Diesel Voltage issues?

dieselsub98

New Member
Joined
Sep 17, 2006
Location
Harrisburg, NC
'98 GMC Suburban with the 6.5L TD. Everything is stock right now....

OK, driving home tonight from work the dash voltmeter was reading 11V. Everything electrical was off except lights and cruise. The last time this happened was about 2 months ago and it was the alternater dying. Got a new alt from Napa at that time. Batteries are 2 years old. This morning it was fine, I remember checking the gauges because I had just changed my oil and scanned the gauges.

I whipped out my voltmeter and checked both batters. Both read at 12V. I checked the alternator and it was pegged at 15V.

Any ideas? Bad dash voltmeter? Is there an external voltage regulator?

Thanks in advance!

-Mike
 
is a possibility the gauge is flaky, they are not real accurite to begin with.

For giggles and grins, clean you cable ends ( both ends) especially the grounds at the rear of the engine ( near fire wall, right side, including the braided ground strap)

Loose grounds on the diesel trucks can really make things goofy with the electricals and the engine performance.

Also, check the firewall bulkhead connector ( 7MM bolt head ), make sure it is tight ( very common issue in this body) can cause some funky electrical issues as well
 
You Said the Batteries were at 12v but the alt was at 15v... They should both be the same voltage when the engine is running. If they are different the you have a high resistance (ie bad connection) from the Alternator and the battery. I know that some cars have a fusable link. you could have one that is blown. if you have a Voltmeter that has a OHM meter built in the measure the resistance between the battery + and the output of the Alternator With the battery disconected. It shoul be close to 0 ohms but a reading of up to 500 ohms could be expected without causing a major issue.
 
Resistance between battery + and alternator output read 0, but that was with the battery connected. Why does it need to be disconnected? (Too dark and cold to mess with it too much last night).

I checked the grounds and they looked ok. I wiggled the cables with the engine running and the voltmeter didn't move. I will check the other connections on the firewall tomorrow when I have more time.

One other thing. This morning when I came in, the voltmeter read 14 and didn't budge the whole drive to work.
 
voltage regulater could be going bad, also if your glow plugs are on alt won't charge, glow plug relay could be bad. did the glow plug light come on randomely
 
Cperry said:
voltage regulater could be going bad, also if your glow plugs are on alt won't charge, glow plug relay could be bad. did the glow plug light come on randomely
Click to expand...

The glow plug light cycles on and off after first starting. Other than that it stays off. It also cycled on and off after starting it again once I got home from work. The engine was still hot, so I was suprised when it cycled. Back when we had warmer weather, it wouldn't cycle once the engine was running. It would just come on when you first turned the key.

This morning when I started it and the glow plugs cycled on and off, the voltmeter dropped significantly at each cycle.

So the voltage regulator is external to the alternator?
 
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