Fuel Gage Always On Full

[TheRecorder]

I have a problem with the fuel gage pegging lightning fast to beyond full when the accessory switch is turn on. This problem existed from the transition from the Military to me. Initially, I replace the fuel gage with a NOS gage, but the problem remained the same. Troubleshooting this specific issue from the TM by the numbers and with the voltage readings from tests accordingly, it says to replace the fuel gage.
Has anyone had this same issue?
To me, it almost seems like the gage is something less than 24 volts. Are there gages with applications and ratings other than 24 volts? The original gage and the one I purchased are identical.
Any Thoughts?

 

[emmado22]

make sure the connections to the gauge are tight, and there is a good ground fronm the gauge to the metal "dashboard" that the gauge sits "in". Meter things out, and see what happens.

Is the gauge metal or plastic?

I've never seen a 12V fuel gauge for a M series truck, so I think you have the correct one.

 

[TheRecorder]

I think the gage is plastic, but the bracket that attatches the gage to the instrament panel and to the dash is metal of course. Let me go and check that.

 

[sloryd]

here what I have found......

Remove the instrument plate from the dash board.

Flip the 2 wires that are connected to the fuel gauge, then turn on the acessory switch. One of my trucks was connected backwards and always indicated full. If not return the wires back to there original position.

If that does not correct the issue remove the plug from the fuel tank sending unit. Connect and Multimeter to the sensor and the other lead top ground. If the reading is 20 ohms or better (max of 36 ohms) the sensor is working. If it indicates 0 or 9999 ohms then remove the sensor from the tank because the sensor needs cleaning (algae buildup on the sensor) Gumout will clean the sensor off.

Every truck I have had this problem it's always been one of these issues.

Yes..... it can be a bad fuel guage but try this first. I have bought new guages and thought that was the problem, now there just spares.

 

[emmado22]

Some excellent advice from Sloryd...

 

[chgofiveo]

here what I have found......

Remove the instrument plate from the dash board.

Flip the 2 wires that are connected to the fuel gauge, then turn on the acessory switch. One of my trucks was connected backwards and always indicated full. If not return the wires back to there original position.

If that does not correct the issue remove the plug from the fuel tank sending unit. Connect and Multimeter to the sensor and the other lead top ground. If the reading is 20 ohms or better (max of 36 ohms) the sensor is working. If it indicates 0 or 9999 ohms then remove the sensor from the tank because the sensor needs cleaning (algae buildup on the sensor) Gumout will clean the sensor off.

Every truck I have had this problem it's always been one of these issues.

Yes..... it can be a bad fuel guage but try this first. I have bought new guages and thought that was the problem, now there just spares.

My M35 has the same problem... I havent had time to trouble shoot it yet, but with this advice...I think Ill get it fixed...Cant wait...Thanks

 

[jimmcld]

My 816 had a simmilar problem, except that it was intermitant. The guage would read correctly for a few minutes and then start wildingly swinging over to past full. The military had already replaced the guage and added a ground wire to the fuel tanks on both sides. Obviously, this had been a persistant problem.

I noticed that it only seem to do this when switched to the left tank, so that narrowed down the search. I then turned the switch on and started wiggeling wires. I found that the wire to the sending unit was broken, under the insulation, just before it got to the sending unit. I just pushed the wires back together within the insulation and fixed the problem. I still need to go back and splice that wire, but it hasn't done it since.

 

[emmado22]

I've also found that it's a great idea to put a dab of dielectric grease in the wiring connection so they dont get stuck together and you wind up destroying the rubber. I do it on every connection that I take apart... It's MUCH easier getting them both apart and togther with the grease.

 

[TheRecorder]

Here is some more info:
A week ago I pulled the sending unit out of the tank, cleaned the entire unit to include disassembling the mounting plate and cleaning the wire connection etc.


Also, I just checked for good grounding from the fuel gage to pannel.
With wire #28 removed from the gage (which is attatched to the "send" terminal on the gage), and which continuity is good to the sending unit, and when the accessory switch is turned on, the needle still pegs to full.


Taking an ohm reading on the sending unit per Sloryd's direction, it reads 54 ohms. The tank is a little over half full.

 

[emmado22]

Here is the info from the TM.

 

[cranetruck]

Adding to Mark's post...
Note that the simple ground symbol in the wiring diagram can be misleading, the actual path of the return (ground) is from the case of the sending unit, via a screw to the tank, via a screw on the fuel pump assy to the fuel pump housing, to the connector on the fuel pump housing and to the frame and finally from the frame to the "neg" terminal of the battery.

 

[TheRecorder]

I have replaced the sending unit with a NOS unit. Before installing, I hooked up a ground lead to the unit and to the frame of the truck and wire #28 to the terminal on the unit, turned the accessory switch on and the fuel gage is now working as intended.

It was a bad sending unit.

Thanks to all of you for great info in helping me resolve this problem.

 

[hooyahs]

I had that problem, until I added several gallons of gas to the tank for the upcoming cold weather. Next day the fuel gauge was working! Must have been some old gelled diesel gumming up the sending unit.

 

[Mhodge]

I also had the problem of the fuel gauge going to full right away. When I cleaned the grounding wires on the fuel tank I noticed a small plastic washer under the grounding wire on the fuel sensor. Don't know how or why it was there but once I removed it the fuel gauge began working.

 

[porkysplace]

I also had the problem of the fuel gauge going to full right away. When I cleaned the grounding wires on the fuel tank I noticed a small plastic washer under the grounding wire on the fuel sensor. Don't know how or why it was there but once I removed it the fuel gauge began working.

The only thing that lies more than a Deuce fuel gage is a politican.