Buzzing and low voltage - Ford Powerstroke Diesel Forum
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post #1 of 3 Old 11-12-2013, 09:57 AM Thread Starter Rookie
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Buzzing and low voltage

It is getting cold here in MI and I am having a new problem with my truck. I have a F-250 99 7.3L about 170,000 on it.
When I turn the key to warm the glowplugs, the vacuum pump will buzz for about 30 seconds. It only does this if it has sat for awhile. My issue, and concern, is that when i first turn the key the voltage is very low. As the vaccum builds up pressure, the volts start to climb. After the vacuum stops buzzing, the voltage is up to the normal area, BUT when I turn the key to start the engine, the volts dip really really far down. Sometimes almost to 8. The truck will still start, but I am wondering why the Voltage is all messes up???? I am just afraid one of these times it won't have enough juice to fire.

With all of that said, I have 2 brand new interstate batteries that are only 1 month old. I have a brand new alternator that is less than 1 month old. If it gets 40 or below, I plug both batteries into the tender at night. When I go to start it in the morning, both batteries are fully charged (indicated on the tender) and that doesn't seem to help it any.

Any suggestions????

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post #2 of 3 Old 11-12-2013, 10:12 AM
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Go have those batteries checked... Load tested at a High Rate (250-300A not on one of those dinky 100A testers the AutoParts Stores have for ricers). Something's Not Right!

The vacuum pump draws an insignificant amount and should have no effect on voltage what so ever on TWO fully charged 850aH batteries. BUT the 150A draw of the GPs will. 8v is WAY Too Low, in fact, on your truck, the PCM won't even trigger the IDM to fire the injectors at that voltage (not positive, it could be as low as 10 and not fire I think...).

Good batteries will hold above 10v with the GPs on for some time. You may have a bad one and a Load Test is the best way to find out.
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post #3 of 3 Old 11-13-2013, 12:56 PM Thread Starter Rookie
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Thanks for the help!!!!!

I had a feeling it was the batteries. When my old ones went bad it was dipping really low to start. I knew they were on their last leg. When I got the new ones my voltage barely moved when I started. Now it dips down really low, and it takes about a minute for the voltage to raise up to about halfway. If I plug them into the tender it is not as bad, but I don;t have that option when I am at work for 9 hours....

Thanks for the info!!
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