how to tell if its the alternator or VR?

[AVIMAX]

I have a charging problem with my '86 3.2. At idle and below 2,000rpm, the battery light comes on. I checked the voltage at the battery. With the car off, it reads just over 12V. After I start the car, it reads around 11.10V regardless of engine speed. It does not budge as the engine is revved. The more accessories I turn on such as the lights, heater blower, etc. causes the voltage to drop more, down to around 10V. I can let the car run for 30 mins. and the voltage at the battery will not change, it does not drain, it just stays constant, so it seems that the battery is charging but at too low a voltage. How do I tell if it is the voltage regulator or the alternator without taking it somewhere?

[Joe Bob]

VRs typically keep the alt from over charging. Low voltage can be bad, brushes, failing diodes, loose belt, bad grounds, ecetera.

Batteries that are going south can mimic alternator problems.

A FULLY charged battery should read 12.5v at rest.

Most local parts stores will load test a battery and can diagnose an aternator. Call first as some zites have to bench test each item.

[notmytarga]

Take it somewhere. To AutoZone etc. They will hook it up to a tester which senses the diode current from the alternator. This info would be the best way to point to one or the other. I wish I had done that rather than trust my evaluation. (I replaced a desent alternator when the VR was not pushing for voltages above 12.5 V or so)

With the good analysis you have done with voltages either the regulator or alternator could be the problem. It could also be loose or corroded connections in the tangled current paths. Get a diode analysis.

[David E. Clark]

It sounds like you may have a faulty alternator diode. "Ripple voltage" or AC voltage can leak past faulty diodes and cause the battery to discharge.

If you have a digital multimeter you can check this at home. Configure the meter to AC and then apply the test leads to the battery with the engine running. The "Ripple voltage" at the battery should be less than 0.6 volts. Be sure to rev the engine up to around 2,500 rpm to make sure that elevating the rpm doesn’t cause this number to get worse.

You may want to also load the system by turning on the rear window defogger, seat heaters,headlights and anything else that uses electric. If the voltage reading inches up this also indicates damaged alternator diodes.

[AVIMAX]

I removed the alternator to check the VR and found that both the VR and the alternator are bad. The brushes in the VR are worn down and the slip rings in the alternator are also very worn, the top ring is worn right down to the point that there is nothing left. Can I replace these slip rings myself?

[T77911S]

all good advice. but as you have found, the ONLY real way to tell is to either take it to an alt repair shop or as you have done, and they will do, take it apart.

[McLaren-TAG]

minor hi-jack.

Is a voltage of 13.8~ 14.1 still considered normal? It doesn't go above that ever, but I know it's on the high side given some of the things I've read here.

[KNS]

At 2000 RPMs or above 13.8-14.1 is considered healthy.