I'm a lot more familiar with Chrysler's computer-controlled transmission, and on those, they go back to pre-programmed shift schedules after the battery is disconnected, then the computer goes through a learning process to know the volume of fluid it takes to apply each clutch pack. It updates those schedules to create nice crisp, solid shifts. That learning can take a few miles and a dozen shift cycles.

First drive the car a few miles to see if the shifting gets better. The speed sensor could be causing a problem but given the fact you've just had the battery disconnected, and the fault code started with the charging system problem, I suspect it's more likely the shifting problems are related to that and nothing in the transmission.

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