Odd voltage spike issue

[dk8019]

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?

 

[Suprman]

Add a new ground wire from the batts negative to a sanded spot on the main frame rail. Alt ground strap. Polarity box connections. Alt terminal connections. Could be a bad voltage regulator.

 

[GeneralDisorder]

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.

2008 A1R with 260a charging questions

Currently has a brand new N3221 voltage regulator (2012 vintage I believe - was new in the box with papers). The truck had a 2013 regulator when I picked it up in Houston. It died about 100 miles from Portland - 2600 miles into the trip back. I put in the brand new spare and drove the last 100...

www.steelsoldiers.com

And here's what my LBCD looked like inside:

LBCD replacement due to loose studs. CARNAGE!!

When addressing loose and corroded connections at the LBCD I noted that several studs were loose in the housing. After confirming with several members that this was a clear indication of failure (the unit is no longer sealed against moisture at the very least), I procured and installed a...

www.steelsoldiers.com

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

 

[Ronmar]

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]

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.

 

[Ronmar]

Or it could be an alt or regulator fault driving full field at random… gotta rule out all the connections/basics first though…

 

[dk8019]

Thanks guys, it's a M1084, need to check the model of the alternator. I'm betting from conversations above, I've got bad grounds. Thanks for the photos above, I'll chase each of these, and report back!

 

[dk8019]

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]

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.