Alternator output low

[Hacksawbob]

2.1 DJ
measured the output of my alt whilst searching for the running problem, (very underpowered driven but revs fine) found it to be just shy of 12V from the big bolt on the rear to the body. So I borrowed Aidans spare and fitted it, exactly the same, flummoxed how could it be that the alt doesn't kick out the right juice? (no change while revving the van)

[billy739]

with the engine running and the volt meter in place remove a battery connection and see what it jumps too.

you will prob have a duff battery / poor wiring connection or a large power draw when the ignition is on!

mostlikely poor connection , check for frayed wires between alternator and starter motor, and check connection on starter is clean.

did you check voltage at alternator and battery? or just the battery?

[R0B]

could well be the voltage regulator.as it controls the alternator's output..

[Hacksawbob]

regulator is in the alt? I have the same reading on both alternators, battery has the same reading as the alternator out put I'll try diconecting the battery and see.... Starter motor is only a couple of years old really hope its nothing to do with that as its a syncro

[gti mad man]

add some new eartyhs? rev the engine and measure?

[billy739]

yes when you check the voltage at the alternator make sure you use the engine, not chassis as the earth.

[Hacksawbob]

ok tommorrow... ta

[bigherb]

First check you multimeter against a good one or on a known good fully charged battery which should read 12.6v. It's not unkown for multimeters to be 1.0v out.

[tencentlife]

Do you get the idiot light when you key on? If so, go pull the DF connector (small blue wire) from the back of the alt. Key on again, there should be no idiot light. Now with key still on and DF still disconnected, check V between the DF wire and the alt chassis. You should see voltage, somewhat below battery V.

The field windings sometimes hold some residual magnetism but it can be lost, so V is applied at DF to magnetise the field windings when it spins up. If no V is available at DF when the alt spins up, it may not self-excite, so there will be no output and the + pole will just read battery V.

You can also manually excite the field with the engine running by just jumpering between the + pole and DF connector. Just a brief touch is all that's required, the V should rise immediately. If that works with either alt, you might want to run thru the idiot light-to-DF circuit.

It's unlikely that both your test alts have bad regulators or brushes. If an alt fails, though, often it's the brushes worn down, and they are held in the regulator assembly, so a new reg gets you new brushes, too, all in one swell foop. Not a bad idea to have a spare reg in waiting for the day it fails or the brushes wear down. It can be changed in minutes with just a screwdriver, it inserts into the back of the alt chassis and is held in by two small screws.