Fuel Gauge Voltage

[plainside]

HI,

The truck is a 1990 M925A2, new to me. The fuel gauge reads full, and from reading on the forum on searches it sounds like a ground issue. Anyways, I have 24VDC on the back of the gauge when I unplug it, so I assume that is right. Then at the sender I have 6VDC from the plug on the sender to ground, is that right? When I wiggle that connection around It works in the cab till I let go then back to full. I undid the connection and put electric grease in it with no success, no rust or issues by the looks of it. now what?>

 

[Coffey1]

Mine only in cold weather stays on empty then when it warms up it reads correct.

 

[clinto]

Assuming the sending unit in the 939 works like everything else, you need 24V at the gauge to energize it, but the sending unit functionality is measured in ohms. The TM will show the "range" of ohms for the gauge. Some vehicles are 0 ohms empty and 90 ohms full, so you can deduce 45 is 1/2 tank, etc.

You need to ensure:

Gauge is grounded.
Gauge is getting 24V
Sending unit is properly grounded
Sending unit, when grounded properly, displays the correct ohm range throughout the float's arc of travel
Sending unit wire, where is goes into the back of the gauge, is seeing the same ohm range the sending unit is sending.


Once you know the answer to all those, you'll know where the problem is.

 

[plainside]

Thanks, I get 35- 36 ohms when I push through the sending wire by the sender and to ground (I get the same ohms at the back of the gauge.) but when I unplug it and put the meter in the female part on the sender connection to ground I get readings all over the place!?!? Gauge has 24 volts. Not sure what else here. Gauge goes to Full but not pinned out full. Truck around half a tank looking in there!

 

[cranetruck]

The fuel gauge is a problem for many. The problem has to do with the fact that is runs on relatively low resistance values (0 to 35 Ohms, or so) and that the ground circuit is shared by many other circuits on the vehicle.
This is a schematic of the Faria instrument.


There are three wires involved, 24VDC, ground and the connection to the sending unit.
The problem is that the grounds (earth symbols in the diagram) are not directly connected. Other "users" of the ground path may disturb the operation of the fuel gauge, by creating small voltages not related to it.

The ideal solution is to run a dedicated ground wire from the fuel gauge to the sending unit.

 

[plainside]

Thanks, I will play with that! So it saying 35ohms would be a bad sender then being the tank is around half full. I will pull that out next.

Update: took the sender out and took it apart and cleaned up any rust and bench tested it. Seemed to work OK up to 29 ohms on full and 1 ohm on low. Put back in the truck and it reads ok now, hopefully it holds up to the bouncing around it gets out in the fields.

 

[74M35A2]

The fuel gauge system on these trucks is the lowest cost thing I have ever seen. My gauge and sender were both bad. Gauge was shot for whatever reason, and my sender has a copper/brass float that was full of fuel (pinhole in it). I replaced it with a newer design that uses a plastic float.