Odd fuel gauge problem

[RIDE-RED 350r]

Truck: '78 K-10 with functional dual tank setup

Problem: Fuel gauge reads more fuel than I KNOW it has. But does not read at the classic problem 3 o'clock position. It does move and now and then will drop down and read what seems correct, but most times it reads more fuel than I know it has in it. Same issue, both sides. Very occasionally, the gauge will drop to empty

Background info: I recently replaced my dash pad and removed the dash AC ducting as the rest of the AC system has long since been removed from my truck. It did take some work to get the ducting all out and I can see the possibility I disturbed some electrical.

I did manage to pop the fuel pump fuse when I was fiddling with my tank switch to install the backing plate. Obviously I shorted one of the contacts on the dash panel and the fuse popped. Is it possible I damaged my fuel gauge? I tend to think the fuse pops before damage can occur?

I have tried a different fuel tank switch and the gauge reads the same. The fuel tank switch is not new.

I have searched this topic but am unable to find results on my particular issue. My plan is to pull my gauges and check my wiring and see if I disturbed anything in a couple days when we get some better weather. Just thought I would ask if anyone else has experienced this particular problem and what was done to correct it.

Again, gauge reads more fuel than I know I have but very occasionally drops down to what I would expect it to have only to stay there a few seconds and go right back up to appx 3/4. Does this on both tanks.

[ray_mcavoy]

Sounds like you have a weak/loose connection somewhere that is adding some resistance to the circuit. A small amount of added resistance in the gauge's ground or the wire going to the sending unit(s) will cause the gauge to read higher than normal. The 3 O'Clock pegged gauge is a result of a lot of added resistance ... such as a completely open circuit.

Since your gauge is still somewhat operational, I don't think the incident with the blown fuse damaged the gauge. However, removing the AC ducting could have very well distrubed the wiring (especially the instrument cluster connector) just enough to cause an intermittent weak connection.

When you pull the cluster out for further inspection, pay particular attention to the cluster plug terminals and the traces they mate up with on the cluster's printed circuit. Also take a look at the metal clips that make the connection from the printed circuit to the studs on the back of the gauge. And finally, check the two nuts on the back of the gauge to make sure they're sung (but not too tight) against the ceramic resistor.

[RIDE-RED 350r]

Thank you for the tips Ray. That is the exact sort of advice I am looking for. Even though the old printed circuit is pretty rudimentary by today's standards, up to this point I have pretty close to zero experience with these units, let alone troubleshooting them.

Thanks again, and will post back what I find when I am able to have a look.

[tucsonjwt]

Checking grounds and connections is always the first step. In my case, that did not help and I replaced both fuel tank sending units (dual tanks). That did not help. I replaced the fuel gauge and that worked. I had the same symptoms you have.
Fuel gauges can be had for about $50, less at junkyard but I would not trust one from there. Sending units are about $40 for aftermarket (Spectra) but the gauge needle will bounce when the truck is moving - ok read when truck is stopped of driving on level road. Delco sending units will not bounce, but cost about $140.
If you are stumped and checked the obvious things, I would spring for a $50 fuel gauge and return it for a refund if it does not work (assuming your seller allows that on electrical parts.) That is a lot easier than guessing for 3 weeks. Following the normal troubleshooting instructions got me nowhere.

[Jeepwm69]

On the reading full thing, that turned out to be a cut wire going to my sending unit. I spliced it back together, and the fuel gauge worked fine for awhile. Then it would drop to 1/4 tank and sit, and sometimes work, sometimes not. I replaced the resisitor on the back of the fuel gauge and it's worked fine since.

Do you have an ohm meter where you can check and see what readings are going to the fuel gauge?

[RIDE-RED 350r]

I do have a known working fuel gauge I can swap in if needed.

Also have a mulit-meter and have emailed a link to a very informative thread to myself I found here that goes into great detail on troubleshooting the system.

I think I am going to find a sketchy connection as a result of my bullishly pulling out the AC duct work.

Thanks for the replies and tips.

[RIDE-RED 350r]

Well, I pulled my gauges cluster and did some checking when I got home from work today.

All connections looked good. I took out the connectors that snap into the printed circuit and the studs on the back of the gauge plug into. I cleaned the printed circuit contacts those connectors touch and slightly tweeked the connectors for a more positive contact with the printed circuit. Also checked the nuts on the back of the gauge and they are snug. I also removed the harness plug on the back of the printed circuit and checked for poor connections and clean contacts. Everything seems in order.

I have 2 fuel tank switches and tried swapping back and forth with no change in the gauge readings.

Since this issue is not isolated to 1 tank I don't think I have a sending unit problem. What are the odds both units would fail at the exact same time in the exact same manner?

I did follow my sending unit wire from the tank switch out the firewall and into the engine compartment and between the switch and engine compartment I found 2 butt splices. I think there is a fair chance one or both of those could be causing my issue. Unfortunately, I started running out of daylight and had to stop there and put everything back together because I need to use my truck to get to work tomorrow.

I plan to redo those two splices and see if that clears my issue up tomorrow when I get home from work.

If that doesn't work, I will have a known good fuel gauge tomorrow to try.

Side note: when I accelerate up hill, the gauge does in fact drop down to lower readings, but when I level out it goes back up. The needle doesn't move erratically or jerky, but nice and smooth as you would expect for normal operation. I am convinced that the sending units are working and that either my gauge is screwed up, or those butt splices are causing some excess resistance in the sending unit wire.....

