Brcmpbl

Few projects on the go at once with the 70 while I have a break in the schedule.

I've been trying to diagnose my inaccurate fuel gauge using the paper written up by Willcox and I'm making some headway.

Here's the deal: the gauge will not read above 1/2 when full, and sometimes flutters around a little. I have verified that the sending unit is not the culprit. Before I had a clue I replaced the unit without really testing it, so it is new. The gauge's temperament is the same exactly with the new unit as with the old. So that's out as far as I can see.

My latest discovery is that if I pull the ground wire off the sending unit, nothing changes. As in, if I was reading 1/4 tank with the ground connected, I read 1/4 tank without it connected. Ring any bells for anyone? New to electrical stuff entirely, so looking for some help.

Brcmpbl

Bump.

Alan 71

Hi b,
Here's a suggestion...
There's a link to the article John Hinckley wrote about the fuel gauge and sender.
If you google....fuel gauge diagnostics john hinckley.... it's the second item that's listed , the pdf. Perhaps there'll be some info in it to help you. It's for 63-67 systems but is useful for ours too.
It's what I used on my 71.
Good Luck!
Regards,
Alan

0Willcox Corvette

When you pull the ohms wire off the sender what does the gauge do with the key on? If it goes to just above the top of the rivet, the ground that wire out and see if it goes to empty.

Its not the resistor, if this were bad the gauge would only read to 1/4 tank max.

It's possible the gauge is no good... but doing the ohm's ground and open test above should confirm that.



Willcox

lionelhutz

Have you tried connecting the sender out of the tank and placing it at different positions in the movement?

Taking the sensing wire and trying the disconnected and short circuit tests is also helpful to narrow down the gauge or sender.

You could also buy a 100 ohm 5W POT and connect it, use the middle terminal and one of the end terminals. It should be able to move the gauge up and down and the pot position should match fairly closely with the gauge position.

Similarily, you could get a decade box like this

http://www.allspectrum.com/store/res...-1w-p-390.html

and connect it and then set different resistances. 0 = empty, 22 = 1/4, 45 = 1/2, 67 = 3/4, 90 = full tank.

Brcmpbl

Willcox, thanks for the video. I'm away from the car until Friday night but I will try the first test then and report back.

If I get into swapping the gauge for a new one, what's that job like? No sweat?

0Willcox Corvette

Its not too bad... The instruction sheet for installing dash pads covers the job for the most part so there is online help.

I probably have a good condition used one cheaper too... Just let me know.

If you do go with new, just remember... the resistor is made on the inside of the gauge so you don't use your old one or the thing will go bonkers.

Lorin D

I have a '71 and I always had a 3/4 tank . My son and I got the babe
out on an very beautiful spring day . On the way home the car spit and
sputter . I called and had my car tolled to a near by garage , there
they began to operate on it . First he replaced fuel gauge , fuel filter . Then he suggested I had to replace the sender . I put 5 gal of gas in and brought her home , dropping it off telling Greg what was needed , Greg check it called me , it wasn't the sender but gauge itself was bad . It has been replaced and no problems . What a relief knowing how much gas is in
tank .
I was giving the same info from Willcox . Check the fuel gauge first
before tearing everything apart .

Brcmpbl

I pulled the ohms wire off the sender and the needle went to the top of the rivet past full like it should have. I then tried to ground out the wire with my ohmmeter and the needle fell about a half inch but didn't go to empty. It is possible I didn't ground it entirely...this is all very new to me, the electrical type stuff. Any thoughts? Thanks again.

lionelhutz

OK, that is a good start. Test some more and make sure the ground is solid. Next step is to get into the dash and ground the sensor wire under the dash.

Brcmpbl

How do I make sure the ground is solid?

Brcmpbl

I finally got back around to testing this a little. When I pull the ohms wire, the fuel gauge needle pegs like it is supposed to. If I then ground out the ohms wire to the frame, the needle drops to empty like it should. I popped the plastic cap off the ground connector and it looks good, no corrosion. I assume the rear harness grounds to the frame at the same point as the antenna, and that connection looks good. The wire at that location has a couple of imperfections, minuscule amount of coating off it in one spot -- like a pin ***** maybe.

The gauge itself will show a hair over half full when the tank is pretty full (given that it's the old style fuel inlet, the pump will stop prematurely so I always lift the nozzle out a little and put a couple more litres in there).

If I make sharp turns, the gauge needle will move with the inertia. I'm thinking this suggests the resistance oil in the gauge has dissipated somewhat. I have a new gauge on the shelf I can install.

Next steps? I guess going into the dash as discussed above, but I wanted to check in again first because I dislike going in there. Thanks.

Brian

0Willcox Corvette

This is every possible scenario for a fuel gauge, just like in the video... So if you don't have one of these issues then I'd take a hard look at the gauge.

If you pull the dash cluster from the car you can verify power, ground, (the ohms of the resistor) and the input ohms from the sender (see if they match at the gauge vs. the reading you get at the tank when you pull the wire). You can verify the ground at the sender too... so everything to diagnosis this issue is there for you.

If you pass all of the above then by all means I'd take a hard look at the dash unit.



And review this article, it covers every possible scenario possible for a fuel gauge.
http://repairs.willcoxcorvette.com/1...-unit-testing/

Brcmpbl

Thanks Willcox. I've been using the article as I've gone along.

Question: when testing sender resistance I don't turn the ignition key on (to accessory power) like I do when checking the ohms wire right?

0Willcox Corvette

Key should be in the "on" position.

7T1vette

You do not need the key to be ON to check sender resistance. You are merely checking the resistance of the slider rheostat on the sending unit. Ohmmeter leads to ground and to sender wire...take meter reading.

0Willcox Corvette

I'm sorry, I'm on vacation and I didn't read the question correctly, Charlie is right, the sender resistance will read with or without the key on.

Ernie...

Brcmpbl

Here's my update, and I find it a strange one (I think I found this last year too and then shelved the project for a bit): My tank is currently nearly full, and the gauge reads a bit over half. That reading stays the same if I disconnect the ground wire at the sender. Whether or not the sender ground is connected or not makes no difference at all to the reading at the gauge.

This is not something that is covered in Willcox's article - in the article, a disconnected ground is supposed to cause the gauge to read empty (as in the pic in post #13 above).

Anyone know what's up?

Brcmpbl

Follow-up question: What's a good way for me to temporarily ground the sender using a another piece of wire, just to check to see if the gauge reading changes?

0Willcox Corvette

Run a temporary ground jumper from the sender to the frame.