928Jeffrey

I have an 84 928. I have been trying to track down a wiring issue in the alternator charging circuit. I thought it was the exciter circuit after having looked through it I'm stumped and could use some help.

Above 4000rpm the alternator produces 13.8 to around 14.6 volts. It only produces voltage above 4000rpm. After the rpms drop below 4000rpm it provides nothing and all I read is battery voltage.

When I turn the ignition key to the on position the red light on the dash voltage gauge turns on. After the engine is started it will stay on until the rpms reach a high enough point to create voltage. Then the light goes out. Of course the light turns back on when the rpms drop.

I tested the voltage at pin #1 on the 14pin terminal in the engine compartment. On the female side of the terminal pin #1 with the key in the on position I have battery voltage.

With the blue wire disconnected from the alternator I also have battery voltage. At the end of the wire where it would be connected to the alternator.

I also disconnected the 14pin terminal and the blue wire from the alternator. Then tested for continuity on the engine harness between pin #1 to the disconnected end of the blue wire. Found continuity. Also tested for continuity to ground. No continuity.

OK. So it seems there isn't a short from the pin #1 to the blue wire that connects to the alternator. At least on the engine harness. Also I have battery voltage at the blue wire while not connected to the alternator. Once the blue wire is connected to the alternator I only read 1.5 volts with the key on. Not sure if something is wrong there. I originally thought there must be a short in the engine harness. Now I dunno.

Mrmerlin

Jeff has the voltage regulator ever been checked for brush condition?
The 14.6 volts is higher than normal.
Is this a Bosch regulator or aftermarket part?

Note the blue wire is usually damaged from hanging the alternator by the wires and it will shear about 4 inches up the orange sheathing from the alternator rear cover

928Jeffrey

The alternator was recently replaced by a shop. And it is a Bosch. Have not checked to see the condition of the blue wire 3 to 4 inches up. I get battery voltage at the blue wire while disconnected from the alternator. So I ruled out damage to the wire. Will check it and see if it is damaged tonight.

Mrmerlin

remove the regulator and inspect it,
if it is not a Bosch part I would suggest to swap in a genuine part

Ladybug83

Also, check the wire bridge on plug "z" on the CE panel. Mine was plugged into the wrong terminals. The correct terminals are labeled in the wiring diagrams, I'd post them but I'm not near my WSM. Once I plugged the wire bridge into the correct terminals, the exciter circuit worked the way it's supposed to.

Landseer

Be very careful bridging alarm on 84. It is different pins than all other years.
Sparks can fly. Btw, sounds like alternator issue, not exciter.

928Jeffrey

Currently heading in to work now. I work nights so I will be back troubleshooting the car around midnight.

The Deputy

What does the alternator do (while rpms are high enough to produce a charge of 13.6) when you apply a load? While it is in a charging state, apply a load of the radio, lights, fan and turn signal...and see what that does to the charging systems output. If the alternator is in working order...it should not even miss a beat and charge 13.6...or marginally less.

Also, be sure to check and clean all of your ground cables...a good ground is crucial to the operation of the good charging system.

Brian.

Alan

What you describe on the exciter line is normal operation. When you start the car - what happens to the exciter line? This is where the issue is. It should go to ~ the jump post voltage - but I suspect it doesn't except at high RPMs. I think you just have an alternator issue - take it in to a good alternator shop for testing. It may not be only the regulator from the symptoms - you may have some fired rectifier diodes too. 14.6 v is too high - it will eventually destroy your battery !

Alan