MMP1

The volt meter on my 1980 928S Euro (5sp) was basically hovering around the bottom of the gauge and the red light was coming on and was told by mechanic that my alternator was shot which I had replaced. This was in addition to getting the timing belt and pump replaced...and while they were working on it discovered that the engine wiring harness was shot so had that replaced (which when installed caused the temp gauge needle to pin to HOT. Mechanic discovered that the wires for the temp sensor had the terminals installed incorrectly and had to remove the terminals and install correct terminals on proper wires).
Long story short I picked the car up last Friday and on the drive home realized the volt meter was still hovering around near the bottom of the gauge and as I drove the red light on the gauge came on. Got home turned the car off and would not start. Jumped car and went and replaced the battery and volt meter still dropping and battery not recharging. Not sure what else to do here as the battery and alternator have both been replaced? Any thoughts on what the issue could be are appreciated. Spent a hefty amount getting the other work done and was hoping to be able to drive the car before the snow starts to fall.

ROG100

Who made the engine harness? Sounds suspect.
It could also be the resistor on the back of the cluster. What alternator was put on the car?

docmirror

As I recall, the dash lamp for the alt field will stop the alt from charging? Just trying to recall one of those. With a 12V charging system NOT attached, and a full battery turn the key to the 'ON' but not 'START' position and look for a red 'GEN' lamp. This is not the low battery indicator in the voltmter. With the engine running, check for approx 12V on the field or D+ terminal of the alternator. Edit to add, there's a resistor on the board as well. Might need to check that.

MMP1

A Bosch alternator was used during the replacement. I have to ask who the wiring harness was ordered from, somewhere in CA I believe. Is the resistor you're referring to the "charging resistor", which was also replaced during the alternator replacement.

Speedtoys

My reman Bosch alternator for my 83 lasted 5 minutes.


The original, sat on the parts car for ~7yrs Paris-Rhone once I grabbed, worked super when I swapped out the failed new rebuilt bosch.

ROG100

All I sell are the Bosch alternators and maybe two warranty returns out of hundreds over the last 10 years.
Warranty returns were for heat damage due to no cooling duct and warranty was denied.

ROG100

The Bosch alternator was a good choice and with swapping the resistor you have done all needed. Is the cooling shroud and air duct from the left front fender in place.
Refer to post #3.
That leaves the harness maybe. CA = Greg Brown or 928 Intl and these are excellent sources.
If not then all bets are off.

Mark Anderson

But the Bosch for 78-84 have no provision for a cooling duct

ROG100

It does if you use Bosch AL325X and add the later 928 603 133 08 shroud.

Mark Anderson

ur a crafty devil Rog

ROG100

You taught me well O great one 8>)

bwarren97

When we bought our 83 earlier this year, I had been trying to troubleshoot an alternator problem for several months. The charging circuit wouldn't kick on until the engine was revved to sound 2k, and on acceleration the oil light would flash.
​​​​​​ Turned out the D+ wire was screwed onto the wrong terminal from the beginning, instead being attached to D-

MMP1

Thanks guys for the advice. Turns out I was incorrect on the brand of alternator. The rebuilt alternator installed was a Valeo and it was 928 Motorsports who built the engine wiring harness.

GregBBRD

Very doubtful that the wiring loom is wrong.

Running a one piece blue wire from the alternator to the #1 pin on the 14 pin connector in a new loom is about as simple as it gets. I'm pretty sure every random homeless person in the country could accomplish that.
.....Although when you said that the temperature sender was wired incorrectly, that's just as simple, which raised my eyebrows.

You might want to pop the cover off the 14 pin connector and make sure there's a blue wire to #1? And that the same blue wire is attached to the D+ post on the alternator. (Post #12....amazingly common.)

All that being said, I can't imagine a mechanic installing an alternator, a wiring loom, and then not checking the charging voltage. That's like taking a poop and not wiping your a$$. Have you called your mechanic and asked him anything?

MMP1

Thanks. I'm going to bring the car back the mechanic on Monday (along with the helpful tips given here) and will update once I get feedback from them.

Thanks again!