Gauge always on F
- Thread starter moknbrd
- Start date
Eric VanBuren
Member
You have a short to ground on the sender wire or the sender itself is stuck or shorted to ground. I would start by checking to make sure that the wire hasn't been pinched or the insulation worn through back at the tank. Of course the short could be anywhere between the gauge and the tank but since this started after working on the tank I would start at that end of the circuit. A disconnect of the rear lighting bhc and seeing what happens when you turn the rig on will quickly tell you if the problem is inside the cab or outside.
Michael Mayben
IHPA Tech Moderator - Retired & No Longer Online
Pull the tank(s), carefully look inside the sending unit and clean if necessary. Temporarily ground the sending unit body and run the float arm slowly through it's entire range of operation while observing the fuel gauge. The float arm May need to be "re-calibrated" (bent) in order to make the analog gauge read in a more accurate manner. Refresh the ground for the sending unit once the tank(s) are re-installed.
Then pull the dash and instrument assembly. If it's a dual tank system, then it's very possible the fuel gauge selector switch portion of the tank selector control will need to be cleaned also. Carefully remove the retainers for all instruments where they make contact with the circuit board. Lightly clean and burnish the contact points on the circuit board and re-install the units after cleaning. Carefully snug up the retainers, do not over-tightened or the board will be destroyed.
All the above is an extremely common issue regarding this type instrumentation package which is powered through the reduced voltage supplied by the cvr. Do not ever supply b+ to those instruments for even a short moment of "testing".
Service manual cts 2303 has a full explanation and diagnostic process for working with the instrumentation packages for these vehicles.
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The fuel gauge on my t/a has not "read full" since it was new.
I do not understand the issue as everyone knows when their tank(s) are full.
Afaic, the important issue is that the gauge read consistent and have the same amount of gas left when the needle hits empty -- 4 gal pass tank; 5 gal rear tank in my case.
I do not understand the issue as everyone knows when their tank(s) are full.
Afaic, the important issue is that the gauge read consistent and have the same amount of gas left when the needle hits empty -- 4 gal pass tank; 5 gal rear tank in my case.
Michael Mayben
IHPA Tech Moderator - Retired & No Longer Online
Yes...a very common/known anomaly!
But...if the tanks are ever removed for "service", that's the time to work with the senders and grounds and see if that issue can be improved upon!
And...this issue is not peculiar to just IH schnizz...accurate fuel gauge calibration was just not something any oem was concerned with back in the day. An accurate fuel gauge is a length of speedometer cable inner chain, snaked down through the filler neck!!
Yes, you are right -- IH is not the only one....
Never thought about using a chain lol...
Obviously, I have run the tanks "dry" to know what is left at e (pass side numerous times). I have put more than 30 gal (out of a possible 34) in a few times -- not something I try to do much.
Eric VanBuren
Member
The gauges work on resistance, 73 ohms e - 10 ohms f so after 35+ years it is very common for the multiple connections in the cir to cause excess resistance so the gauge(s) will never go to full, hot or high. The most common problem that I found on my rigs is the cheap stamped steel nuts that attach the gauge itself to the printed circuit board on the back. The stamped steel nuts engage less than 1 thread on the stud so I replace them with brass nuts and washers after carefully cleaning the cir board and stud. On most of my rigs the nuts were only finger tight. I would start there but any resistance in any of the connections along the way will cause the gauge(s) to read low. The nuts are 10-32. If that doesn't get it to read full, then follow the circuit and clean the connections along the way.
You can also test the system at the various connections along the way. Grab a pack of 10 ohm resistors from radio shack or where ever, 1/2 watt or higher. Disconnect the wiring connector at the chosen point and ground the side that leads to the gauge through the 10 ohm resistor. That should make the gauge read a the top line of the scale +/- 1 needle width.
You can also test the system at the various connections along the way. Grab a pack of 10 ohm resistors from radio shack or where ever, 1/2 watt or higher. Disconnect the wiring connector at the chosen point and ground the side that leads to the gauge through the 10 ohm resistor. That should make the gauge read a the top line of the scale +/- 1 needle width.