Ford pickup low voltage at idle

Nate Nagel · 2007-08-05T20:51:09+00:00

I think I posted about this before, but never really got beyond two suggestions...Problem is this. When truck is idling, if enough accessories are turned on, voltage will start to drop, and keep on dropping as if the battery is discharging. Voltage at battery with key off is about 12.9 (seems high?) with engine running about 14.0, with lights, A/C and fan on starts off at about 12.8 and keeps steadily dropping. If you rev up the engine with all the accessories on eventually the gauge will snap back up to the normal position but will fall off again when the engine returns to idle. It is almost like a regulator issue, the needle moves all at once and too smartly for it to be just voltage falling off because the RPMs are too low.Suggestions made were a) battery may be bad? I doubt it as voltage seems high with key off. b) connector to alternator is bad? possible but alternator itself is too hot to touch after only a few minutes of idling. FLAPS says alternator tests OK. At this point I feel like I should take it to a good auto electrical expert to check the whole thing out, but I have absolutely no idea where to find one. Any ideas of anything else to check? BTW I also checked for a voltage between alternator case and B-, ground wiring appears to be OK even under heavy load.Where is the voltage regulator for this truck? Is it integral with the alternator (and therefore should have been tested when the FLAPS spun it up) or is it hidden elsewhere?This is a '93 F-150 with the 4.9L engine. If I can get this issue and also a cold start issue fixed it'll be darn near done.thanks,nate

Aug 05, 2007 26 Replies

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N8N 18 years ago

I think it's a long shot, but I'll have the battery checked next time I feel like venturing outdoors. (I kind of came home from work and ran and hid in the basement last night. When I got out of my car, I felt like I was drowning and I went blind from the fog on my glasses. I love DC, I really do.) I did clean the battery with a baking soda solution and cleaned the terminals well and slathered them with Sil- glyde when I first got the truck, but I didn't bust the caps off to look inside, and there is no date stamp anywhere on it.

Of course, I don't really know who I can trust to check the battery, as the place that checked my alternator said it was fine and when I disassembled it I found the brushes worn to little nubs. I guess I probably ought to keep my eyes peeled for a cheap used load tester, although that doesn't help me immediately.

nate

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N8N 18 years ago

Oddly enough, I have the same "issue" with my '55 Stude, but I assumed it was because I was using a Delco 10SI on a Stude engine with no real thought having gone into pulley sizing. It's driven off the harmonic balancer (not a bolt on pulley) anyway, so the only way I could fix the issue is to find an undersized alt. pulley...

nate

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Mike Romain 18 years ago

I put a hand throttle in my Jeep. That way I can bump the idle up to

1200 for when I am using my big lights and winching someone.

Meanwhile I have never had the battery go dead from slow idling along trails with defrost, wipers, lights and stereo on for hours at a time.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's - Gone to the rust pile... Canadian Off Road Trips Photos: Non members can still view! Jan/06

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Nate Nagel 18 years ago

Follow up: according to the local Advance Auto Parts, my battery is fine. I'm sooooo confused.

If anyone knows of a good auto electric person who will work on a whole vehicle (not just rotating components) near Falls Church, VA I'm all ears.

nate

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Ashton Crusher 18 years ago

I know it won't last forever. But neither will I. It's just a question of which of us gives out first.

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anumber1 18 years ago

As a side note:

Check the harness and connector where it plugs into the alternator. I have had several harness show significant corrosion and degradation there. The symptoms were erratic charging. If you are over 100k miles you may also have some worn brushes. I have rebuilt several of those alternators on my workbench. It is not particularly hard to do.

I do live in Michigan and vehicles are subjected to extreme corrosion from the amount of salt used on the roadways in the winter months.

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Nate Nagel 18 years ago

The connector LOOKS OK but I have been told before that it's recommended to replace it whenever the alt. is R&R'd so if I can't find any other issues I may replace it. It really doesn't seem like the problem but I am running out of ideas.

I did just replace the brushes (which were worn out) without obvious improvement in symptoms.

Usual use of the truck is after work (i.e. dark) and A/C on full blast is a given considering the recent weather, so this is actually not a hypothetical.

nate

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