My speedometer gear has been replaced, and I am ready to reinstall the instruments into the pod of my 80 928 Euro S. I recall a resistor change being discussed but am unable to locate it in the archives. Symptom: Upon startup my alternator light remains on at idle until I "blip the throttle" slightly. Once it energizes the alternator field, the light stays off when it returns to idle. The resistor at the back of the voltmeter measures 151 Ohms. Can I change it to correct this issue? Can anyone point me to the link?
Charles 80 928S
PS: I replaced all the light bulbs in the instrument cluster. There are quantity 19 of the 12/1.2 - the ones in black plastic holders - OEM was Osram; Wagner # 17035 is a direct replacement. The three (3) at the bottom that illuminate the cluster (and dimmed by the dimmer knob under the dash) are 12V 3W. OEM was Phillips; Replacement is Wagner # T168. If you're thinking of replacing with a higher wattage, don't. I did and noticed that it started to overheat the surrounding area. A couple of these bulbs "chewed up the circuit board" where they connect. The work-around fix that I used was to directly wire them into the same circuit on the other side of the connector. (Black wire with blue stripe, and the brown ground, next to it.) Hope this helps someone, or at least saves a trip to the parts store. My local dudes asked me to bring a bulb in to match it, then had to order them as they didn't have that many. Sheesh, you'd think I was asking for parts for a 23 year old exotic car or something...
According to Tech bulletin 11 in the Pats and Tech book, you want a 68 ohm, 5 watt resistor. This is related to changing the alternator from Marschall to Bosch. (Do I have that right?) Metal to plastic duct, I know.
You might give your alternator a check.
I think you are talking of two different resistors. I had discovered and replaced the potentiometer on the backside of my voltmeter before discovering the internet and Porschephiles. The problem was the voltmeter was wrongly pegged at full scale, sometimes. The wiper had worn a spot.
I don't believe that resistor/potentiometer has anything to do with your alternator light. Additional evidence, Glen Larson says the resistor you need to replace should be a 5W (5 watt) size. Nothing near that much power flows thru the potentiometer/resistor on the back of the voltmeter. I used a hand selected 1/4W fixed resistor about 40 ohms. The original potentiometer was of about 100 mW or 50 mW size.
There may be two resistors back there. The 5 watt one feeds power to the
exciter circuit and is in series with the "warning" light part of the
gauge. It is not used in the circuit for the actual voltage reading.
And in consideration for the audience, a 5W resistor is most likely going to look like a 1/4" x 1/4" x 1" tan/beige ceramic brick. It will not have a screw adjustment as the OEM potentiometer found on the back of my '83's voltmeter. It will be mounted in the open, at least 0.2" above a PCB so as not to cook other components.
Glen gets the Prize for fastest correct answer! His answer triggered some
memory cells that haven't been used in awhile. I checked the old "core
alternator" box in my basement. The old one was indeed a Marshall, the new
one on the car is likely a Bosch - it has a plastic cooling shroud. In the
box with the packing slip is a 68 Ohm 5% resistor (big package likely at least a 1/4 watt). Looks like this update has been waiting since I replaced
the alternator. The packing list says 1998. Price then was $231 for alternator ($198) plus replacement hose and shroud. How does that compare
to today's cost? Anyone want a Marshall core alternator? (or should I keep it for "originality" when these cars become collector's items?! )
Charles 80 928S