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Fuel Sending Unit problem

KikiPhillips

Well-known member
Joined
Jul 23, 2014
Messages
50
Location
Alabama
Corvette
1982
Hello again,Trying to troubleshoot my (1982) gas gauage not working properly, I've narrowed it down to probably my sending unit but in my testing of various things, I came across a few puzzling issues.
1. There are 3 studs on fuel gauage: Power, Ohms, Ground. Power is good, ground is good but when I turn on the ignition, the ground continuity goes away??? With the key off, the ground has continuity with the Chassis. The ohms wire has continuity to the gauage also.
2. Testing the Pigtail connection at the tank. If I unplug it and test the 3 wires going into the tank, the power wire for the pump has continuity with the ground. That doesn't make any sense to me???
So, I'm trying to keep from pulling the sending unit, until I know it's what's bad.....Any ideas on this?
 
The gauge should always have a ground. I would test this usng a voltmeter when the circuit is energised rather than an ohm meter though. Only the one gauge giving you problems? Grounds for the other gauges are normal?

When you say the fuel pump has continuity with ground while the three pin plug is disconnected; is there continuity between the power wire and the chassis, or the ground wire on the plug?
 
Good questions....It turns out that all of the grounds act the same way, they all ground to the chassis till I turn the ignition switch on, then they no longer test for being grounded. I also noticed on the Printed Circuit Board, a flimsy part with no push down nut on the Oil Temp gauage acting weird. I tried pushing it down a bit and when it made contact, the warning sound the car makes when the door is open and the key is in the ignition (not on though), plus the lights on the cluster came on. Strange huh? The door was open when I did this.

As for the Plug leading up to the sending unit, the hot wire did NOT have continuity with the ground wire. The ground wire is still grounding to the chassis. So, if the sending unit plug has the hot wire grounding with the ground is that normal? It doesn't seem right but sometimes the continuity tests like that on some things and still works. Because the pump does work and the car cranks ok. You could test yours really easily right? : ) Test the continuity between your ground and Hot wire leading into your sending unit?

Thanks again for help knowledge...
Kiki
 
The flimsy part by the oil temp gauge is most likely for the clock. (Depending on what radio option the car came with it had a clock or the oil temp gauge) it should be taped up so it can't contact anything. I'd have a look at your gauges fuse, you may have blown it. If your gauges are loosing continuity to ground this should be picked up by the voltmeter. Does this read anything odd?

It would easy to test the sending unit on mine, unfortunately my car is away at the mechanics.
None if the circuit diagrams I look at show the fuel pump connection either.
 
Ignition on.

When the ignition is on you should not see ground but voltage since the circuit is now completed. If you are seeing 12 plus volts things should be good. So run your voltmeter from that contact to ground and what do you see?
 
Easy test for a fuel gauge. (or the temp or oil pressure gauges). Disconnect the wire from the sender and put a 12v test light between the lead and a good ground. Turn the key on to power the gauges. The test light should glow and the gauge should read about 1/2 scale. Not 100% accurate for calibration, but a quick and dirty test to troubleshoot gauge problems from my days as a dealer tech. If the test light does not come on at all, there is no power- possible blown fuse or bad gauge. Light glows, the circuit TO the sender is good.
GM gauges are 0 to 90 ohms. Most test light bulbs are somewhere around 40-50 ohms resistance, giving you 1/2 scale on the gauge.
 
Easy test for a fuel gauge. (or the temp or oil pressure gauges). Disconnect the wire from the sender and put a 12v test light between the lead and a good ground. Turn the key on to power the gauges. The test light should glow and the gauge should read about 1/2 scale. Not 100% accurate for calibration, but a quick and dirty test to troubleshoot gauge problems from my days as a dealer tech. If the test light does not come on at all, there is no power- possible blown fuse or bad gauge. Light glows, the circuit TO the sender is good.
GM gauges are 0 to 90 ohms. Most test light bulbs are somewhere around 40-50 ohms resistance, giving you 1/2 scale on the gauge.

Great test. Thanks for the quick lesson? :beer
 
Easy test for a fuel gauge. (or the temp or oil pressure gauges). Disconnect the wire from the sender and put a 12v test light between the lead and a good ground. Turn the key on to power the gauges. The test light should glow and the gauge should read about 1/2 scale. Not 100% accurate for calibration, but a quick and dirty test to troubleshoot gauge problems from my days as a dealer tech. If the test light does not come on at all, there is no power- possible blown fuse or bad gauge. Light glows, the circuit TO the sender is good.
GM gauges are 0 to 90 ohms. Most test light bulbs are somewhere around 40-50 ohms resistance, giving you 1/2 scale on the gauge.

Handy to know. The other one is with the sender unplugged the gauge should read high, if the sender wire is grounded it should read low.
 
