Stumped on Fuel Gauge

[Corts60]

Hi folks,

I used to think I was pretty decent at solving electrical gremlins, but this fuel gauge in my '86 is kicking my butt. It all started this summer when the gauge would only read 1/4 tank when it was full. This happened out of nowhere as it always worked before. It would move to E as you used fuel, i.e.: 1/8 tank would actually be 1/2 tank or so. I thought that was weird so I pulled the sender wire and put it directly to ground to test the gauge. That pegged the gauge out so I thought OK, bad sender.

I pulled the tank and replaced the sender. before I put the new sender in the tank, I hooked it up to the sender wire and ground and manually moved it and watched the gauge respond. OK, all good? Then I put the sender in the tank, installed the tank and added 6 gal of gas or so. The gauge will not come off of E. If I ground out the sender wire, It will peg full as before. I rechecked all grounds from sender to frame, frame to battery, battery to body, etc. All are 1ohm or less.

Any one have any ideas of what I am not doing correctly here?

Thanks in advance,

Cort

[JJH Jimmy]

My first guess would be that the float had a hole in it or it is hung up on something.

[Corts60]

Originally Posted by JJH Jimmy

My first guess would be that the float had a hole in it or it is hung up on something.

That’s what I was a feared of. Now I get to pull the tank again…ggrrreeeaaatttt. Haha.

[Dead Parrot]

Had something similar on my 84 CUCV. Gauge would only rise to 1/8 when full but would read E when empty. Saw a thread on the Steel Soldiers site on a similar problem that stated replacing the gauge fixed the problem. Swapped in a different gauge and that one worked normally. Only downside is it says Unleaded instead of Diesel.

It was an easy fix at the time as I had the gauge cluster out chasing issues with several of the idiot lights that were standard with the CUCV trucks. Never checked what happened if I grounded the sender wire.

The SS thread stated that one of the resistors failing in the gauge can cause that behavior.

[Corts60]

Alright fellas, I need some help here. I’ve swapped two senders, two gauges, ohm’d out the sender wire, checked all grounds. I’ve manually moved the sender by hand and watched the gauge respond. When I first start the truck for the day, the gauge reads correctly, but when the engine gets warm, the gauge goes back to E….

I’ve troubleshot a lot of wiring issues in my time, but I’m stumped. Only thing I can think of is something is heating up and causing resistance to change. What could it be?

[JJH Jimmy]

Where did you ohm the sender wire from? All the way to the gauge cluster?Possible getting resistance in the bulk head connection? I would probably start by running a temporary new wire from the sender all the way to the cluster. If that doesn’t solve it then you narrowed it down to the printed circuit on the gauge cluster.

[Corts60]

Originally Posted by JJH Jimmy

Where did you ohm the sender wire from? All the way to the gauge cluster?Possible getting resistance in the bulk head connection? I would probably start by running a temporary new wire from the sender all the way to the cluster. If that doesn’t solve it then you narrowed it down to the printed circuit on the gauge cluster.

Thanks for the reply. I ohm’d it from the connection at sender to the back of the gauge through the circuit board. This is the 2nd circuit board as well…I installed the tach upgrade kit from LMC. This problem was happening before the install and is still happening.

[ray_mcavoy]

The gauge topping out at 1/4 even when the tank is full and the sending unit & float are okay is often a sign that the resistor on the back of the gauge is bad or not making a good connection.

[Corts60]

Originally Posted by ray_mcavoy

The gauge topping out at 1/4 even when the tank is full and the sending unit & float are okay is often a sign that the resistor on the back of the gauge is bad or not making a good connection.

Ok thanks for that. At this point it has to be something I’ve already touched since I literally have touched everything. Haha. Maybe the little spring clips in the circuit board did not get seated well enough.

[Corts60]

Ok reviving this from the dead as I still have not resolved this issue and need some help if anyone has ever seen this issue:

The fuel gauge works fine when the truck is cold, it reads the correct fuel level. The second the thermostat opens on the engine (or thereabouts) the gauge just goes to E. And stays there until the next day after everything has cooled off. I think the sender wire is opening up when the engine is warm and I cannot figure it out.

Anyone ever experienced this? I’m about to run a whole new wire to the gauge and be done with it. But this truck is so original, I don’t want to start hacking wires up.

Any help is appreciated!

[Shark_13]

Sounds frustrating, especially since you replaced the sender and the gauge.

Will be watching this, as I am going down this road now.
I'm the process of putting in new senders.

Might be worth the effort of trying a temporary new wire, just to rule out the old wiring.

[Pontiac1976]

Check fuse to see if filament is not split heat up it open up cool down make contact again. As said check your wiring both side that got good wire and good connection to the sending unit. have someone help sitting in the seat, you wiggle the wire when hot see if any change if does not. Then recheck the cluster main plug you may have not have a good connection at the main plug, maybe bent circuit board lead at main plug or gauge.

[Getter-Done]

Here is a wiring diagram for the plug if you didn't have it already.

[Loco83]

My 83 Silverado gas gage only goes up to half tank when full, been 2 yrs since it started acting up, now I just live with it, add gas when the gage is between empty and a 1/4 tank.

[66gmcguy]

The very first thing I do when trying to diagnose electrical problems is clean the crap out of grounds. This tends to solve most of my problems. Clean the metal underneath the terminal on the ground wire on the sending unit. Make sure the metal is clean and not rusty. Hit it with a flap disc or wire wheel if you have to. Put in a new bolt and new star washer too for good measure.

Sending units in these trucks are 0 ohms empty and 90 ohms full. So if it's showing a lower level, that means there is less resistance than there should be. If the insulation on the sending unit wire is worn away and the copper is touching metal, that means it's effectively bypassing the resistor in the sending unit and giving you a false low reading on your gauge. So the first thing I would do is check the sending unit wire for chaffing. On some trucks, the sending unit wire runs along the back of the engine and down through a metal tube on the back of the engine above the starter, then over to the frame. Seems like a possible wear point (and an area of high heat which could change things enough, thus why the problem doesn't show up until the engine is warm). If you've already changed gauges and sending units to no avail, that's the first thing I would check after checking the ground at the sending unit.

If there's no obvious damage I would run a new temporary wire from the bulkhead connector to the sending unit (hold wire directly on the terminal on the sending unit) and see if that changes anything. You can remove the sending unit wire from the bulkhead connector by pinching the sides of the terminal with needle nose pliers and pushing it through the plastic on the connector out the front, then stick your new wire in. You can also buy new terminals from Napa auto parts. They are male Packard 56 terminals. You will have to crimp them onto your new wire. I like to keep some of those on hand for doing exactly this kind of stuff.

If that doesn't work, I would put a wire direct from the ground terminal on the battery to the sending unit housing (not to the ground wire on the sending unit) and see if anything changes. If that's not it then I would just start checking for loose wire connections inside the truck and on the gauge cluster.

Other than that I have no clue what it could be lol.

[Sabaka454]

Agreed, clean the grounds. Near the parking brake you have grounds, clean the body, engine and frame grounds as well.

[Honest Bob]

Have you gone through and checked every inch of the wire? I know on my blazer there was a splice point (kinda looked like a miniture tow hitch ball and socket) that I had to clean and rebend some of the fingers to get a better connection. With it being heat related the wire may be coming close to the exhaust somewhere.

[Corts60]

Thanks everyone for the input. But, I've decided to move on and posted the burb for sale last weekend. It's time for new projects. The new owner comes tomorrow to grab it.