jheis

Not seeing any alternator light when I turn the key.

Looks like lots of things to check this weekend. I don't relish the prospect of having to pull the instrument cluster to replace a light bulb!

James

John Struthers

Failing that.
Easier to yank the alt. take it to a automotive/electric shop -Safer than than the auto parts store, don't ask- and have them test it.
If it's the Paris Rhone - god awful contraption- they usually won't rebuild it. The Bosche they will usually do. I've ordered replacements from Checkers, Auto Zone, and Advance ... recommend they load test before you take it home, might have to swap your old pulley, too.
If you are pulling the pod there used to be several how to's with flicks in the archives and one or two guys have a pictoral on their home sites. Good luck
OOps
P.S. If you do have to swap alt's charge up the old batt, before cranking.

docmirror

there appears to be a resistor on the pod board that would allow current even if the bulb is dead? Not being picky, but it would be a bad design if a failed lamp faulted something as important as the charging circuit. Also, the bulb handling the entire current of the field surge would also be a bad design. Ref - print X/38 from pins 3R to 11R. On another note, if the bulb caused the failure, it's pretty rare for a bulb to be intermittant, although it does happen. Could be a dual failure of the bulb and the associated resistor at X/38?

Doc

SharkSkin

Doc, turns out that's the case. I looked back over my notes and '81 & earlier did not come with the resistor, though many cars have had it added so they can use a later alternator. On the early cars with no resistor a failed bulb will normally prevent the alternator from charging.

The 3W bulb limits exciter current to ~.25A. The 68Ω resistor only adds another ~2.2W, or ~.18A, for a total of about .43A. The bulb really does the larger part of the work in the exciter circuit and you can't do without it; later alternators won't work on a car without the resistor, and more current is going to the field in that case than a car with a good resistor and no bulb.

In any case, if the alternator light does not come on when you turn the key, you can't expect the system to charge -- whether the light doesn't come on because it's not getting power, the bulb is burned, or the wire is disconnected from the alternator, you have the same symptom and the same result(not accounting for PO wiring here ).

As Dr. Bob often points out, an alternator may "self-excite" and work OK without much(or any) exciter current. It's also possible for an alternator to "self-excite" only part of the time, because conditions are on the fringe between having enough exciter current or not enough. It sounds like this may be happening with James -- but I only mention it as a possiblity. I'm only trying to point out what could possibly be the root cause so James can look at those things -- positive diagnosis isn't going to be possible without a DMM in one hand and his shark in the other.... I don't claim to know the exact cause.

docmirror

Thx Dave, now we know why they added the resistor. That'll still should provide enough current to the field for his 82 to charge even with a dead bulb. Maybe rev the car quickly for a sec. Looking like the bulb needs to be changed anyway, or resolve why it's not coming on at key-on.

The pins listed in my prev post will help. I think I would work backward. With key-on but engine off verify ~12V at T14 pin 1, then ~12v at CEB O8, Z1, Z6, then H8, H7. If present on H8 and not present on H7, move to the pod and check 11R, then 3R. If yes at 11R and no at 3R do the bulb, and check the resistor. If yes all the way to 3R, go to the alternator and do the brushes, then bench test.

Doc

SharkSkin

Doc, not to be a PITA, but the resistor was not added to keep things working in the event of bulb failure -- though it may have that effect if the alt has enough residual magnetism.

The bulb did not provide enough field current for '82+ alternators to charge. The resistor alone would provide less current than the bulb alone. The resistor was added so that, combined with the bulb, enough total current would flow to the field.

I didn't confirm the terminal designations you mention, but yes -- it's down to tracing out the circuit. Note that the field wire, if disconnected from the alternator, will have ~10V potential to ground anywhere between the bulb/resistor and the terminal that attaches to the alternator -- probably 6-8v if connected to the alt.

jheis

Thanks for all the suggestions! Trouble shooting the charging circuit/alternator is my project for tomorrow.

Picked up a 1,000# engine stand from HF this am and my project for today is to get my spare engine up on the stand so that I can access my spare alternator (just in case I need it).

