ALT light troubleshooting. FIXED, Specifically, what makes it come on? Options

[malcolm2]

I noticed the last time i parked that the red "G" light had come on slightly. Got in yesterday and it was on bright and the volt meter was at 10.5. Revving the engine did not help either.

I checked the belt tension and decided to tighten it.

Took a 20 minute drive to Nashville and all was well. At the last stop i saw the dim G light again. Then bright for a while. Then out.

What is the prognosis? I have about 20k miles on the alt wire harness and gel battery. Maybe 10k on the rebuilt alt.

[worn]

I noticed the last time i parked that the red "G" light had come on slightly. Got in yesterday and it was on bright and the volt meter was at 10.5. Revving the engine did not help either.

I checked the belt tension and decided to tighten it.

Took a 20 minute drive to Nashville and all was well. At the last stop i saw the dim G light again. Then bright for a while. Then out.

What is the prognosis? I have about 20k miles on the alt wire harness and gel battery. Maybe 10k on the rebuilt alt.

Try:
http://www.pelicanparts.com/techarticles/9...roubleshoot.htm
works well even if it is troubleshooting.

[malcolm2]

Try:
http://www.pelicanparts.com/techarticles/9...roubleshoot.htm
works well even if it is troubleshooting.

the following is the TS part of your link:

To trouble-shoot the problem, you need to check the various sections independently. Thus the first check: Connect +12 volts from the battery to the DF terminal on the relay board. This is the maximum field current situation, and should result in maximum output of the alternator. Note that this checks the B+ diodes, the alternator windings, and the common diodes. It does NOT check the D+ diodes.

To check the D+ portion of the system, it is necessary to find out if the D+ output can produce enough current to drive the alternator to full output. To do this, short the D+ and DF terminals on the relay board. This will provide the maximum field current to the alternator that the alternator ITSELF can supply (not the battery, as in the earlier check) and so checks the remainder of the circuitry. If this test puts the light out, then the alternator is good, and the trouble is elsewhere. If it doesn't, then the alternator is almost certainly bad...

Paragraph 2 is what SPOKE suggested and it gave me >16 volts at the battery, but did not put the light out until the output was > 13 volts. But then the light stayed out at all RPMs.

My question is para #1. I don't follow what to do, how to test and measure. It says to put 12v on the relay board DF. I can do that BUT:

is the car running?

am I measuring volts at the battery?

Should the light go out?

[Spoke]

Paragraph 2 is what SPOKE suggested and it gave me >16 volts at the battery, but did not put the light out until the output was > 13 volts. But then the light stayed out at all RPMs.

My question is para #1. I don't follow what to do, how to test and measure. It says to put 12v on the relay board DF. I can do that BUT:

is the car running?

am I measuring volts at the battery?

Should the light go out?

Yes you should be measuring at the battery; you're basically measuring the primary side of the alternator.

How many times have you repeated the DF/D+ shorting test? Do you get the same results each time? You can let the car run for a couple of minutes to charge the battery while you're there.

Paragraph 1 is the same test you're doing except the the power to D+ is from the primary side of the alternator connected to the battery.

The test you've done tests the ability of the alternator secondary voltage to power the alternator with the VR.

One other test to do when you have DF/D+ shorted: Measure the voltage from DF to chassis ground and battery to chassis ground. I want to see if the secondary is producing the correct voltage. The 2 voltages should be very close to each other.

[malcolm2]

How many times have you repeated the DF/D+ shorting test? Do you get the same results each time? You can let the car run for a couple of minutes to charge the battery while you're there.

One other test to do when you have DF/D+ shorted: Measure the voltage from DF to chassis ground and battery to chassis ground. I want to see if the secondary is producing the correct voltage. The 2 voltages should be very close to each other.

I have done the shorting test 2 times, yes I get the same results each time.

VR out, jumped DF/D+
key on, light on
car start, light on, battery 12.3-ish at 800 rpm
2500 rpm, light dimming, battery at 12.5-ish
3000 rpm, light out, battery at >16
back to idle and the light stay out, battery at 13-ish.

I left the VOM on the battery for the last few days. After I did the last test the reading was 12.53v, this morning it was 12.43 volts. So I would say the battery is almost fully charged, correct?

I'll try your "other" test this afternoon. BTW, I bought a new VR to have, incase that is the final verdict. went with the cheaper German one, BEUR.

Thanks so much,

Clark

[Spoke]

How many times have you repeated the DF/D+ shorting test? Do you get the same results each time? You can let the car run for a couple of minutes to charge the battery while you're there.

One other test to do when you have DF/D+ shorted: Measure the voltage from DF to chassis ground and battery to chassis ground. I want to see if the secondary is producing the correct voltage. The 2 voltages should be very close to each other.

I have done the shorting test 2 times, yes I get the same results each time.

VR out, jumped DF/D+
key on, light on
car start, light on, battery 12.3-ish at 800 rpm
2500 rpm, light dimming, battery at 12.5-ish
3000 rpm, light out, battery at >16
back to idle and the light stay out, battery at 13-ish.

I left the VOM on the battery for the last few days. After I did the last test the reading was 12.53v, this morning it was 12.43 volts. So I would say the battery is almost fully charged, correct?

I'll try your "other" test this afternoon. BTW, I bought a new VR to have, incase that is the final verdict. went with the cheaper German one, BEUR.

Thanks so much,

Clark

The battery voltage looks ok. Battery voltage when sitting should be around 12.6V.

When running, after first start up like you described, how does the voltage increase with RPM? The first startup you describe seems it takes a lot of RPM to get to 16V. It could be for the first rev up the battery is charging heavily and taking voltage slowly. This is why subsequent revs may show better how the alternator output voltage changes with RPM.

BTW, did you try the new VR? If the alternator is ok, installing the VR should bring your charging system back to service.