The GMC CCKW

Am trying to work out if my fuel gauge is correctly paired ( or mismatched) with my sender unit.
...or if I just have a fault somewhere ... ?
The gauge will only read a minimum of halfway when empty and full when full ( ie gauge never gets less than half way).
any thoughts/help appreciated

Are you running 12 volts?

no ... 6 volts only

The gauge needs a ground on the dashboard to operate properly

yes have done that
resistance across the sender unit runs from 13-14 ohms at empty to 32-34 ohms when full
also gauge was operating back the front until I turned the float arm around

TM9-801 (Apr 1944) has a pretty handy section on troubleshooting the fuel gauge. It is located in Section XII, paragraph 100.

There is a passage in there that talks about grounding the sender terminal and checking to see that the gauge goes to empty - which implies that the sender should be at zero ohms as well, when the tank is empty.

If you are seeing 13 - 14 ohms across the sender with an empty tank, I wonder if the sender arm could possibly be bent or hanging up?

Also, the 13 - 14 ohm measurement is taken directly across the sender, correct? (As opposed to between the sender and the frame, or some other ground?) If the latter were the case, then I would be check the sender ground circuit for a poor connection.

Lastly

also gauge was operating back the front

. Not sure that I follow you...

I'm curious to know what you find out. My gauge was disconnected when I purchased my truck. The fuel system is next on my list, once I finish up with the brakes.

Best Regards...

Hi Mudflap
actually I have the sender out of the tank and hooked up to a 6 volt battery
Initially when I put the float in the "full" position the gauge read half empty ... and when the float was put in the empty position the gauge read full .... so I switched the float around to the opposite side of the rotating arm ... as described on this site from a great post in 2006 or 2008. ....when I did this the gauge then worked in the correct direction ie full when the float was in the full position but only halfway when the gauge was in the empty position. I am beginning to suspect I might have two problems ... I believe the resistance across the sender ( which is what I have been measuring ie from the terminal to the float or float arm) should in gmc's range from 0 ohms when empty to 30 ohms when full ... given mine only reads 13-14 ohms as the minimum perhaps it is no wonder the gauge only gets to the halfway point ???... the other problem could be a "wrong" gauge in the vehicle ... other mv's such as jeeps run the opposite way eg 50 ohms when empty and around 0 ohms when full which could explain me have to switch the float around in order to get the gauge working in the right direction ....
so am trying to work out if I have a faulty sender unit as well as the wrong gauge type ....???
HELP!! :0)

Hi Sixbysix,

If the operating range is 0 ohms = E and 30 ohms = F*, then I could see 13 or 14 ohms causing the needle to show half a tank. If the sender is in the empty position, and the sender resistance is 13 – 14 ohms, then it looks like there is a problem with the sender.

If you still have doubts about the gauge, a quick way to check the gauge would be to ground the sender wire and see if the gauge needle goes to "E". Then, to check the full position, insert a 30 ohm resistor in between the sender wire and ground. You could either buy a resistor with a known resistance value, or set the sender to a position that indicates 30 ohms resistance and reconnect it to the circuit. The needle should be at the "F" mark.

(BTW, if you use the resistor method, make sure that you use a one with sufficient power rating. I'm not sure what the typical current is in that circuit, but I'm guessing that if you try to use one of those dinky 1/8 or 1/4 watt carbon resistors, it's liable to getting really hot really fast. A ceramic resistor with a higher power rating would be better. Something like this, perhaps:[url] http://www.radioshack.com/product/index ... d=12573244[\url])

Lastly, and I'm sure that you already know this, make sure that any of the resistance measurements performed on the sender are made with it disconnected from the circuit to avoid any influence from the other components. This has got me several times in the past, like when I forgot to remove power from the circuit and my meter showed negative ohms...

Best Regards...

* Does anyone have a reference for the 30 ohm = full value? I have seen it published on a number of websites, but I have never seen it referenced in an actual manual of other definitive source. It must be in an Army TM someplace.

just to let anyone interested know - the problem was a fuel gauge sender unit mismatch.
I ended up getting an NOS fuel gauge which was quite different from the fuel gauge in the truck.
The NOS unit is is 24mm ( 1 inch) deep compared to the old unit which is 37mm (1.5 inches) deep.
the back is not stamped near the terminals and has a brown "insulation"strip under the terminals whereas the old one has "ign" stamped near the ignition terminal and "GPA" stamped near the other terminal.There is no brown "insulation" strip. I am guessing this old fuel gaugeis a repro unit ( as it looks so clean) and perhaps designed for jeeps???
the NOS gauge works in the correct direction with the original sender unit without having to turn the float around (see my previous posts) and goes from just above empty to full with the sender unit running from 12-13ohms ( at empty) to 32-34 ohms (at full). I have tested an NOS sender unit as well and it does register exactly empty and full and has a resistance running from 5ohms at empty to 32 ohms at full, so am guessing if my old sender unit did function at its "empty" end below 10 ohms it would probably read correctly as well ... but it works well enough for me to keep it anyway.
So hope that helps for anyone that might have the same problem