47-55.1 gas gage

[nvrdone]

ok, so my gas gage is showing full at all times. if im reading the wiring diagram
correctly, power goes to the gage & back to the sender at the tank, then thru the sender and to ground. Im getting no reading with a test light or ohm meter.
Any ideas where to look?

[mongocanfly]

you should be able to read ohms across your sender..if you have it pulled from the tank
as you move the arm up and down the ohms should change

[mr48chev]

You should get a reading with the ohm meter just by putting one lead to the post the gauge goes to and one lead to the top of the sender. Make sure you have it set on ohms, no need to ask why I said that.

With the key on you should also have voltage to the sender though the gauge.

It's kind of a step one, step two step three process.

1. make sure you do have voltage to the sender with the key on.

2. if that is good pull the wire off and check across the sender with the ohm meter.
No reading, pull the sender, if you get a reading, you may have to run a ground wire from one of the screws on the sender to a good ground on the body. Some tanks seem to have metal to metal contact and a good ground with the body and others are insulated from any contact with the body metal.

As Mongocanfly said, you may have to pull the sender and check it with an ohm meter but I would do that after running the other checks.

[nvrdone]

ok guys, I will start doing the other checks first. I hate to take the tank out to get to the sender because that requires draining the tank and then dropping it. I just thought there would be a quick check. as you know getting to the gage under the dash is a royal pain.

[mongocanfly]

Stock tank or donor tank?

[vintovka]

Just remember original gauges are 6 volt. I forgot this when i converted my 54 to 12 volt. The gauge swept over to full momentarily and then to empty even faster. That was 1969 and it hasn't moved since.

[nvrdone]

its an aftermarket tank mounted under the rear of the bed. I do have a resistor on the input power lead to drop from 12v to 6v. Its worked fine for many years, but the truck has been parked for about 3 years and now the gauge only goes to full. the gauge didn't work at all and I found the power lead had come un hooked from the ignition switch. after I hooked that back up, it now only goes to full. so that's this weekends project.

[nvrdone]

ok, so I got hold of an ohm meter. set it at 20v dc. checked for power into the resistor feeding the gage. got 10.6 v into the resistor, 6.1 out and 3.6 out of the terminal that the wire to the tank connects to. hope that is clear. do those numbers seem right. and yes the ignition switch is on and the pump runs.
got under the back and can just reach the screws on the top of the sender. I get no reading at either terminal. I may not have had a good ground when testing so I will fix that tomorrow and re test. I may have a broken wire somewhere in that circuit. I seem to remember when I took the tank out of the cab, I just extended the original wires to the rear to hook up the float.
more later. wish me luck.

[mongocanfly]

To verify the sender is working you need it out of the tank..hook the ohm meter to it and move the arm up and down....that's how you check that...maybe you'll find a loose wire somewhere and thatll fix it.

[nvrdone]

im trying to avoid dropping the tank if possible. I would really like to not have to drop the tank if possible.

[nvrdone]

shouldn't I have some kind of ohm reading at one of the sender terminals?

[mongocanfly]

I understand...pulling a tank sucks...and it always seems like they're full when I need to
But yes there should be some ohm reading on the terminal...but without knowing where the arm is inside the tank you'll have no idea if your ohm reading correlates with tank level..
https://m.youtube.com/watch?v=435kwVAANxU

Maybe you'll get lucky and find a loose wire somewhere...

[nvrdone]

ok, I think I will trace wires before I drop the tank.

[mr48chev]

That's the best thing. do the simple easy stuff first. Are you getting a voltage reading of any kind at the end of the wire at the sending unit when you have it disconnected from the sending unit? Power though the gauge if not?

[Second Series]

I think the quick and dirty test is to disconnect the wire going to the sender from the back of the gauge, short the gauge to ground it swings full one way, open swings full the other way. You could also use a potentiometer attached to the gauge to see if the gauge works. You replace the sender with the pot. for that test. I ran my original 6v gauge and sender on 12v for years, I finally put in a dropping resistor, but it didn't change much.

[nvrdone]

getting 3.6 volts out of the gage. nothing at the input for the sender. but I may not have gotten a good ground. I will short the gage first and see what happens. then check for power at the sender. I will also run a ground jumper from the sender to the frame and see what happens. I cant disconnect the power wire to the gage with out dropping the tank. no room to get my fat hands in there. id rather work on the truck than paint the back deck, so here goes …...

[nvrdone]

ok, one step forward, now many steps backward?
The first thing I checked was the wire from the gage back to the sender. at the gage, the nut that holds the wire on was only finger tight. removed the wire & shorted to ground. the gage shows just above empty. re installed the wire & tightened the nut. no change. added 5 gal of gas & no change. got a jumper wire on the ground stud of the gage & grounded it. no change. so im thinking that the problem is in the sender, but im going to trace the wire from gage to sender & back to ground. I know those wires are original & extensions just spliced together. if I don't find anything, its time to drop the tank. Oh, dread!!

[vintovka]

Gauge problems? had plenty. This time i had opportunity to bench test gauge and sender both in an out of new tank. First gauge was no good (chinese junk), then connection was iffy. Once replaced all was fine when installed. If you have to pull tank and sensor do bench test if you can

[nvrdone]

I will do a bench test, after I test the wiring. right now, after adding gas, I think I may have a bad float because the gage is showing almost empty. I think I have another sending unit somewhere around her that I may swap in. its suppost to rain thru tues so good time to figure this out.

[mr48chev]

You can hook the other sending unit to the wire , ground the housing and move the float to see if the gauge works the way it should. After testing it with the ohm meter to be positive it works.

[nvrdone]

that's a good idea. I could run a jumper wire from the positive terminal on the in tank sender to the positive on the extra sender. then ground it. I think I will try that tonight. now I just have to find the extra sender. thanks.

[nvrdone]

ok, im looking at an ohm meter and since im not good with electrical, what setting do I use to test the sending unit?

[mongocanfly]

The one that looks like a horseshoe..

[nvrdone]

ok on my meter the "horseshoe" shows settings for 2000k, 200k, & 20k. whats the best choice???

[mongocanfly]

20k I think will work