Another Alternator Question..I have search but found no answer..All Is Good Now

[Peter]

the red light (diode) in the dash Does not come on at anytime..also the tach will remain dead until 3000rpm at which time the alternator "excites" and puts out a near steady 14V in all RPM's..I have exchanged the voltage regulator on the alt and checked and cleaned all possible connections..also exchanged the red bulb in the dash...any ideas as to what I can still do..could it be that the dash components are failing or is my problem more related to the alternator...is there a way to check with a simple volt meter the "blue" wire...

[bajacalal]

Blue wire should show about 12v with the engine not running, key in "on" position. The exciter voltage is routed through the LED. Make sure the LED is installed with the correct orientation as it only operates in one orientation...but would then glow once the alternator excites if installed backwards, right?

Rephrased from the Bentley manual: Test by grounding the connector on the blue wire to the engine (with the key in the on position, engine off). If the light does not come on now, you have a bad connection at the ignition switch or faulty wiring somewhere along this circuit. If the light comes on, the regulator or brushes are faulty or not making good contact.

My vote is faulty connection around the ignition switch or in the dash/cluster somewhere. To narrow it down you might have to do some probing with an ohm-meter, checking for a break in the circuit somewhere.

[Vincent Waldon]

I have exchanged the voltage regulator on the alt

A couple of questions to help diagnose:

- was the voltage regulator you put in new?
- how did the alternator's commutator look?

Failure to excite until 3000 RPM means that it *is* exciting, which means the blue wire, dash, cluster, etc are pretty much working.  Some things to check:

- voltage regulator, unless you know for sure the one you installed was good
- worn commutator
- alternator not grounding correctly... meaning one of the big grounds to the tranny perhaps

[bajacalal]

I could be wrong but I heard somewhere alternators would self-excite without a 12v source given a lot of rpm (even if they are not the self-exciting type). Mine seems to require some rpm above idle to get the alternator going but it's more like 1400 rpm, not 3000.

That's a good point, the alternator case must be grounded properly for the exciter circuit to work, make sure it's making a nice clean contact to the engine block and that the engine to chassis ground is making good contact. I'm thinking it wouldn't hurt to have another ground right off the alternator, would be easy and cheap to do.

[Vincent Waldon]

I could be wrong but I heard somewhere alternators would self-excite without a 12v source given a lot of rpm (even if they are not the self-exciting type).

Yup, good suggestion and I believe you are correct... and so I went back thru the original post... I thought I had read that the light eventually went out at 3000 RPM (hence my thinking that the exciter circuit must be OK) but that's not actually there.

So, OP, does the ALT light come on at all, or it stays dark the whole time but the tach wakes up at 3000 RPM?

If it stays dark the whole time then I recant my comments about the blue wire... you'll want to take a close look at that circuit.  If you remove the blue wire from the alternator, ground it, and then turn the key to "on" the ALT light should come on.  If not... could be the connector, the wire back to the cluster, the cluster harness, or the LED itself... they go in two ways, and only one way is correct.

Mine seems to require some rpm above idle to get the alternator going but it's more like 1400 rpm, not 3000.

Yeah, standard operating procedure for Bosch alternators... my 1986 owners manual mentions this behaviour specifically.

I'm thinking it wouldn't hurt to have another ground right off the alternator, would be easy and cheap to do.

Also a good suggestion IMHO... you'd think with all that metal-to-metal contact grounds would never be an issue... but sadly, the more grounds the better.  I always add a redundant alternator ground whenever I relay the headlights... better safe than sorry I figure?! 

[Peter]

I could be wrong but I heard somewhere alternators would self-excite without a 12v source given a lot of rpm (even if they are not the self-exciting type).

Yup, good suggestion and I believe you are correct... and so I went back thru the original post... I thought I had read that the light eventually went out at 3000 RPM (hence my thinking that the exciter circuit must be OK) but that's not actually there.

So, OP, does the ALT light come on at all, or it stays dark the whole time but the tach wakes up at 3000 RPM?

If it stays dark the whole time then I recant my comments about the blue wire... you'll want to take a close look at that circuit.  If you remove the blue wire from the alternator, ground it, and then turn the key to "on" the ALT light should come on.  If not... could be the connector, the wire back to the cluster, the cluster harness, or the LED itself... they go in two ways, and only one way is correct.

Mine seems to require some rpm above idle to get the alternator going but it's more like 1400 rpm, not 3000.

Yeah, standard operating procedure for Bosch alternators... my 1986 owners manual mentions this behaviour specifically.

I'm thinking it wouldn't hurt to have another ground right off the alternator, would be easy and cheap to do.

Also a good suggestion IMHO... you'd think with all that metal-to-metal contact grounds would never be an issue... but sadly, the more grounds the better.  I always add a redundant alternator ground whenever I relay the headlights... better safe than sorry I figure?!  

thankyou for re-reading my post...the red dash light does Not come on at all, but the the tach "wakes" up at 3000rpm... the red was exchanged with a good known replacement..I am waiting for a new one from VW..hopefully tomorrow I will be able to investigate further into what you have said...also wanted to mention that the voltage regulator is brand new out of the box

[Peter]

Blue wire should show about 12v with the engine not running, key in "on" position. The exciter voltage is routed through the LED. Make sure the LED is installed with the correct orientation as it only operates in one orientation...but would then glow once the alternator excites if installed backwards, right?

Rephrased from the Bentley manual: Test by grounding the connector on the blue wire to the engine (with the key in the on position, engine off). If the light does not come on now, you have a bad connection at the ignition switch or faulty wiring somewhere along this circuit. If the light comes on, the regulator or brushes are faulty or not making good contact.

My vote is faulty connection around the ignition switch or in the dash/cluster somewhere. To narrow it down you might have to do some probing with an ohm-meter, checking for a break in the circuit somewhere.

tomorrow I will testing the above and thankyou

[Peter]

I could be wrong but I heard somewhere alternators would self-excite without a 12v source given a lot of rpm (even if they are not the self-exciting type). Mine seems to require some rpm above idle to get the alternator going but it's more like 1400 rpm, not 3000.

That's a good point, the alternator case must be grounded properly for the exciter circuit to work, make sure it's making a nice clean contact to the engine block and that the engine to chassis ground is making good contact. I'm thinking it wouldn't hurt to have another ground right off the alternator, would be easy and cheap to do.

the grounding was checked..no real voltage difference anywhere

[Peter]

Well, I had Success today, first of all, I followed up on all the advice given and still the problem was not solved, but I was heading in the right direction..lastly I looked inside the alternator and saw that the little spring contact that pushes up against the voltage regulator was slightly tarnished, and I mean slightly tarnished...shined it up with 3m green pad and all works fine..the alterntor now "excites" at 1200rpm with 14.10v..something so small to chase down, but at least I learned something for next time...Thanks to All

[Peter]

I have exchanged the voltage regulator on the alt

A couple of questions to help diagnose:

- was the voltage regulator you put in new?
- how did the alternator's commutator look?

Failure to excite until 3000 RPM means that it *is* exciting, which means the blue wire, dash, cluster, etc are pretty much working.  Some things to check:

- voltage regulator, unless you know for sure the one you installed was good
- worn commutator
- alternator not grounding correctly... meaning one of the big grounds to the tranny perhaps

  the off and on failure to excite the alt. until 3000rpm has been corrected..pulling the #3 fuse for the "clock" in the cluster has made this alt.  now work flawlessly