Low battery voltage when in idle

I have a 1996 ford f150 that when the weather is cold or damp the voltage runs really low almost to the point of stalling the truck. in warmer weather or dry weather it runs fine. i have replaced the belt & spark plugs. i have to keep my foot on the gas so it doesn’t stall.

Check pos and neg battery connections, both ends of the cables, battery, belt tension, or tensioner, and charging circuit in that order. Coincidentally cheaper to more expensive. Then go on to spark plug wires fuel filter, my plan of attack.

You might get someone to test the voltage and amperage output of the alternator and the electrical demand of the truck. There are many possible causes for the situation you relate here.

This is a question, not an answer. Is it possible the idle speed is dropping way down near the stalling speed, and the alternator simply can’t produce the current at that low speed? Not that I can suggest what with cold and damp would slow the engine. It’s just that low current from alternator does not seem likely to kill the engine as long as the battery is still there.

Well said Red Knox. Good advice from the others too. Having a load test done on the charging system will reveal some light on this trouble.

Many auto part stores will check the battery and charging system for free while you while you wait.

In cold weather, are you running your heater fan on high? If so, try turning it down one notch and see if the problem goes away.

Also, does you idle speed go below normal on cold days? That could indicate a bad IAC (idle air control motor) or TPS (throttle position sensor) or simply someone “adjusted” your idle speed with the throttle stop screw. If the latter, you don’t need that “mechanic” working on your vehicle any more.

How do you know it is an indicated “low voltage” situation that is causing the “almost stall” Has the indicated “low voltage” situation been verified by an outside measurement? With GM vehicles of the time period we speak of GM has issued a TSB that states an indicated low voltage situation at idle when many electrical systems are in use is to be considered normal and no repairs should be attempted.