Fuel and Oil Pressure only working halfway....

[sassriverrat]

Hey guys- I appreciate all of the help.

So I got the truck over a year ago- long story short- all of the gauges were dead but the ABS and CTIS systems worked like a charm!

I've put all new gauges in (all from a yard-should be good). I had a ground issue initially that I think I tackled but...

1. I have a fuel gauge that when turned on, goes to E. I've tried shoving a ground against the clamp and that didn't seem to work. I figured the next step was to a. take the gauge out and clean it and b. open the tank (I noticed one guy had a float come off...). Any other ideas?

2. I have an oil pressure gauge (assuming 0-120psi gauge is correct) that, when turned on, comes to an imaginary zero point and when the engine is running slightly comes up/can slightly move with tach but doesn't move a lot like a mechanical gauge would. Again I played with putting a ground wire against the clamp (all while in the dash) but didn't seem to get anything. One guy suggested I switch the wires on the back of the gauge but that didn't do anything for the gauge. Now the interesting part- if you disconnect the gauge from the sender, it pegs out, and if you then ground the sender wire the gauge goes to zero.

3. I had a battery with a bad terminal connection- it ended up sparking the system the other day before I could replace it and now the ABS light is on steady and I definitely don't have ABS. Has anybody opened one of these boxes up to trace circuits?

Now for a side note: The front axle lock light is dimly lit....

I'll be tracing an electrical diagram because I foresee a bad ground somewhere.


Secondary: I have a "warning" switch. What's that for?
And I have a two prong female plug with no connector and a lot of wire under the driver door dash. Ideas what that was from?- It seems to have 12 volts in it but I don't have a 12v setup to the batteries....

This is an M923A2 converted to an M925A2 (winch conversion).

Finally- What have people done to mount a set of marine speakers in the cab? I thought about grenade boxes to the inside of the door but 6.5" marine speakers are *just* too big. Thanks!

P.S. The pictures are power off and power on.

 

[sassriverrat]

Well I figured out the fuel gauge- the sender works but the float was cracked and therefore not floating! Carefully soldered together and it actually works (surprising)!

Still not sure about the oil pressure.

The axle light was a grounding issue (solved) and I haven't found where the "Warning" switch goes to yet. Two prong plug goes to the axle light and had become unplugged.

 

[74M35A2]

My float also filled up. Newer ones are poly.

Stock electric oil pressure gauges typically read only 3 positions: below 0psi, a needle above 0psi (idle), and 30psi (above idle). Don't expect anything more than that. Go mechanical if you want much increased accuracy and resolution. But honestly, a Cummins really doesn't even need an oil pressure gauge. If it has oil, it has oil pressure.

 

[71DeuceAK]

While I'm new to actually crawling around on M939s in person, what are the odds that the "warning" switch was for something else the truck had in the field?

 

[Those Military Guys]

For the oil pressure gauge go to the passenger side behind the hose behind the generator your sender is there i bet the wire or connection is bad or broken from the hose pressing against it so i would check that out and if you have someone in the cab unplug the wire and tap it to ground to see if the gauge jumps to 120 if nothing could be a bad gauge but before you get a new gauge bench test it to make sure we have lots of really good take outs if you don't mind a take out plus we will be having a clearance page coming up soon with lots of parts that are just sitting in the shop and we need to clear them out so you might want to look at that when it comes out it should be next week.
Thank you
TMG