hupp

For a '79 5-speed: New battery, aftermarket alternator (approx 4 year old Valeo from one of the big 3)

The voltage reading on my dash has suddenly dropped very low. While the car is running the gauge will read just above the 10V line. I know these gauges are not always that accurate, but there is a problem here because there are obvious signs of discharge -- slow windows, radio cut-off etc after driving. No draw down over night only while running. I'v done some thread searches and edjucated myself on some of the common charactistics of the OB electrical gremlins and have done some trouble-shooting:

1st thing this morning, car off:
- Across battery terminals - 12.7v
- Across + terminal and ground to body - 12.7v
- At jump start Terminal - 12.7V

With car running:
- At jump start - 11.7V

After a short ride (about 5 miles) car off:
- Across battery terminals - 12.3V
- At jump start - 12.3V

With car running:
- At jump start - 11.7V
- Across battery terminale - 11.9V

Bottom line from what I could see is the battery is definately being drawn down after a drive. Also evident is an approximate voltage drop when the car is running as measured at the battery and jump start post.

My guess now is that my alternator/voltage regulator has gone south. What do you guys think?

ROG100

Either the regulator or the exciter circuit.
Do a search as there was a similar topic a couple of weeks ago with plenty of indepth info from the electrical BOFFIN Alan and others.

Alan

Since you readings with the car running are lower than the battery alone at rest - you are not charging at all. The battery is running the car - it won't for very long. Your new alternator is either faulty, not connected correctly or your charging exciter circuit has a problem. As Roger says - see the previous post on some more detail of that. Avoid some of the pitfalls there.

Alan

hupp

Did some research and got back under the car to test the exciter wire. My intent was to remove the wire to perform the voltage test on the unattached wire with the key off (should be 0v) and key to on (should be 11v-12v). Well I did not get that far...

When I removed nut, which fastens the exciter wire on the terminal, the terminal screw all but disappeared into the alternator. In other words, I have a loose terminal post/screw ratteling around inside the alternator. I'm no genius, but I'm assuming that this is not ok and that the terminal is no longer connected to what it's supposed to be connected to inside the alternator ... if that made sense.

hupp

One thing I forgot to mention before is that the red low voltage warnig light does not illuminate during bilb check. All other bulbs function ok and shark runs fine otherwise -- it just does not charge.

So something is not right with the exciter circuit based on what I've read. I still need need to check the detached exciter wire, but I do have a question until then -- can a faulty exciter termination at the alternotor cause the "no warning light" symptom, or is this an issue behind the cluster (i.e. the resistor/bulb)?

Jim M.

The voltage light is in series with the exciter circuit. You've found your problem, it's with the alternator. Replace it or fix the loose wire inside.

Jim Mayzurk
93 GTS 5-spd

ZEUS+

With the wire disconnected you have an open circuit and light will not illuminate. Fix alternator, make sure exciter circuit is good and re-test.

Mrmerlin

DISCONNECT the BATTERY. remove the alt ands retrieve the screw also add a nut to the screw that fell into the alt so it is always positivly nutted and get a secoind nut to secure the loose wire

hupp

Made some progress this evening and I believe the answer all but smacked me in the face.

I detached the exciter wire and tested with the key turned to on (bulb check mode) -- 0V at exciter so the problem is here.

Before diving into the pod, I decided to detach all of the alt wires and slide off a portion of orange wire loom to have a look at the wires. Well, about 4 inches of exciter wire slid off with the loom. Someone had apparently spliced this 4 inches using a crimp coupler at some point in the shark's life - this splice located very close to the sharp bend in the wire bundle. Thinking back, I have had the drivers side cam tower gasket replaced and the mechanic could have disturbed this splice by reaching up to the cam tower bolts or by possibly moving the alternator out of the way.

So, the plan is to do a better job splicing back together, test and hopefully all is good.

John Struthers

Jeez...
Like the creme' de le creme of the Lucas Auto-Electrical Engineering Community College-employed briefly- at Porsche couldn't inflict enough grief upon us.
I'm just tickled pink to learn, yet again, that there are those who would endeavour to raise the bar of 928 electrical dead-ends.
Pour me a double, please...
Hang in there Scott.

hupp

I pieced the exciter wire back together and tested -- 11V

Re-attached all alt wires, re-attached battery ground and turned key to "on" position -- warning light is now on.

Re-installed alt cover and plumbing and cranked her up. Started fine but no charge; however, the warning light was on as it should be.

Gave the shark a quick rev-up and bingo! Needle pegged in middle of gauge above 12V line.

Tested at the jump-start post and we have charge -- approx 14V. All seems good for now.

Sailmed

It's not wise to tempt the 928 Gods!

Dennis Wilson

Scott,

You may still have a problem with the exciter circuit. If your new alternator is a Valeo, it self excites at rpm's above 2500. You may need to replace the resistor to properly excite the newer alternator.

Dennis

hupp

Dennis,

I'm fairly confident that this solved the issue (tempting the 928 Gods again). My reasoning:

- Prior to my repair, the gauge would not show above 10v at any rpm. I drove around quite a while thinking I had a gauge problem and not an alt. problem.

- I tested the wire before repair and 0v were present, after 12 v were present.

- Alternator charges now without the need to rev-up.

But just in case the gods are not kind -- where is this resistor located?

Dennis Wilson

Scott,

It is on the circuit board in the dash. Glad to hear that it starts charging without reving.

Dennis