Dynamo Kaput..?

Early Bay Forum

Help Support Early Bay Forum:


Well-known member
Jul 12, 2007
Reaction score
North Beds
Having a few issues getting the van back up after 4-5 years 'resting'.. New fuels lines filter and pump and now she runs.. but I noticed gen light stays on.. Drat! Brushes look good and the failure seems quite abrupt.

DVM measures only about 4.5 V on the AC scale, but it it even an AC output? I noticed the regulator is only ever referred to as this and not a Rec/Reg... Anyhow I see no rise in volts at the battery when the engine is revved so I guess das dynamo ist kaput.

Are there any other tests I can do?

I note there are both 30A and 38A options but are they physically the same? Would my 1970 have had 30 or 38 originally.
I'm not inclined to replace with alternator due to cost.

Hints and tips welcome.


Staff member
Oct 5, 2006
Reaction score
Have you tried depolarising it? Usually works if the car has been sat for a while.

From vw-resource.com:

Questions are often asked regarding the need to polarize a generator. If the generator is charging (the battery stays charged) then you don't have to polarize it. When a generator has been left unused for a long time (months or years), the metal core looses it's magnetism, and it NEEDS a little residual magnetism to start the charging process. Polarizing just gives it enough "boost" in magnetism to start it charging properly. When it's being used, it keeps its core polarized core so no need to do it again.

Note: Why do generators need to be "polarized"? Automobile generators need some magnetism to get started. This "residual" magnetism remains in the field pole pieces even after the engine has stopped. The next time the generator starts up, the residual magnetism creates a small voltage in the armature windings. Not enough to charge the battery, but enough to allow the field windings to draw current. As the field current increases, the pole pieces create even more magnetism. That makes even more voltage in the armature, and the cycle continues until the generator is capable of producing maximum output.

What happens though to a generator which has been stored a long time or is freshly rebuilt? The residual magnetism may have decreased to the point where it can no longer get the generator started producing voltage. In the case of a new generator or one which has been mis-treated, the residual may even be of the wrong direction (North and South poles reversed). Polarization is a simple process used to restore the field pole residual magnetism and ensure the magnetic direction is correct.

To polarize a generator, connect a jumper wire from the (DF) terminal on the generator to the generator frame. Remove the fan belt, then connect a wire from the positive terminal on the battery to the (D+) terminal on the generator. The generator shaft should start to spin.

Note: Don't run the generator this way for more than a few seconds to avoid overheating.

The generator will now be properly polarized. If the generator did not spin during this process, the generator is most likely defective.

Put the fan belt back on and re-test the generator voltage with the (DF) terminal grounded. If the output voltage is still low, the generator is defective.

Note: Voltage regulators do not need to be polarized -- they are not polarity sensitive. Even if voltage regulator came with instructions to polarize it -- these instructions actually polarize the generator, not the regulator. The regulator manufacturer simply wants to make sure that your generator will work properly so you don't blame the voltage regulator.

Neither to alternators need to be polarized. Alternators use a trigger current to start the process of charging - electromagnetism, so don't need any internal magnetism to begin with, like a generator does.

That trigger current is supplied via the alternator dash light (it's a tiny current and won't make the light glow when the engine is running) so if that bulb blows - the battery won't get charged and it will go flat. Curious circuitry VW used, but it works just fine. So that means that with an alternator equipped VW, you should always glance at the dash as you turn the key and make sure the Alt light does glow with just the key on, just to make sure that that circuit is intact.


Well-known member
Apr 2, 2019
Reaction score
One day I called in to my local VW shop and the owner was struggling with the electrics on a van as the indicators were not working correctly. Everything else seemed ok. We tested this and that and checked earths and all the usual stuff. Anyway the owner went for a cuppa and I carried on and then after more faffing about I did a polarity check and I found it was ass about tit. Went to the battery, disconnected it and sure enough the battery had a reverse polarity. I found another battery and the indicators worked. I had never come across this problem before and when I told him he knew exactly what was wrong. He depolarised the generator and with a new battery it all worked. So the moral of this story is, no matter how much experience you have CHECK THE EASY THINGS FIRST.

Latest posts