Easy test for a fuel gauge. (or the temp or oil pressure gauges). Disconnect the wire from the sender and put a 12v test light between the lead and a good ground. Turn the key on to power the gauges. The test light should glow and the gauge should read about 1/2 scale. Not 100% accurate for calibration, but a quick and dirty test to troubleshoot gauge problems from my days as a dealer tech. If the test light does not come on at all, there is no power- possible blown fuse or bad gauge. Light glows, the circuit TO the sender is good.
GM gauges are 0 to 90 ohms. Most test light bulbs are somewhere around 40-50 ohms resistance, giving you 1/2 scale on the gauge.



That is true, but ONLY with an incandescent bulb test light (which is what I like to use myself). Electronic test lights with high impedance ratings, so as not to damage computers, will not work the same. :)
 
Great Test, the test light glowed a little and the gauge went to a little below half. So, the gauge and ohms wire are good, which means the sending unit is bad. I didn't want to buy one till I was sure. (I've bought enough things that don't fix problems)

The test on the Grounding issue (ground disappears after turning on the key) didn't prove out, maybe it was me though. My Multi-meter sucks bad! It does a thousand things but just not what I expect. I did however, hook up my very lightweight Test Light to a good ground, turn the key on and tested the ground posts on the gauges and it did not light up...I thought if there was current it might light up a little? Something tells me they're ok though because the gauges do work.

Also, on the Flimsy PCB connection. I do have the Oil Temp gauge not the clock. I'll do as you suggested and put a piece of tape on it. It's seems strange to me that it came from the factory that way though?

Thanks everyone for the insight & skill...I'll be in touch soon, I'm sure....
Kiki
 
Great Test, the test light glowed a little and the gauge went to a little below half. So, the gauge and ohms wire are good, which means the sending unit is bad. I didn't want to buy one till I was sure. (I've bought enough things that don't fix problems)

The test on the Grounding issue (ground disappears after turning on the key) didn't prove out, maybe it was me though. My Multi-meter sucks bad! It does a thousand things but just not what I expect. I did however, hook up my very lightweight Test Light to a good ground, turn the key on and tested the ground posts on the gauges and it did not light up...I thought if there was current it might light up a little? Something tells me they're ok though because the gauges do work.

Also, on the Flimsy PCB connection. I do have the Oil Temp gauge not the clock. I'll do as you suggested and put a piece of tape on it. It's seems strange to me that it came from the factory that way though?

Thanks everyone for the insight & skill...I'll be in touch soon, I'm sure....
Kiki



Make sure that your sending unit is grounded properly, I don't see where you state that you checked that. You may not see the test light glow when checking a ground source to the gauges, as the current is so little. I usually just run a known good ground wire to the source and see if that changes anything, which is what I would do to your fuel sender if you haven't done so yet. Good luck with it. :)
 
The test on the Grounding issue (ground disappears after turning on the key) didn't prove out, maybe it was me though. My Multi-meter sucks bad! It does a thousand things but just not what I expect. I did however, hook up my very lightweight Test Light to a good ground, turn the key on and tested the ground posts on the gauges and it did not light up...I thought if there was current it might light up a little? Something tells me they're ok though because the gauges do work.

Also, on the Flimsy PCB connection. I do have the Oil Temp gauge not the clock. I'll do as you suggested and put a piece of tape on it. It's seems strange to me that it came from the factory that way though?

If you tested between a good ground and the ground at the guages and your light stayed off it means you're ground is fine. Your test lamp needs voltage to operate. Between two good grounds you would have 0V.

The tape on mine is old enough to have come from the factory. Yours may just have come off over the years. They came with that little tab as it meant one part could be used on all cars.
 
The thing that tells me the Sending unit is bad, is because: when I unplug it, the supply part of the plug has a good ground, the ohms tested good and the power has 12 volts (for the pump) All 3 are good.

But the other end of the plug (feeding into the tank) The ground is good (grounds to the tank and chassis) but the power wire shows continuity to the ground? Which doesn't seem correct. So I'm thinking the sending unit ground in the tank is Fudge-up or bad somehow. right?

I'm getting ready to pull it and see if slapping it around a little will help, if not then buy a new one.

Thanks again...
Kiki
 
The thing that tells me the Sending unit is bad, is because: when I unplug it, the supply part of the plug has a good ground, the ohms tested good and the power has 12 volts (for the pump) All 3 are good.

But the other end of the plug (feeding into the tank) The ground is good (grounds to the tank and chassis) but the power wire shows continuity to the ground? Which doesn't seem correct. So I'm thinking the sending unit ground in the tank is Fudge-up or bad somehow. right?

I'm getting ready to pull it and see if slapping it around a little will help, if not then buy a new one.

Thanks again...
Kiki

If the gauge tested fine then it probably is he sending unit.
when you say the power has continuity to ground do you mean the power for the sender or the pump? Also what is the actual ohm reading? Continuity to me can be anything up to a few thousand ohms.
 
Keep in mind that an ohm meter is not a good way to check an electrical circuit as it won't tell you if the circuit can carry any current, a circuit can show a complete circuit on an ohm meter, yet the circuit may not be able to carry any current which is what gets the work done. A voltage drop test is the best test for a circuit check, as it will tell you if the complete circuit is good. :)
 

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