All,
I had a previous post about this issue on 8/3/21, but decided to make a new thread after following advice given in that thread and others.
Original thread at; https://rennlist.com/forums/928-foru...-3000-rpm.html
Since, I've conducted more tests and have more data for you provide insight. Sorry for the long post, just trying to be thorough and not waste your time.

Issue: 1979 Auto TMU. Starts and runs, idles well, drives well (after much work), but alternator does not "excite" (or charge) until I rev the engine. More details below.

Background: After doing a lot of underhood work, the car starts and runs great. However, some of the gauges and warning lights in the cluster were non-functioning or absent. So, I installed a new Porsche printed circuit film. Bench tested all bulbs prior to install. After that, most of the warning lights worked, but not all. So, I swapped out the Central Warning Unit (CWU) with a known good unit from my other '79, and 'boom' everything in the pod worked.....except I still have to rev to about 1300 rpm for the alternator to "excite". (Prior to CWU swap I had to rev to about 3000 rpm.) And yes, I have DeOxit'd every ground and the connections (again).

More Background: I observed that my alternator appears to indeed be a rebuilt Porsche unit. I've posted pictures below. The first shows the "blue exciter wire" (which does not appear blue in the pic, looks black) and the part #. The second shows the "Valeo" label that was placed over the alternator part #, I'm assuming during rebuild?

A. Continuity tests. The "blue exciter wire" follows an interesting path from the pod cluster to the alternator, according to current flow diagrams (and my observance).
From the 2R plug at the pod cluster, to H8 at the Fuse/Relay Board (FRB) (not my terminology, from Porsche) to Z6, to Z1, to O8, to Terminal 1 on 14-pin, then to D+ at alternator.
(Sidenote: when disabling the alarm on a '79 you unplug "Z" plug at FRB and bridge Z6 to Z1...so basically you are just "saving" the blue exciter wire circuit.)

All of these continuity tests were performed (twice) and had good continuity (battery disconnected):
1. 2R at cluster to H8
2. H8 to Z6
3. Z6 to Z1
4. Z1 to O8
5. O8 to Pin 1 on 14 pin.
6. Even this very long test....2R plug at cluster all the way to to female 1 at 14pin. Continuity check. "Inside" is confirmed.

Removed cover from alternator and inspected everything. All looked good. (Except I noticed a suspect connection at the oil pressure sender that may need attention...later.)
All wiring in the Front of Engine Harness (FOE) was inspected and is intact and not exposed or damaged.
1. Removed exciter wire from alternator, perfect continuity to male pin #1 on FOE at 14pin.

B. Voltage Tests: This is where it gets interesting.
1. Jump post at rest: 12.35v (battery fully charged overnight)
2. Jump post after I excite alternator: 13.8v
3. Female #1 at 14pin with key at Run: 9.15v
4. Z1 or Z6 (remember, path of blue wire) with key at Run: 9.15v
5. Plug in the 14pin, start car, running, but alternator not excited yet: at Z1: 1.2v
6. Plug in the 14pin, start car, running, but alternator is excited/working: at Z1: 13.8v

So, the exciter circuit (blue wire) is at 9.15v when key turned to "Run". Once car is started, without exciting alternator it is at 1.2v.

I have not removed the resistor from the back of the cluster. I tried to test it, without removing it all, but readings were not consistent. I am getting 9.15v through the blue-wire-exciter circuit when ignition is at run, so I suspect the resistor is doing it's job? (Note: even though the printed circuit film is new, the resistor is not. I had to transplant it from the old.)

Finally, bulb check. After the circuit film and CWU replacement, all warning lights and gauges come on when key is turned to Run.....except for the voltage "idiot light."
I haven't replaced the pod cover, so it was easy for me to remove and bench test the bulb. All good. Replaced it twice with two bench-tested bulbs. Still nothing.
Ground the #1 female at 14pin, turn to Run, and still no bulb.

Sorry for the delay....this testing took some time. (Heat index in Dallas is 103 right now.)
Thanks in advance for any help/direction.

