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Odd voltage spike issue

dk8019

Active member
799
54
28
Location
Lovettsville, VA
Having an odd issue with my FMTV. At random I will get a voltage spike that will peg the volt meter and put the transmission into limp mode. I’ve checked the connections to the regulator, only one post is connected, and cleaned them up, but the issue persists. Seems to happen more often when there’s little to no load on the system. For instance turning on the fan and high beams will reduce the chance for it to occur, but higher extended RPMs seems to cause it. Any ideas on where to start?
 

GeneralDisorder

Well-known member
Steel Soldiers Supporter
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113
Location
Portland, OR
What year is this truck? Which alternator? Which voltage regulator?

Does the regulator go into OVCO? Flashing red LED?

Read through my thread on my over-voltage problem. Check all your connections at the LBCD (if you have one) and check for leaking capacitors.


And here's what my LBCD looked like inside:


I suspect that the LBCD was somehow discharging the caps into the field circuit of the alternator and driving it high.
 

Ronmar

Well-known member
3,782
7,352
113
Location
Port angeles wa
What general said. This sounds like it could be a bad connection back to the batteries. Or as suggested a bad ground. Alternator and battery work as a team. You drop your battery load and the alternator can surge in voltage before the regulator can reign it back in. your comment about it being not as bad with other cab electrical loads on also supports this…

If you have not already done so, drop to 2 batteries.

The ground connections are generally crap on these trucks. Pay particular attention to the alternator to passenger frame, and the starter motor to drivers frame ground straps. Disassemble, Clean the attach points to bare metal, re-assemble and coat with paint or galve coat. Then work on the grounds from pax frame to cab and inside cab to dash and power dist panel…
 

MatthewWBailey

Thanks for this site. My truck runs great now!
Steel Soldiers Supporter
794
1,446
93
Location
Mesa Colorado
I had OVCO +32v on my Neihoff before I pulled it, After having spent 800 on a new regulator. Still Haven't had it rebuilt yet. Was quite a random problem. Swapped to the Delco alternator. Not a single issue since. But I did redo all the ground cables as suggested above.


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