Hello,

My wife's 1993 940 nonturbo's engine hesitates when she has the radio and AC fan on while idling at an intersection.

I cleaned the Idle Air Control Valve, and the problem persists. So I'm leaning toward a problem with the alternator.

I put a digital multimeter on the battery.

Symptoms: with engine at idle, no load, voltage measured across battery is 13.70 V. With engine at idle and lights on, AC fan on high, voltage drops to 12.70 V. It stays at 12.70 V while the AC fan is on high.

I know the regulator works to keep the voltage from getting too high, usually under 15 V, but does the regulator also work to keep the voltage ~13.5 V or so? If the voltage drops under load, is that a symptom of a faulty regulator? Or, could it also be the brushes?

Any help would be appreciated.

Thanks.

 

well, at 700 rpm or whatever idle speed, the alternator can barely put out enough current for everything-on-at-once. actually, the ECU should be boosting the idle speed slightly when the AC comes on, mine goes up to 800 or almost 900 when the AC kicks on. if you give it a touch of gas when you are reading 12.7 or whatever, does it pick back up to 13.5?

the radio shouldn't matter much, the big loads are the headlights and the AC compressor (the compressor is a mechanical load, not an electric one). the interior ventilation fan is a medium electric load, as is the engine cooling fan.

its possible your alternator is tired. maybe its brushes are worn, so its not delivering full current.

its also possible you have a marginal ground somewhere. check the alternator ground to battery ground. put your voltmeter on battery - and the alternator body with everything on and the car running, do you read even a few 10ths of a volt?

 

Thanks, pierce. Will do your suggested tests soon.