Intermitent engine stalling and failure to start

CHEMICALFACIST

MEMBER
2001 GMC SONOMA
89,000 MILES

My truck stalls occasionally while running then fails to start and needs a jump.
The battery and alternator checked out okay by the tow truck guy who jumped it for me recently.
It's a new battery.
The gauges occasionally all flash on and off and the speedometer and tachometer jump up and down. When this happens it won't start again with out a jump.
When it does run the idle seems low and then I have to drive with my foot on the gas and brake to keep it from dying.
Sometimes it runs just fine for days, but this problem has become more frequent this last few days.

3 Replies

CARADIODOC

MECHANIC
33,873 POSTS

The need for a jump-start is a clue to have a load-test performed on the charging system. The generator should be tested for maximum full-load output current and "ripple" voltage. If it has one bad diode of the six it will only be able to produce exactly one third of its rated current and that's not enough to meet the demands of the electrical system under all conditions. The battery will have to make up the difference until it runs down.

While you're at it, follow the smaller battery negative wire to the body and the smaller positive wire to the under-hood fuse box and be sure both of those connections are clean and tight.

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Tuesday, May 28th, 2013 AT 9:11 PM

CHEMICALFACIST

MEMBER
2 POSTS

I checked the battery with a multimeter and it read low. So I put a jump box on to start it. Then I tested the alternator to see it's output. It read between 13 and 14 volts. The guy who jumped it last said that something was "on" in the truck but I can't find anything that is on. I tested the alternator with and without all my accessories(ie garmin, lights, radio, and phone chargers) on and off and there was no change. How can I go about looking for other parasitic drains?

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Wednesday, May 29th, 2013 AT 9:36 AM

CARADIODOC

MECHANIC
33,873 POSTS

Measuring output voltage is fine to determine if the generator is working vs. Not working but that is not the question here. It has to be tested for full load maximum output current. That has to be done with a professional load tester.

A lit match is really hot but it won't heat your house. A garden hose can have lots of water pressure but it won't put out a forest fire. With one bad diode the generator's output voltage will usually be perfect, between 13.75 and 14.75 volts, but if it can't meet the current demand of the electrical system the battery is still going to run down while you're driving.

(Gotta run out of town. I'll check for your reply as soon as I get home).

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Wednesday, May 29th, 2013 AT 10:04 AM