checkmate1996

IMHO, I'm leaning toward belt slippage just because everytime I tightened the belt a little more, I got better results...

Alan

Imre - read on - seems later with pulley cleaning and additional tightening Brad got quite a bit better results at 800rpm - so I'd say looks like it really helped.

I agree rpm does make a big difference too - so if its adjustable by all means raise it up a little and see if that helps.

Alan

dr bob

Each time you tighten the belt, it rides lower in the little pulley and the alternator spins a little faster. For grins, look at a crank pulley on a GM car that had that alternator installed originally. Is it bigger or smaller than the one on your 928? I'll speculate that it's not any bigger than the one on your 928. FWIW. So the Camaro or Caddy that spawned the one you have would likely not be making any more idle volts than you are seeing. Also, if you are actually measuring at the battery terminals, you have voltage drop between the alternator and battery that the GM cars don't. A few tenths of a volt when charging at idle wouldn't be at all out of line.


Just some thoughts.

The general thought might be that there are few cases where the car sits at full electrical load at idle speed for extended periods. I guess on a very hot rainy night at the In-N-Out drive-through, or the post office late tomorrow night, it might happen. Generally though, if the alternator can maintain 12.5 or so it's keeping up with the external load. That isn't enough to recover charge in the battery very fast, but it also isn't draining the battery.

ew928

Measure base battery voltage. (engine off, accessories off)
Measure voltage with selected load and car at idle.

If idling voltage is lower than base battery voltage, then the battery is helping run the car with the selected load. That's how I come to understand the electrical charge system.
The killer electric load I found was being stuck in traffic at night with the rear defrost on.
That rear defroster is a killer.

Don't most GM vehicles have the alternator and battery in the engine bay so there's a very short wiring connection between the two.
Is price the only reason for the AC Delco experiment.

Think I have a Ford (Mopar) or GM (AC Delco) voltage regulator on my Bosch factory alternator. Car had been electrically unhappy and the foreign regulator gave a happier (higher) voltage. Voltage was checked at battery terminal with a DVM and also at cigarette lighter with a KMart digital voltage gauge.
The cigarette lighter voltage meter was good for impromptu spot check of battery voltage.
Now I've got the Passport 8500 radar detector set on voltage display mode.
Really ossifer. The radar detector is there to check my car's charging system.

checkmate1996

[QUOTE=ew928;5312245]Is price the only reason for the AC Delco experiment.

QUOTE]

I think price is a primary driver. When looking at alternator options you have:

rebuild a bosch 75amp (OEM '79) for $150
purchase Carl's @ 928motorsports for ~$600
purchase a rebuilt 928 alterantor - $200-$300
GM conversion mod 105A- $71

Alan

The reason I chose 12.7v as a reference level is because a healthy low maintenance wet lead acid battery at 100% state of charge will be charging at this voltage at any temperature.

Its actual open circuit voltage will vary based on temperature and its state of charge.

12.7 volts ensures charging of the battery under all conditions while at 12.5 volts a healthy battery is supplying some current and will until it is down to approx 80% SoC (depending on temp.) This would also be acceptable in typical use but does cycle the battery more.

The voltage drop over the battery cable is still not much because it is sized much larger than on most engine compartment located batteries.

Note that the voltage drop will usually be negative if the battery is charging e.g. the battery terminal voltage will be slightly below the system voltage at the alternator/jump post. However if the battery is actually supplying power (system voltages below 12.6v) then this voltage will be positive - the system voltage will be below the battery terminal voltage. In either case the differences will be small.

This is complicated by some direct seperate feeds from the battery terminals (esp on later cars) while most major loads are connected chained off the alternator directly.

Alan

borland

charging voltage is highly dependent on battery condition/charge. So your idle voltage is subject to the condition of the battery.

I rebuilt the 115amp Bosch alternator on my 90' S4 for about $30 in parts. That included a new aftermarket regulator which maintains 14 volts on the dash gage at idle with the battery at full charge. The gage only dips when the radiator cooling fans kick on but then immediately returns to 14 volts.

hacker-pschorr

What year did the 928 switch from a V-Belt to a 6 rib?

Alan

Its only dependant on the battery condition if the battery requires more charging current than the alternator can generate after supplying the rest of the car loads...

With minimal accessories the battery would have to be quite heavily depleted to cause most alternators any major trouble meeting their reference regulator voltage. However a severely depleted battery and/or lots of additional accessory loads switched on will have this effect - its just a total current load question.

A significantly depleted battery will certainly consume more current at a given system voltage - good reason to ensure its always staying well charged.

If you start the car (depleting the battery) and immediately pull into stop & go traffic where you are idling much of the time - this would be a bad scenario to be running at <12.5v without ample high rpm driving mixed in to make up for it.

Alan

checkmate1996

This is certainly a viable option IF you already have a 115amp alternator and one that fits your car...

6mil928

I'm doing the mod in the AM. The connector I got has red, black, and brown wires. Anyone happen to know which wires goes to what. If anyone has a pic of the painless connector wires attached I can figure it out I believe. Jason

Landseer

On mine, the connector plug was labeled.

Retain only the connector wire market "L". Solder or otherwise splice it to the small wire associated with your wiring harness. Rest of them are not used.

You can remove the others and fill the space with silicone gasket material. You can wait until you are sure it works before you make that final mod.

6mil928

Was the "L" wire the larger of the 3 wires?

Landseer

Let me check, be right back

Landseer

It goes SFLP....S is big.....Use L, ditch the rest.