Will post back what I accomplish tomorrow.....

[ray_mcavoy]

Originally Posted by RIDE-RED 350r

Since this issue is not isolated to 1 tank I don't think I have a sending unit problem. What are the odds both units would fail at the exact same time in the exact same manner?

I agree ... it's highly unlikely that both would fail at the same time in the same manner.

I did follow my sending unit wire from the tank switch out the firewall and into the engine compartment and between the switch and engine compartment I found 2 butt splices. I think there is a fair chance one or both of those could be causing my issue.

...

I plan to redo those two splices and see if that clears my issue up tomorrow when I get home from work.

Yes, those butt splices could definitely be a problem. If they're not sealed with heat shrink or anything, moisture and/or road salt can find it's way in there and cause corrosion. That'll obviously increase the resistance in the circuit and cause the gauge to read higher than normal.

If you continue to follow the sending unit wire back along the frame, you should come across a single inline connector where it plugs into the dual tank harness. Should be located somewhere near the tank switching valve. Checking that connection to make sure it's tight and free of corrosion wouldn't be a bad idea either.

From that connector, the sending unit wire enters the dual tank harness and basically makes a U-turn heading back up to the switch on the dash. So check that section of wire over for butt splices or other issues as well. The wires running from the dash switch back to each sending unit are most likely okay since a problem there would only affect one side.

[RIDE-RED 350r]

Thanks for the advice..

I wasn't able to dog too deep in the sending unit wire as it left the engine compartment and headed down toward the tanks, but it seems there is only 1 wire?

I could be mistaken, but either way, I think my problem is likely in that area......

Another oddity: Initially when I bought the truck there was no backing plate for the tank switch on the dash that indicates left and right side tanks. I figured out by putting fuel in one tank that when the switch is flipped up, I am running the left side tank and of course down is the right side. This is the opposite of how that plate is labeled. I think it is just a matter of a pair of wires reversed but I'm not sure which ones. There was a Painless brand complete harness installed in this truck by the PO so I think something got reversed. No big deal really. I'm more concerned with getting my gauge to read correctly.

I did verify the valve is switching when the switch on the dash is flipped. I crawled underneath and had the wife flip the switch and I could feel and hear it switching.

Once I get this problem straightened out I need to figure out why I only have high speed on my windshield wipers...LOL! That's annoying

[ray_mcavoy]

Originally Posted by RIDE-RED 350r

Thanks for the advice..

I wasn't able to dog too deep in the sending unit wire as it left the engine compartment and headed down toward the tanks, but it seems there is only 1 wire?

You're welcome.

Yes, there is 1 wire that connects the gauge's sending unit terminal to the selector switch. It takes a rather roundabout route to get there though.

On a single tank truck the wire basically runs from the gauge, out into the engine compartment, down along the frame, and connects directly to the sending unit. That same wire routing is used on dual tank trucks but instead of the wire plugging directly into the sending unit, it plugs into the dual tank harness instead. That harness routes the wire back up to the front of the truck to the switch. And bundled together in the dual tank harness are the wires that run from the switch back to each sending unit as well as the wire that operates the solenoid valve.

Another oddity: Initially when I bought the truck there was no backing plate for the tank switch on the dash that indicates left and right side tanks. I figured out by putting fuel in one tank that when the switch is flipped up, I am running the left side tank and of course down is the right side. This is the opposite of how that plate is labeled. I think it is just a matter of a pair of wires reversed but I'm not sure which ones. There was a Painless brand complete harness installed in this truck by the PO so I think something got reversed. No big deal really. I'm more concerned with getting my gauge to read correctly.

Yeah, someone probably had the connector apart and got the two sending unit wires reversed. GM did make some changes to the switches over the years. I can't remember if they reversed the left/right configuration at some point or not. And I'm not sure if the label plates will even interchange between the different year switches. But if they do, I suppose that could be a possibility.

Once I get this problem straightened out I need to figure out why I only have high speed on my windshield wipers...LOL! That's annoying

I'm sure you'll want to start a new thread for this, but to give you a head start with some troubleshooting info, the switch is on the ground side of the circuit. There is a 3 terminal connector down near the bottom part of the motor. One terminal is a power feed that is live with the key on. The other two terminals are the switched grounds. The switch grounds one of them for low speed and the other one for high speed. The other 2-terminal connectors on the motor are for the parking switch and the washer pump.

[RIDE-RED 350r]

UPDATE: I would like to take this opportunity to announce to the world that I am an idiot! LOL!

So, I conducted a little test today (payday) and filled my left side fuel tank all the way full..and guess what... The gauge reads perfect full for the left tank.

So, I had to make a run to a parts store to get a new set of plug wires and it's about a 40 mile round trip. I kept running on the right side tank which started the trip reading a little over half. Well whatya know... by the time I got back home and parked I was at 1/4 tank.

Now either the truck gets a little better fuel economy than I gave it credit for, or it's just one of those deals where it takes awhile to get down to reading 1/2 on the gauge, but once it does it goes quickly from there....

Anyway, I would like to offer my most sincere thanks to those who took the time to offer me advice, tips, and information.

And now, on to my windshield wiper issue....

[ray_mcavoy]

I'm glad to hear that it's working okay!

On the plus side, it sounds like the instrument cluster got treated to a good inspection & cleaning in the process