gas gauge question
1962 with 340hpI have been thru all the archives on this issue and run all the tests.Ground the dash area.Ground the black/white wire from the sending unit.Checked all ground straps.My issue is that when key is off the gauge goes below empty mark.With key on, gauge comes up to empty mark and stays there.I even added more gas to see if the gauge moves, which it did not.Question: Should I have 12volts at the brown wire on the back of the gauge?I don't. What should it be?Should it be 12volts at the sending unit?thanks
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Re: gas gauge question
i'd check the float in the sending unit. mike -
Re: gas gauge question
The sending unit is basically a potentiometer. Input is 12V and as the float moves the wiper across the resistance wire wind output voltage changes, which is what moves the needle on the gage.
As Mike said, it's probaby time to check the sending unit and float. Be sure the float "floats". Carefully bend back the taps that hold the wire wind cover, then with an analog ohmeter attached move the wiper across the wire wind and you should see a smooth transition from zero to 80 ohms and back.
Over time the resistance wind can see deposit buildup, which can cause flaky readings. They can often just be cleaned with acetone, tested, and put back into service. I don't recall offhand whether zero is empty or full, but if you're in the habit of filling the tank at no less than about a quarter remaining, if you let it get lower the gage may not respond due to deposit buildup on the wind because the wiper rarely travels that far.
DukeComment
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Re: gas gauge question
I believe you should say "0-30 ohms"( it's a 62) . Also, he can check the resistance from the sender terminal, rather than removing the sender to test.Comment
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Re: gas gauge question
0 ohms is empty. 30 ohms is full.
You can use an ohmmeter to check the resistance, but do NOT under any circumstances connect a hard 12 volts to the sender. The results would be devastating, as the sender resistance wire would burn up.
You could check the fuse. One of the fuses protects both the gas gauge and the temp gauge. If the temp gauge does not work either, then it is probably the fuse.
Your best bet is to just pull out the sender. It is not hard, and ethanol gas could have destroyed it.
-Dan-Comment
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Re: gas gauge question
OK, all good advice. But for one thing. The sender is brand new and at one time I believe it worked for me.I am reluctant to pull the cover to the tank. It is all painted and has new double sided tape sealing the cover.If the float is stuck is there any way to dislodge it without taking the cover off? I hate to ruin the paint.What about my question of the voltage reading at the gauge? What should that be? All other gauges are working fine.Comment
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Re: gas gauge question
I know this is going to sound wierd, but does the fuel gauge move if you step on the brake. This could be a bad ground problem.Comment
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Re: gas gauge question
Dan, The fuel and temp gauges are not fused. Power comes directly from the Ignition switch when ignition on. There is a Pink wire on the back of the Ign switch which splits to two pinks which go directly to the 2 gauges(right lug looking from rear). The only fuses are for the tail lights, brake lights, dash lights, radio, heater, and Park Brake alarm.
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David, connect a ohmmeter to the Brown wire under the dash at the single male/female plug near the E-Brake shaft. It'll be next to the 4 way connector to the rear harness, and it's part of that harness but splits out to that single connector. Connect the other meter lead to the black ground wire in the 4 way connector. BTW, that little black ground wire goes to the fuel sender, and the rear lights. It has a big job to complete the current path of all of those needs.
Meter should read 0 to 30 ohms or something in between. You'll have about 1 ohm or less in the meter leads. If you're getting erroneous readings, add another ground to the tank from a known good clean chassis source and take your readings again. You can just make a jumper wire to test it. It's a common problem to have a poor ground at the tank. It's connected to one of the sender attaching screws, so tighten those up if needed to get a better connection and retest.
It's possible you have a bad sender, but the ground for the tank sender is just that wire from that 4 way connector, which originates from the Master ground at the Engine Left Inner Rear Valve cover. That's a Black/White Tracer #12 wire. It splits in the harness to provide grounds for everything else, including the cluster gauges and dash lights & lighter, radio, heater panel & heater, clock, etc.
Regardless, it's a good idea to add another Ground wire to the tank.
RichComment
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Re: gas gauge question
looks like we were all typing at the same time....
See my post above to check the sender without removing the cover, but if it's a ground issue..........off it must come.
To answer your voltage question.....No, you won't see 12 volts at the sender, but you will see a potential, based on how much resistance is being seen by the gauge. It'll vary based on the fuel level(resistor value from 0 to 30 ohms). Don't worry about voltage there, measure it statically(power off and sender wire disconnected at the single plug) using the ohmmeter.
