ckabee1

I'm posting this for a friend of mine. He will be reading and responding:

Wondering if anyone has experienced this problem before and what I might try to fix this issue. I have a '85 928S 5sp US stock except custom exhaust. I am having alternator problems causing the car to run erratic. The battery is reading 12.0V at start up and idle @ 800rpm. The voltage stays consistent thru the rpm range from 800-2100rpm. At 2100rpm the voltage raises to 13.4V (alt. starts working) and the car becomes erratic and will choke out if you let off the accelerator. I can cycle the key and the car starts right up, idles fine, and the issue continues. If I unplug the alternator the car starts and runs fine all the way thru rpm range up to 5k as it should. I replaced the alternator with a new Bosch and the problem continues. Does anyone have a clue what could be causing this? Thanks in advance for any input

jwillman

Not sure this is the case on all models but on older cars the Alternator low voltage light is in the alternator excitor cycle as is a small resistor on the back of the instrument pod wired into the alternator guage system.

Porsche changed the resistance rating from 250ish to 68 ohms for this very issue. With the higher ohm resistor the alternator may not begin charing until 2000 RPM. The low volt light must also be operational. At ignition on the light should be illuminated. If not the bulb maybe burned out / inop. There was also a change to that bulb assembly in the guage at some point.

I did not have the idle issue but did have the no charge till 2000 RPM and resolved that by replacing the old stle resistor with the newer 68 OHM.

risaac928

Can you post a pic if what that resistor would look like. ..

WallyP

The alternator-caused misfire is a known problem.

From Page D 24-27 in the Factory Workshop Manual:

Test Point 21: Alternator, regulator
Engine misfiring may be caused by alternator voltage peaks.
Remove the drive belt and start the engine.
If this eliminates the fault, check the alternator and regulator.

Changing the alternator should have fixed the problem. Possible, but very unlikely, to have two successive alternators causing the same unusual problem.

Perhaps a poor connection somewhere in the charging system - don't forget the ground connections.

Jim's answer is the basis for the slow start on the alternator. Don't forget that a poor connection anywhere in the string from the ignition switch, thru the light/resistor combo, thru the Central Electric connections, thru the 14-pin connector near the jump start terminal (prime suspect!) can cause the slow start in charging.

WallyP

Standard resistor, mounted on the back of the dash voltmeter.

jwillman

Here is the back of my instrument pod. The alternator resistor is at the far left runing top to bottom mid way inside the red circle. The one pictured is the old ~250ish ohm resistor. The new 68Ohm resistor I got from 928sRus is larger and square.

risaac928

Thank you for posting that pic of the resistor...this helps me understand things much better. You know on my 82 928 I was going through some trouble shooting on an unrelated issue and found a similar "resistor" tucked away in some wiring in the engine bay. I wonder if the P.O. had the same problem.

soltino

This Video is Private

WallyP

If I remember correctly, the video shows an attempt to change the fuel/air ratio by making the control system think that the engine was cold.

risaac928

Sorry...got it PUBLIC....