fuel gauge: full to 3/4 full only
 

Okay so I had the old flatbed at work yesterday hauling away some racking. Anyways I noticed that the fuel gauge isn't working properly. Talked to dad and he confirmed. I installed a new sending unit thinking it was the culprit but nope. The gauge pretty much stays at 3/4 tank all the time unless you fill it up. If I unhook the plug on the tank it drops to empty. I checked the wiring and it all seems fine. Its almost like the float is stuck in the tank but when I installed the new float it was confirmed it wasn't. Plus it does go to full when its filled up. Has anyone had anything similar? If so what was the issue? Thank you guys

My guess is an internal shunt (calibration) resistor in the gauge is bad. Looks like you'll have to replace the gauge,

What does it say if you actually run it out of fuel?

[coffee] [laugh]

What about your oil pressure gauge and temp gauge? If your other 2 gauges work OK I would suspect a loose connection at the instrument panel. The prongs on the instrument panel are simply crimped into the circuit board which is a single sided board without plated through holes. They WILL corrode and make a bad connection. I polished and soldered every pin to the circuit board and also tightened the female pins in the plugs. I still have trouble with them.

All 3 gauges run off a 5V voltage limiter on the back of the instrument panel. The voltage limiter seems to be a pulse type rather than a linear regulator. From the looks of it it could probably be replaced with a 7805 type 3 terminal regulator which is linear. If you try to measure the voltage with a simple voltmeter it probably won't give you an accurate reading unless you put a big 100 uF capacitor in parallel with the voltmeter to smooth out the pulsing If you see 12 V or 0 V the limiter could be bad.

I have wiring diagrams for the 89 that I'm looking at. I don't think it has changed that much.

The other gauges work fine. Thank you Edwin

Well whether it's the gauge itself or the loose connections you're probably going to have to pull the panel out. I'm an expert on that I've had mine out so many times I lost count. [laugh]

EDIT: I was thinking, those gauges are cheaply made and aren't sealed very well, I've seen tiny little particles of iron or rust get on the magnet and freeze the gauge so you might check that if all else fails. After re-reading your initial post, did you try working the sender while it was hooked up to everything to see if the gauge moves like it should?

Edwin

PM Sent

Want to hear something weird? I don't know if this'll help your situation, but worth looking into-
Back in 2003 when my rear left tire tread peeled off at 70mph freeway speeds, it not only mashed up the wheel well area, but took out my tranny cooler with fan, and bent upwards the fuel fill steel pipe.

Got the tranny cooler replaced, and straightened out the fuel fill pipe. But since that mishap, my fuel gage would never go all the way down to empty, and would never go all the way to full, when filled. The needle would hover around 3/4 to 1/4 full. Then one day while under the truck, I saw that rubber doughnut seal that is on the fuel tank, that puts a seal on the steel pipe as it enters the tank, was badly torn and all weird looking. I replaced that part from the dealer, and my fuel gage immediately worked perfectly and normal after that.

Could there be something weird going on with the air pressure in the tank? Or is it just a coinkidink? [roll]

Air pressure wouldn't affect the sender, but the filler pipe out of position and interfering with the float might.

J Martin you may be on to something. When dad had me switch from the single wheel flatbed to the dual wheel flatbed back in the Summer we ran into a snag. I had made a holder for the black piece the filler neck sat in where the cap goes. Well on the dually flatbed there was a recessed hole in the side of bed for the three screws that the round part of the filler neck screwed into. Well the problem I ran into was that the factory fuel inlet had a long bend that caused fuel to trickle into the tank very slowly and if you tried to run the fuel nozzle to fast it would run out of the neck onto the ground. So I found a stub filler neck that didn't have a bend but did have a long rubber hose. I might have shoved the metal inlet to far into the tank which is interfering with the float from working. Does it come close to the float/sending unit inside the tank in factory form? I don't recall pushing it in that far but I may have trying to get it all to work right.

Key word was "might". The operator is probably a spiraled flat iron with about a 3/4 twist full length. If something were to be touching it, it would limit the rotation. As far as the actual positioning of the parts, I don't know for sure.

Maybe one of the parts guys on the board could take a quick look at a tank and tell us if the gauge/pickup port in the top and the fill port low on the side are in line.