928 flucuating idle with lights on

saabwonkka · 02-12-2006, 06:56 AM

hey guys, i have a 1979 euro model 928 automatic that had a leaking waterpump. i replaced the water pump and timing belt and the car runs awesome. the only problem i'm having now is that when i turn the headlights on, the idle starts going up and down when the car is parked and the lights get very dim.
does this mean my alternator is failing because it didn't do this before i did the waterpump job. any help would be great.
thanks

928ram · 02-13-2006, 04:50 AM

How's your idle speed? (keeping in mind that a almost 30yr old dash tach may be off a little) A really low idle speed can bring the fuel dist. to a point where it's not metering. Also, a slightly lean condition at the fuel dist. will give a surging idle.

saabwonkka · 02-13-2006, 06:00 AM

the idle speed seems perfect. if i leave it parked with the lights off, it is smooth and doesn't waiver. but as soon as i switch the lights on it goes up and down to the point of nearly stalling and the lights get dim. as soon as i drive off the lights get good and bright and drives smooth.
wondering if the alt belt is too tight or loose or if the alt is failing?
i just don't get why the lights have that much affect. do these alternators have a servicable voltage regulator. before that waterpump job it didn't dip at idle with the lights on!!
thanks

928ram · 02-14-2006, 04:35 AM

AFAIK, the regulator is internal and replaceable as well as some diodes(?); I'm a bit weak on knowledge in this area.
I do recall that on my '79 that the alternator belt, though tight, was slipping and needed alot of tension, even with a new belt. I don't recall whether the idle was affected, but it's been several years ago.

saabwonkka · 02-14-2006, 07:43 AM

thanks for your help!! i'll check the tension on the belt for starters before i do any replacing. hopefully that will take care of it. if not i may have to buy some parts!!!
seth

did you keep that large belt driven fan on your 79 or would you recommend replacing it with an electric unit?

928ram · 02-15-2006, 05:02 AM

I've retained the stock fan myself; to convert to electric you'd have to either find some bolts to retain the stock fan's pully or get a belt that fits crank-to-air pump. Shouldn't be to tough to go electric.