Fuel Gauge

[sonicbluezx3]

This may have been visited before at one point, but I am not very good at using the search function to find specific results.

As some of you may remember, I had a similar problem with my temperature gauge. That has since been fixed and is working fine as I changed over to a mechanical gauge. I will be doing the same with my oil pressure gauge in the next few weeks.

I have done a full frame off restoration on my 86 K10 diesel. I replaced the sending unit, fuel tank, and lines. I found when I went to fill up my truck the other day (because the gauge read 1/4 tank full) that I could only put 17 dollars worth of fuel in and then it overflowed. I came home and didn't do anything with it for a couple weeks, but last night I went in and checked the fuel vent and all the lines - everything checks out ok. So next I dropped the tank to see if the sending unit was hooked up wrong, and of course everything looked okay and I found out the tank was FULL which is why it was coming back out at me.

Basically I want to know how does a fuel gauge work (specific to this truck).... and how do I go about fixing it? The sending unit has a brown wire that connects to the middle of the sending unit and there is also a ground.

Thanks!

 

[philjafo]

The gage measures resistance to ground. The sending unit is an adjustable resister, 0 to 90 ohms. As the float arm rises inside the tank the resistance goes up as well. If there is a short somewhere you get 0 or close to 0 ohms to ground resulting in a reading at E. If the connection is broken as in no connection to ground the resistance is infinite and the reading goes past full. Of course dirty or loose connections anywhere in the circuit results in incorrect readings, usually its ground connections and can be ones seemingly unrelated to the fuel gage circuit. The adjustable resister, or sending unit is exposed to the inside of the fuel tank and gets dirty also. The gage in my truck reads full and nearly full for most of the tank, then it drops like a stone from 3/4 to E. Dirty resister, and with only a 16 gallon tank I just watch the odometer anyways.

 

[sonicbluezx3]

Okay thanks. I am going to look at the wiring and see if anything is messed up when I get home from work today.

I think it's one wire (brown) coming off the sending unit and going into the cab, and another wire (black) going from the ground post of the sending unit to a frame mounted ground.

So technically if I wanted to test the gauge itself, I could pull it and put the brown wire on one post and ground out the other post correct?

If the gauge is bad I am going to just get an aftermarket one... and I am guessing that i need to find one that matches the sending unit ohm rating right? 0-90?

 

[philjafo]

Yes

 

[sonicbluezx3]

This is weird, but it's the sending unit.... I bought a brand new sending unit too when I replaced the fuel tank. I haven't tried filling it until now since I haven't been driving it, so that is why I am just now finding this out.

The wiring is fine and it's not a bad ground.

So I guess I'll try to find another sending unit. I'm almost positive the one I got was correct it just had 3 prongs instead of 2 since it was cheaper. Maybe not.

 

[sonicbluezx3]

Diesel is the same ohm range as gas right? Everything works except when it should read full it reads between 1/2 and 1/4.... however, it does read E when it should. It doesn't go further past E like you would think, so that is why I don't believe it is the gauge. Also, when I ground out the wires at the sending unit the gauge reads like it should.

I guess my next step is to test the resistance on the sending unit.

 

[philjafo]

If the sending unit is new, I would suspect the gage. Are you using a multimeter to measure the output of the sender? You say you replaced the tank, 87 fuel injected tanks have baffles, is the float hitting anything? GM used 0-90 ohm sending units in everything till sometime in the 90's, then they switched to 30-240 ohms (I think not certain) for better accuracy. How's the accuracy of the rest of the gages, could be the clusters ground or connections to the printed circuit. There's a gage cluster rebuilding thread on here somewhere that might help.

http://www.gmsquarebody.com/forum/showthread.php?t=6035

This is the one, note the way the gages connect with the clips. You might have a dirty connection there.

 

[sonicbluezx3]

The gauge worked just fine when i grounded out the wires at the tank though. That leads me to believe the gauge is fine. All the other gauges work fine too.

Im going to take the sendng unit to work tomorrow to test it and see if its reading right.

 

[sonicbluezx3]

The sending unit is a bit erratic but at full it read 95 ohms and was close to zero at empty.

Reason i thought it was the sending unit and not the gauge is when i grounded out the connections the gauge showed full. That seems like the opposite though...

I think im going to go get another sending unit and see if it does the same thing.

 

[sonicbluezx3]

Okay well I went and bought a new sending unit... it reads 0-90 ohms much smoother than the old one. However.... the gauge still reads the same way. That's why I just took it out of the box and hooked it up rather than putting it in the tank.... now I can go return it lol.

Sucks becuase I really thought that would fix it after testing the new one vs the old one on the multi-meter.

So now I'm back to square one, although I can rule out the sending unit. The wires don't seem to be a problem since the gauge actually gets a signal. So Im kinda lost....

Right now I'm trying to figure out how to test the fuel gauge (pulled it out of the cluster) along with the sending unit. However I don't know which pins in the back are power and whatnot. Trying to find a wiring diagram right now.

 

[sonicbluezx3]

If the sending unit is new, I would suspect the gage.

Well, I was wrong. I pulled the gauge out and hooked it up to a 9V battery and guess what? Does the same damn thing. Who woulda thunk it. Lol.

SO needless to say I will be visiting the u-pull-it yard to get a new gauge.

This truck makes me

Lol...