Fuel sending unit or gauge?

[CanonNinja]

Murphy's Law bit me again. Not a big issue though, this time. After the truck sat for a few days, my fuel gauge is on the fritz. It will read the correct level, but when the truck is running, it's all over the place. I'm wondering if there's some wire I need to check, or start replacing parts outright.

 

[houdel]

A). Wire brush the connectors at BOTH ends of the ground wire between the fuel sending unit and the frame. Coat both ends with CONDUCTIVE grease (NOT dielectric grease, I generally use Plumbers silicone grease, though most any decent chassis grease will work) and reassemble. If gauge still is funky, go to step "B".

B). Refer to TM 9-2320-316-20, Unit Maintenance for 2-1/2 ton, 6x6 G44A2 Series Trucks (Multifuel), http://www.steelsoldiers.com/index.php?module=pagesetter&type=file&func=get&tid=1&fid=file&pid=13. Go to Chapter 2, Service and Troubleshooting Instructions, Section VI, Electrical Systems Troubleshooting, Malfunction Code 32, Fuel Gauge Inoperative (page 2-124), and follow the test procedure

Be wary of the published voltages. I NEVER got +24V at lead 28 at the fuel sender. I tested least 28 from the fuel sender connector to the fuel gauge connector, read zero ohms resistance. Checked the instrument spider connectors, still could not get +24V. Ran a ground wire from a clean chassis ground to the gauge panel, still did not get +24V. I did check the resistance of the fuel gauge sender to ground, it gave a reasonable reading for the amount of fuel in the tank. The published reading is zero ohms for an empty tank, 31.5 ohms for a full tank.

My gauge would read from empty to a little over 1/2 tank, even with a full tank of fuel (back when diesel was reasonable affordable!).

If really in doubt, get a couple of jumper wires, connect one from the +24V terminal of the battery to the +24V terminal on the fuel gauge, the other from the fuel sender output to the fuel sender connector on the fuel gauge and see what happens. I could never get the voltage readings suggested in the TM, but since my resistance test of the fuel gauge sender seemed reasonable I elected to buy a new fuel gauge from Saturn (way cheaper than the fuel gauge sender). I installed it and the new gauge reads perfectly.

I would guess I have either a bad primary to chassis ground, or bad chassis to cab ground, or bad cab to IP ground, or some bad ground in there someplace. Deuces are noted for electrical grounding issues ("earthing issues", as they are known over the pond). I guess I'll have to look into this problem once I replace my leaking master cylinder and the other 9,999 issues on my deuce "To Do" list!

Now, if I could only figure out a way to lower the fuel pump pickup nearer to the bottom of the fuel tank, so I don't have that unusable 10 or so gallons of fuel just sitting in the bottom of the fuel tank!

 

[cranetruck]

1 image=1,000 words

Remember that the ground return for the sending unit is via the fuel pump connector (wire #79).

 

[acetomatoco]

The old rule of thumb is that the amount of fuel in the tank is directly proportional to the amount of shake in the needle....as the world turns... Jeeps were among the worst to display the symtoms... ACE

 

[acetomatoco]

Regarding the lower pickup... remove your pump and carrier and see if the pump is in the lowest physical position to fit your tank... they are adjustable to fit a myriad of tank depths....rows of bolt holes along the sides of the bracket... you may have to lengthen delivery hose by a like amount.. ACE

 

[pa.rich]

Yes it is adjustable, I raised mine because of the rust flakes floating around in the tank. It was 1/2" and now is 1 and 1/2". The bolt holes were about 1" apart.

 

[houdel]

I guess I'll have to order a new fuel pump gasket and see if I can lower my fuel pickup. I don't THINK I have a rust or sludge problem in the tank. I just hate being nearly out of useable fuel when my fuel gauge still reads 1/4 tank of fuel!

 

[acetomatoco]

Next time you are in the tank, so to speak, pull the bottom drain plug and filter the fuel thru a good solids and water funnel and rinse the tank a coupla times with some more fuel... and put the plug back in and return the filtered fuel to the tank and you are good to go... the primary filter on the frame is a great junk collector... just service it often...and you will have no worries...ACE