shadowknight

Over the weekend the old alternator was replaced with a rebuilt bosch. while driving home tonight with the lights and hvac on, the voltmeter within the dash cluster indicated less than 11V's while at idle. Above 1000RPMs it would indicate 13.5 volts. Once home I ran a multi-meter from the jump post and checked it again. With lights and HVAC on the alternator is generating at idle 12.1 volts. With the lights off and hvac on 13.1 and with lights and AC off 13.6V. Anyone have a suggestion on what my problem maybe? The previous alternator was having a similar issue as this new alternator but not to the same extent. It would indicate just past the 12V mark with lights and HVAC at Idle and 13V at or above 900RPM's from the dash cluster voltmeter.

Mrmerlin

to get the most out of the charging system you should clean all of the grounds and the hot wire connections,
the only other ways to increase charging system output is swapping the pulleys
IIRC Alan has done this

Alan

Your exciter circuit seems to be disconnected - did you connect up the small connector on the back of the Alt?

Alan

shadowknight

The exciter wire is connected, I checked some older posts concerning the battery strap and ground points. Thinking it might be the battery strap last night I added another cable which had no effect. I believe I have narrowed to the radiator fans. When the car is cold, the AC is off and I have the following turned on: Blower Fan, Headlights and Radio (two amps and capacitor) the voltage is 13.5 at idle. Once the fans kick on, either due to AC or engine temp the system drops to 12.1. Not sure why replacing the alternature would cause the fans to act up. Where do the fans ground out at?

17prospective buyer

Could it be that the rebuilder doesn't/didn't test the rotor and stator windings? That could cause low charging voltage, but this could also be caused by the regulator. So you'd need to maybe bench test the alternator on a machine to see if the regulator is regulating the exciter current. But check the charging circuit wiring for cleanliness/corrosion as well, could just be that. However even with bad/dirty/corroded terminal connections, the output should still be 14-14.8V. Measure open circuit batt voltage with a DVOM after a drive if possible, the dash gauge is just a dummy gauge if anything, too inaccurate for diagnosis.

How does the car start? Does the starter seem to be cranking at the normal RPM? Check the "30" terminal on the starter solenoid. Should be a red wire going from the alternator to this terminal.

Alan

Its not a ground issue, its an alternator issue, its possible there is a problem elsewhere in the exciter circuit - that's where I'd look...

Does the charge light come on in bulb test mode (on the dash voltmeter), does it go out whenthe car is running?

Test the voltage on the exciter line at the alternator (small connection). With key out it should be 0v, with key in ignition position car not running should be about 1-2v, with car running it should be 12+V.

Test car running exciter voltage at cold idle, 3K RPM and ideally also hot idle...

Also actually perhaps the simplest yet - is the alternator belt tight, really really tight, tighter than you think it could ever need to be...

Alternator belt slip is worst at idle with high electrical load (like the fans for example)....

If slipping the belt will be getting very hot... If this was the issue - get a new belt - its probably been toasted

Alan

Lorenfb

"the voltmeter within the dash cluster indicated less than 11V's while at idle."

Not a good indication of what the alternator's voltage really is, i.e. the 928
voltage gauge because of voltage drops rarely measures the actual voltage
of the alternator. One ALWAYS needs to start by measuring the voltage
right at the output post of the alternator with respect to the case of the
alternator.

Dictys

What condition is the front engine harness? particularly where it crosses the front of the engine.

Joe Twill

Could this be normal? Isn't an alternator at idle putting out less than half rated power? If my car was doing this, I would have thought that it is normal, since you are only getting the low voltage with a high electrical load....blower, lights and rad fans. You say you only need to go up a couple hundred rpm and the voltage is good. Are others holding 13.5 volts at idle with all this stuff turned on?

dr bob

My similar symptoms were eventually solved with a new ground strap. Plenty of testing caused me to replace the battery, the alternator, positive cable, re-clean grounds, etc. Ultimately the ground cable was the solution. Here's why... With the ground cable getting tired, the alternator couldn't deliver full charge to the battery. The battery wasn't charged enough to maintain system voltage above 12 at idle with load. The alternator really doesn't have enough capacity on its own to supply all the car's electrical needs, so the battery is used to supplement the alternator under that condition. As soon as engine RPM's go up above say 1500, there's enough to supply those needs and charge the battery again with 13.5+ volts.

The collective wizzdumb here has identified a 2ga HD ground cable as a suitable replacement for the original. You can buy a bare one yourself and add heat-shrink insulation to it, or buy one already prepped from Roger. My local POLAPS does not carry the strap, had to let my fingers do the clicking to find one. Original straps are available from 928 International as well as from the other Usual Suspects if you crave originality.

PC-85-928S

have your old/original alternator rebuilt.
That is what worked for me.
I think the original alternators were under rated by factory

Joe Twill

A bad battery will run a lower voltage under load, so it might be that at idle when power is needed from the battery (by design), the voltage drops off just because it is old. You might try charging the battery and then having it tested.

longleader

If you need a new ground strap PM me and I can meet you somewhere close to 60; I work in Tampa

dr bob

Looking at some basic battery diagnosis, consider pulling the battery and verifying that electrolyte levels are good. Then charge the battery completely with an external charger. "Smart" chargers aren't expensive, or use your old-school charger and keep an eye on the battery to make sure it doesn't boil out the electrolyte. Once it's as charged as it's going to be, use a simple specific-gravity tester to see if the battery is OK. The tester is available in a simple floating-***** model for a dollar or two at most places. More elegant models have a calibrated float in them that shows numbers in addition to colors, and come from better parts places like NAPA stores. They serve the same purpose so the little one with the floating ***** is more than adequate. If the battery is marginal, this will tell you right away. FWIW, this test and maintenance is part of teh annual electrical system maintenance, at least the testing part. Clean the terminals WYAIT, and coat them with Vaseline after they are mechanically secure. There's a vent hose connection that should include a tube to route charging vapors outside the battery well, good to install if you don't still have the original installed.

Of course, if the old battery fails the floating-***** test, get a new one.

Lorenfb

"Plenty of testing caused me to replace the battery, the alternator, positive cable, re-clean grounds, etc. Ultimately the ground cable was the solution."

- dr bob -

Again:

"One ALWAYS needs to start by measuring the voltage right at the output post
of the alternator with respect to the case of the alternator."