James

Alan

BTW be aware a 3W bulb limits current in a strange way. While at the rated voltage (say 12.8v) it will draw a current of approx: 0.24A (P/V = I) ~matching the power spec.

And its resistance at that time (burning bright) will be approx: 55 ohms (R= V/I).

However when it is not hot (either because its only just been turned on - or because it is supplied with much less than 12v... brightness to voltage is very non-linear so at 6v it will be much dimmer (cooler) not half bright) then it will have much lower resistance. When cold it will probably have about 1/10 its resistance at steady state rated voltage & power.

This phenomenon is actually useful some of the time - I suspect so here - an initial boost in current and also a larger than linear maintained current as the voltage differential reduces as the alternator starts generating.

We consisder bulbs to be very simple devices but they are actually quite complicated to understand fully...

Though of course you can usually simply plug in a replacement!

Alan

Lorenfb

"Looks like lots of things to check this weekend. I don't relish the prospect of having to pull the instrument cluster to replace a light bulb"

Always 1st just ground the field wire at the alternator and check for the alt
light with the key "on" and the engine not running. Also, you can always check
the alt in the car, i.e. bypassing the alt. light by just connecting a test light
(incandescent) between the B+ and the field. This should excite the alt. and
start charging.

"later alternators won't work on a car without the resistor"

Not true! All the 928 alts, including the Bosch 115 amp, will always start with the PROPER
alt bulb. That's the case for ALL Bosch alts used on Porsches, BMWs, and Mercedes.
There's nothing unique about the 928 Bosch (or the Paris Rhone - Valero). The only problem
that might occur is when one uses a junk replacement regulator or the Bosch regulator is bad.
Also, bad brushes or slip rings can cause too low of an excitation current.

Brian B

I have a similar problem with my Alt. Dash bulb. Mine does not go on when I turn the ignition key on (when all of the pthers light up) but it does go on when I shut the car down. The car seems to charge alrright, but I was wonfering if I should pull the pod and replace the bulb.

I added the resistor when I changed the alternator a year or so ago.

ceedee

to add something interesting...
to find the bulb - the one that illuminates the battery symbol.
i had to take out the pod (again mine is an 83) unscrew the resistor and four more screws and then
pull out the two black brackets and then the whole assembly, inside are two white caps holding the two lightbulbs, one for the oil pressure and one for the power indicator. have your soldering iron ready, cuz they don't just pull out.
i did all that to find out my bulb was A-OK...

jheis

Well, I was going to try and trouble shoot the problem today, but it's working again!

I am getting a light at the bottom of the gauge (about the same position as the reserve light on the gas gauge) when I turn the key on and when I turn it off, so I know the bulb isn't burnt out. (No light in the battery symbol - but I don't think there's supposed to be one there on an '82).

With the engine running, I'm reading about 14.6v at the underhood + terminal when it's charging (only about 11.6v when not charging).

Guess I'll have to wait until it does it again.

James

SharkSkin

Intermittent problems suck. James, early cars have a battery symbol on the charge light itself, sorry if that confused you. Sounds like you may have a loose connection. If, the next time this happens, you turn the engine off then turn the key back to "on" and the light comes on, you can rule that out -- in that case I'd bet on the regulator.

jheis

Dave:

Yeah, intermitent problems really suck - especially since you don't know when they're going to strike. If I'm just hopping in the car for a run to the P.O. - no big deal. If it happens on a day I need to be in the City, it can be a real big deal - don't want to risk a 110 mile RT trip running on the battery (of course, there's always my old rusted-out Toyota pickup).

Just to be clear, when it's working - the alternator light comes on when I turn the key. When its not working there's no light and no charge. It's hit or miss. If the alternator light doesn't come on with the ignition, turning it on and off repeatedly doesn't make any difference - tried that about 15 or 20 times.

Anyway, got my spare engine up on a stand (by the way, the "1,000 pound" stand that HF sells is REALLY marginal for a 928 engine. Anybody who expects to support a thousand pounds with one of these things is nuts). Pulled my spare alternator & got it all shined up, so I'm ready to go. Just not sure where I'm going.

James

SharkSkin

OK, it sounds 99% sure that either the D+ connection to the alternator or it's internal connection is flakey, or it's the regulator.