Not weird at all, as that's a telltale of a ground problem. The stop lights use the same little #14(or is it #16 I forget) ground wire as the sender, and the sender resistor terminates at the ground point. Lights on will draw current, and flows through ground to complete the circuit. If lots of current is used by the sender and lights, the small wire gets slightly overloaded and will make the gauges flicker. Any bad connection is resistive and then is a load(like a resistor) and draws more current.Comment
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Re: gas gauge question
Not a weird question if you know this issue with Corvettes. No, the gauge is steady when the brakes are applied, signals as used or lights are turned on.Just to clarify, the gauge does move slightly, but not erratically. Just a flicker of movement.I have run a direct ground wire both from the dash frame metal to the negative terminal of the battery, and from the black /white wire coming off the sending unit.I'm not convinced it is a ground issue. If it were I think I would see an inconsistent reading on the gauge and some change when using the brake and signals and lightsetc. I'm thinking more the float sticking and how to unstick it.Comment
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Re: gas gauge question
David, I doubt it if it's a new sender, but.........
If you really think it's a stuck float, take the car for a ride on a bumpy road and do some figure 8's in a parking lot. Assuming you're in a seasonal driving climate like mine here in Florida that is.
Oh, and if the car is in running condition too.
RichComment
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Re: gas gauge question
Thanks Richard, I will use your very clear instructions to test the sender.If I run a second ground wire to the sender, can I cut into the black/white wire where it comes out from under the tank cover behind the drivers seat andtap off it there and run it to a good ground on the frame? Or must I solder a new wire to the top of the sender unit and run it to a good ground?Comment
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Re: gas gauge question
David, Your best ground will be a new one connected directly to the tank sender screw where the present one attaches. This will also help your grounds for the rear lights. Run it down to the frame close to the tank. You can use the existing (5/16 NF) bolt for the fuel line clips, with a external toothed washer to get a good bite on the wire terminal.
I wasn't clear about one thing, but mentioned it somewhat in my follow up........
When you test the sender at the Brown wire underdash single connector, disconnect it from the female terminal first, so you don't get a false reading with the gauge connected to it. The terminal going to the sender through the rear harness is the bare brass male terminal. The female terminal going to the dash harness/gauge has the plastic housing enclosing it.
RichComment
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Re: gas gauge question
Hi Rich, I based my fuse statement from looking at the wiring diagram. The pink wire does indeed go to the ignition switch, but it also goes to the fuse panel, and appears to be attached to a fuse. But the wiring diagrams are not all that clear. So I guess at this point I am not sure just how they are wired, and I know you have a lot of experience with these cars - a lot more than I do. I have been repairing some light switches this winter and I am getting quite familiar with how these cars are wired. I also based this on the fact that if it is mis-wired at the connector back of the kick panel, then it is possible to explode the gas tank, so I thought that GM may have put a fuse in it to prevent this from happening.
But Dave, it is scaring me a little when you are being told to measure several things with gas gauge wiring. So far you have not been told anything wrong, but gas and electricity do not mix.
My guess is your problem is with the sender. When you say it is new, that just reinforces my argument as some of these aftermarket senders are not very good. I posted on this not too long ago. If your gauge reads near empty and does not move, then that is saying the sender is near ground potential. If you pull the brown sender wire out of the kick panel connector, then the gauge should peg at past full. (this will not damage the gauge).
What else you can do is go to your local Radio Shack and buy a 15 ohm resistor and a 30 ohm resistor. Remove the brown sender wire at the kick panel connector and connect one end of the resistor to the wire going to the gauge and the other end of the resistor to ground. With 15 ohms the gauge should read about half full, 30 ohms will be right on full.
Just please be careful with what you do here. I assume you have gas in the tank.
-Dan-Comment
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Re: gas gauge question
Dan, The GM wiring diagram is incomplete, as it doesn't show the correct connections of the source and load wires at the fuse box. Yes that other pink goes to the fuse panel, but to the source side of the fuses for the radio(yellow) and heater(brown) as seen below. IIRC, the other pink for the gauges also goes to the flasher for the directionals, evidenced by requiring the ignition switch to the "On" position for them to work. It can be seen in the lower part of the picture.
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Edit....Added corrected wiring diagram to fuse panel showing Feeds.
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When testing fuel gauge wiring, safety is very important. Testing the sender wiring with a ohmmeter under the dash is safe. Similar to what the fuel gauge is actually doing all the time when powered. Applying high voltage to that sender wiring is not safe and can turn the resistor into a toaster and cause a major problem.
In your description of what happens when you pull the brown wire sender connector apart.....
EDIT.....Dan, I'm wrong, my mistake. You are correct, with the power applied and the sender wire removed from tha gauge, the meter will Peg Full to the right.
RichComment
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