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Odd voltage spike issue

dk8019

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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

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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

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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!
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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.


IMG_5408.jpegIMG_5546.jpegIMG_5538.jpegIMG_5551.jpeg
 

Ronmar

Well-known member
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Location
Port angeles wa
Or it could be an alt or regulator fault driving full field at random… gotta rule out all the connections/basics first though…
 

dk8019

Active member
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28
Location
Lovettsville, VA
Ok, so quick update. It appears my truck only has one ground wire of note running from the alternator to the frame of the truck. Nothing from starter side to the frame could be located, and nothing from the frame to the cab! The one from the alternator to the frame was corroded enough to be worrying on both ends, so pulled it apart and cleaned up everything, ground the frame connection point and polished the alternator connection (100A). I then made a new block to frame ground cable up for the starter side. For the frame to cab, I'm somewhat mystified on how it is done currently, I assume through one of the wiring harness, but thought I would try it as is to see if the issue was corrected. Starter her up and she jumped up to a steady 28V after starting with no issue. Driving around for 15 minutes showed no issues, so fingers crossed she might be fixed. We shall see, and I will report back if it comes back!
 

Ronmar

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Location
Port angeles wa
There should be a 1/2" braided strap from front of drivers frame rail running up behind the grill to the cab body near the front of the engine tunnel/hump. there is another one inside the cab that runs to the dash and yet another that runs to the power panel ground point and the main ground Terminal Board TB2.

There should be a wide, perhaps 1" strap from the front of the starter motor housing to the drivers frame. That is the main return path back to the batteries so i don't see anything working without that, unless some bright light performed a "Chief alt" and ran one from somewhere else, like the battery - to the frame, or somewhere else along that large gauge ground wire that runs from battery - to the front of the starter motor. like at the instrument shunt up between air filter and spare tire near the polarity protection device.
 

MatthewWBailey

Thanks for this site. My truck runs great now!
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Location
Mesa Colorado
The one on the front of the starter motor is not easy to see. Easier from underneath I think…
I'll go digging around for those, I may have missed them somehow!
I remember doing this. That one is behind this bolt on my A1. This is drivers side looking in. There's a bundle of air lines on the backside, with the ground strap and nut under those. You'll need earplugs bc of all the loud cursing.

the strap is long enough to relocate to the frame next to the compressor. You only need to loosen that bolt/nut bc as you can see the ground strap has a fork terminal, also shown that way in the TM.


Old:
IMG_5409.jpeg


NewIMG_5408.jpeg
 

MatthewWBailey

Thanks for this site. My truck runs great now!
Steel Soldiers Supporter
809
1,460
93
Location
Mesa Colorado
I'll go digging around for those, I may have missed them somehow!
As Rick explained to me, the cab ground is nearly impossible to access on the frame end, unless you
Manufacture a special box wrench lol. Best to just add one as in my pics above. But the cab end is here... (take grill off to see on passenger side)


This cable was under the hex screw on the right..
The CARC paint is not a good conductor of electricity IMG_5624.jpeg
 

Ronmar

Well-known member
3,805
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Location
Port angeles wa
yea the bottom end of the cab strap behind the grill is a PITA to get at also. the one on my A0 didn't look as large as the ones Matthew has pics of. moving the frame end attach point, or the whole strap to an easier to service location like he shows, is a good idea...
 
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