Howdy folks, been trying to get my gas gauge functioning again. Just got done putting a new gas tank in, as the old one was gumming up my carb with nasty gas. Figured a new float and sending unit would fix the gauge, and while I had it all apart I fished the float out of the tank and found it totally separate from the sending unit. So I figured hey that's what was wrong with the damn thing. Get it all back together, put gas in it, and sure enough still stuck on empty. So now I've got my whole dash torn apart and been tinkering. I don't think it's the voltage regulator as my oil pressure works(before I broke it taking everything apart that is), and I've established that the gauge works with a 9 volt battery. So it's probably the wiring? My question is what voltage should I be getting at the harness plug? How do I go about fixing the wiring? The only thing I know about electricity is that it hurts and I'm scared of it!

Thanks!

 

Did you test the sending unit before you put it back in? If you hook it up and manually move the float arm it should move the needle on the guage. If you jumper the leads in the connector the needle should max out if the guage is good. If the sending unit tests good out of the tank but not while in the tank then your float is bad. They can get cracks or pin holes and fill with fuel.

 

What you can do is put everything back together in the dash electrically, and then go back and take the wire off the sending unit. Get a scrap piece of wire and hook it to a good ground and have the other end near your sending unit wire. Turn the key to run but do not start the engine. Watch the gauge, with the sending unit wire off hanging in the air it should go full scale empty or full (I can never remember which). Then go back and put the sending unit wire on your ground wire. The fuel gauge should swing full scale the opposite way. If it passes this test, the wiring and the gauge are good.

You didn't say what engine you have. If you have a fuel injected truck you will have some extra wires at the sending unit for the pump. Pick the correct wire If you have a 460 you will have a pump also, and you also have a switching valve on the frame that can go bad and cause gauge problems.

Another test; Go back and unplug the sending unit. Make sure the dash is all hooked up electrically. Turn the key to run, engine off. Go back to the sending unit plug, take a meter on dc volts and probe the wire to the gauge. With one probe on the correct wire, and the other probe on a good ground, you should get a flashing voltage, much like a turnsignal flasher.

 

You don't have duel tanks do you?
Before you went and pulled the dash all apart there was an easy test to see if the wiring from tank to gauge was good.
Ground the sender wire turn the key on and the gauge should move to full. If it does not then you will have dig deeper.

If that did not work there is a plug joining the rear wiring harness to the trucks main harness under the master / booster.
Pull this apart and check if it is nice and clean. Then using a ohm meter check the wire from the sender to that plug.
If good using a test light or volt meter check the main harness if you have power there for the sender. With a test light it will blink, depending on the meter used the needle should be around 5 volts the other type will go up & down.

If you do not have power there you will have to go deeper. Duel tanks you will need to check the switch (what year is this?) on some later truck the gauge switching is done at the tank switching valve so the above ohm test may not work as a bad switching valve could cause a bad reading.
Any way on early trucks the dash switch does the gauge switching so you need to check if power in & out of switch.

If not go back to the gauge checking the wiring from dash switch to the plug at the gauge cluster. Then from the plug to the gauge, got power into the gauge? Power out / in to the IVR?
Power into the IVR should be solid 12 volts, out should be about 5 volts or blinking with test light.

Also as said you could have tested the sender before you installed it, ohm meter and move the float arm.
Dave ----

 

I put a new float and sender from LMC trick in with the new tank. Felt silly for not playing with it plugged in as soon as I realized it didn't fix it. I've got an 86 4.9. it used to have dual tanks but the rear had been "disabled" by a previous owner. It was filled with ancient gas that started to leak out so I did away with it. I won't be surprised if it has to to with the tank switch. I'll try some of these test this weekend and report back.

 

If you had dual tanks, take the dash switch and flip it up and down rapidly about 10 times and then see if it starts working. I had to do that to my old 1980 till finally that didn't work and I had to go in and wire around the switch.

 

Well ran the sender ground test. Nada, needle stays put. And I tested for voltage at the sender plug with a voltmeter--and it read steady at less than half a volt, no flickering whatsoever. Like I said the gauge pegs when hooked to a 9 volt battery so I'm figuring its the wiring some place. (If it narrows it down any I've got the same situation with the temp gauge, pegs with 9-volt, nothing when hooked up) I'm not really sure what to look for at the wiring harness, nor how to test the tank select switch. Any thoughts or resources there would be appreciated!

 

By golly that did something! Gauge slowly crept up to 1/2 full. I feel like I oughta have more gas then that as I just filled it up the other day- and only driven like 30ish miles tops.
EDIT: Filled 'er up at it went to full, whoda thunk!

 

Looks like the weak link is the fuel selector. As for the oil. Do the same test, ground the lead and watch the guage. Helps to have a long alligator test lead, Harbor freight has a kit of different sizes. I typically put a set on one or both ends of a vacuum cleaner power cord, they get tossed out all the time, just stop and cut the cord off. If you use a lead like I described then you can watch the needle. Ground it for as little time as possible so you don't harm the gauge. If needle pegs out, bad pressure sender, if not, you've already tested the guage so it will be in the wiring. That doesn't mean the sender is good, but you can retest after fixing the wiring.

 

When you posted that and the duel tank up front we could have given a little better way to try and fix it.
I say this because if the 3 gauges: oil, temp and fuel gauges and they don't work it could point to the ICVR as it feeds 5 volts to all 3 gauges. This ICVR is on the back of the gauge cluster.

Then you posted this up so I would say the ICVR is good (so far).
As for the low reading if the fuel tank switch is not making good contact I would think that could be why the low reading? You can try flipping the switch a bunch more times to see what that will do.
If you can get to the back of the switch and get electrical contact cleaner and spray it in the switch and flip it a bunch to see if that works also.

For the oil and temp gauges not working do as posted and ground the sender wire. You can ground the wire then turn the key on and see if the gauge moves up just don't let it peg out for a long time. It is best to turn the key off when it hits half gauge and let it come back down.
Good luck
Dave ----

 

Yeah, the ICVR can cause problems, but I've only seen the gauges all wig out in unison, not fail altogether. My suggestion is to jiggle the ignition key a bit, twist it towards yourself just a bit and see if the dash gauges start working. The lube in that mechanical assembly gets old & crusty and the spring can no longer overpower it.

~~

I like this stuff for cleaning switches & contacts, TV tuner cleaner from Radio Shack:



https://www.radioshack.com/products/...-and-lubricant

 

That is good stuff, glad to see they are still selling it. If your volume control on your radio is scratchy and makes noise when you turn it, get some of that stuff sprayed in the control (you may have to take the radio apart a little bit) and it fixes it right up.