 

Sorry. Just looked up and that part # on my alternator is for 1982+. 92860311401
Could that be the issue on a ‘79?

 

What happens if you put 12v to that exciter wire from another battery source - does the alternator then immediately excite and start charging? Does the dash gauge show it not charging and then excite and start charging?

 

Hmmm. I can try. It Is going to be tricky to apply 12v to the exciter circuit while everything is plugged in and car on. Only thing I can think is send it through Z6 or Z1 jumper in circuit? I just don’t know if that is a good idea. (Don’t want to fry anything )
The resistor is there for a reason. And I am getting 9+ Volts in the circuit until I start the car and it drops to 1v.
And I’m not sure how that would solve my issue?

 

I think what's happening is you're not getting enough voltage on that Exciter wire until you rev up the engine and the alternator speeds up and puts out more voyage, then that voltage is making it through the Exciter wire to trigger the alternator to start charging. I had a Saab turbo with this same behavior and just wired a 12v switched wore to the Exciter so it would get 12v when the car was running and never had the issue again. Thought you might be able to do something similar from a wire off the 14 pin connector. I used one of those on my supercharged 79 to wire a switched 80A relay that feeds a secondary fuse box for my heat exchanger for the intercooler, the hid headlights, and the electric fan controller setup.

 

Thanks for the suggestion. I'd prefer to fix it as intended, and not add more switches or relays.
Does anyone have any suggestions on what is going on?
Do I need a different alt or resistor?
(I'm still stumped why the exciter wire is at 9.15v with key at "Run" but drops to 1.2v when started--(pre-excited)).
Thanks,
-Jason

 

You need to replace the alternator light bulb - actually a circuit board with a bulb - if that is bad the alternator will not excite.

 

The behavior should be that the exciter wire should be at ~2V with the ignition on and car not running - with the (no) charge light bulb on. With the alternator running (after excitation) the exciter line should be driven by the alternator regulator up to approx the same voltage as the jump post (~13V+ when cold). Yours seems from what you say to behave backwards.

Grounding the exciter line with the ignition on - car not running - should illuminate the bulb - does it?. If not there is likely a failed bulb based on your other results.. Connecting the jump post to the exciter line with ignition on and car not running should leave the bulb off.

With the car running at cold idle, temporarily connecting (~1sec) the exciter line to the jump post should cause the alternator to start generating immediately and you should see the exciter line go to and stay at the jump post voltage and the jump post voltage should have risen to ~13V+

Do these tests to see what happens. Depending on the results - you may have a problem with the dashboard bulb end (this is the only place the exciter voltage/current comes from before starting). The resistance (bulb+resistor) may be too high to initiate the alternator (this will affect the voltage you see). Alternatively there may be a problem with the alternator regulator. If what you have said is fully accurate I'd expect there may be issues with both. The checks above will be help to isolate that.

Most likely the bulb (or its connection) is bad - start with testing for that.

Alan

 

Had this issue on an early '78 where I had tested the alternator warning light bulb as good, and then managed to not completely screw in the bulb socket to make good contact on the circuit board. Had to pull the cluster again to fix it, all works as intended now.

Pick a rookie mistake, I'll make it.

 

I've been following this thread because my '79 has the exact same behavior.. i think.

You're saying that a bulb in the dash cluster will prevent the alternator from exciting?

Is this the red indicator at the bottom of the voltage meter? If so, then that's not my problem, because on mine my bulb does light, according to whether the alternator is putting out enough voltage or not. The light is on when I first start it, rev the engine, the light goes out as the gauge needle rises above 12v.

 

In your case you may need the resistor in parallel to provide enough excitation current. For some alternators you need both - if either fails you may not get excitation on start up.

Issues could be due to failure of the bulb or resistor (or poor connections) or could be that a non-stock alternator (for the year of car) has been fitted - this may require adaptation of the dashboard.

Alan

 

its the original model,, rebuilt at least once.

 

Then check or replace the resistor and bulb.

Alan

 

just to confirm.... this is the bulb in the instrument cluster?

 

Correct, the bulb in the gauge cluster.