91 JETTA 1.8L - 2x Alternators = No Voltage

[Mr_Accord]

Charging issues on my 91 Jetta. For The last week I've been trying to fix my charging system with no luck. Started off with the car dying due to the battery being drained down but the low battery light on the dash never came on (???). Checked voltage at the battery and it's below 12v so the car is running only on the battery. Checked the outputs at the alternator only picked up the battery...no 14v in DC and no AC in case reg died. Did a continuity and resistance check of charging cables they're good. Cleaned connections and terminals including blue cable on alternator. Nothing. I have the valeo style unit so I managed to find a new set of brushes...installed still no output at unit. A friend had a rebuilt bosch unit that he lent me to try and that too doesn't put out any voltage. Any ideas?

 

[JHZR2]

I'm assuming the blue wire is the exciter wire?

Do you get the electrical bulb to illuminate under any conditions? If that bulb is out, it may not excite the alternator. Have you tried an alternate source of 12v to that line?

 

[The_Eric]

Without looking at the schematic, could it be the bulb or connection in the dash? It looks like an internally regulated unit, so there will likely be a wire going to the dash for an indicator light- these can play a critical role in the charging system.

I once worked on an old Nissan Pathfinder where the idiot light holder was loose in the cluster- and since it didn't make connection, it wouldn't complete the circuit and the alt wouldn't charge.

 

[RooflessVW]

Originally Posted By: JHZR2
I'm assuming the blue wire is the exciter wire?

Do you get the electrical bulb to illuminate under any conditions? If that bulb is out, it may not excite the alternator. Have you tried an alternate source of 12v to that line?


You're right, it is a failure in the exciter circuit. Run a wire off the + side if the coil and see if that gets you charging.

There is a bulb and a resistor in the cluster that can fail and cause an open condition.

 

[Chris142]

battery voltage at the B+ wire on the rear of the alternator?

 

[Mr_Accord]

Yeah I'll have to look into the Exciter wire more. When you say 12V coil? Which coil?

Thanks

 

[Mr_Accord]

Originally Posted By: rooflessVW
Originally Posted By: JHZR2
I'm assuming the blue wire is the exciter wire?

Do you get the electrical bulb to illuminate under any conditions? If that bulb is out, it may not excite the alternator. Have you tried an alternate source of 12v to that line?


You're right, it is a failure in the exciter circuit. Run a wire off the + side if the coil and see if that gets you charging.

There is a bulb and a resistor in the cluster that can fail and cause an open condition.


Is it ok to jump 12+ off the battery with a cable temporarily to see if the alternator starts. or would that harm the voltage reg?

 

[RooflessVW]

I've always used the ignition coil + lead. (sometimes as a semi-permanent hack/repair with a diode)

But you can use the battery.

 

[mk378]

Turn the key on and don't start, the light must come on. This is through the blue wire sinking current to ground until the alternator starts to generate its own power. The small current through the bulb is important to start up excitation in the alternator.

If you ground the blue wire and the light still doesn't come on, check the bulb and the fuse that supplies it.

The workaround suggested by others is to apply switched 12 volts to the light / excitation terminal directly, if the alternator is good it will start charging then.