M939 Series Battery Ground Cable Attachment Point?

[74M35A2]

Fuel gauge is way off. Disassembled sender, found copper float chamber full of fuel. Replaced with new sender. New sender now ohm's out OK, but gauge still reading off scale. 24-28V power into gauge checks OK, but ground circuit resistance on wire to sender shows 30 ohms. After checking, ground resistance from battery "-" to any body ground show 30 ohms. Since the gauge works in the range of 0-30 ohms from the sender, adding the 30 ohms body resistance will put the gauge off scale the entire time.

Also noticed when a sudden electrical load is activated, the oil pressure gauge needle will change positions simultaneously. This is also indicative of a bad ground (?). The differential ground resistance increases as electrical load is applied (between battery bank "-" and any body or frame measurement point, so I am thinking a bad ground for sure.

Ground resistance as measured from battery bank to back of starter ("-") measures nearly 0 ohm's, so this is OK. There should be a firm ground connection either from the starter ground to the vehicle body/frame (these starters are not grounded through their frame metal like automotive are), or from the battery bank "-" to a body/frame ground. I see a heavy ground cable coming off the starter and going to a bulk harness, but I have not crawled completely under in an attempt to chase it. It is 0F here in MI right now, so I am hoping I can cheat and have somebody tell me where the frame/body ground connection point(s) are, rather than lay on cold concrete @ 0F and try to trace it. I could not easily find it in the TM's, albeit I need to become more fluent in those also.

Thanks in advance.

 

[FullSpecial]

Ground cable connected to the starter comes up into the battery box between the batteries. It goes to the drivers side battery, there is a jumper to the passenger side. Passenger side has a cable that goes to the jump plug. It is definitely grounded at the plug,
you must also disconnect this cable when unhooking your batteries or you WILL get a surprise.

 

[74M35A2]

Sorry, that was no help. Maybe my question was too long. Where or how is the truck body and frame grounded? That is the question. The engine and body are on rubber vibration isolators, so they each should have their own ground cable connection points. I'm wondering where those are. Thanks.

 

[aheilmann68]

Just run a 12" or so wire as the ground from the sender. Terminate one end under one of the screws that bolts the sender down and terminate the other end on the frame somewhere. Use aggressive star washers to get good continuity, I have decent luck scraping some paint off and using a split lock if I have no stars.

 

[74M35A2]

That is likely the path I am going to go down just to save time. Run a new ground from battery to a clean frame, body, and fuel sender points. Thanks.

 

[73m819]

..I would ALSO run a NEW heavy ground from the battery to the frame, from what you describe, this is your problem, running a new ground cable may be easier then getting to the old one.

 

[juanprado]

front driver's side by gladhand has a ground strap from hood to frame. Mine was slightly rusted and 1/4 cut from flexing.

Another ground strap from frame by steering box to injector pump.

Ground strap from block to back of starter.

Mine did not have it, but I saw one m931 with a ground from radiator to frame on driver's side.

 

[74M35A2]

Thank you. That was useful. I will check those points.

 

[KN6KXR]

I know bumping an old thread.....

So I had issues with my new to me M936A2 fuel gauge, found this thread, went on an adventure. Some useful points about the system:
1) The ground for the gauges is the back of the panel via the hold down bracket. Found this out when my new fuel gauge has the hold down at 90 degrees from the old one. Of course the back of the panel hadn't been scraped down there so I wasn't completing the circuit for the gauge and I thought I had a dud. Same for bench testing the gauge because I didn't realize it uses the gauge chassis as the reference. So if your testing it out of the socket or on a bench remember to ground the gauge chassis.
2) If you have a fuel sender going bad it'll likely repeatedly peg the gauge as it sloshes. This will destroy your fuel gauge over time. Best to unplug it until you can install a new sender.
3) Get a DVM with a min/max/avg function (I have a Fluke 289). This way you can leave the gauge connected then go use a rod to reach in the tank and test the range of motion on the fuel gauge. By yourself. This showed me the left tank if NFG and the right one is reading bad (24 Ohm really full). Range of 0 to 30 Ohm is correct. If the sweep of the sender shows a dead spot (my left showed over 200 Ohm) then take it out of service for aforementioned reason.

That's all. Maybe this post will help the next guy when they find this thread in a search.