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1967 Mustang Fuel Gauge Question

3K views 6 replies 3 participants last post by  Ismellelephant 
#1 ·
I am in the process of getting a 67 that has been garaged for 16 years running again, this site has been invaluable to me for this task. So far all has been going well but I am confused about troubleshooting the fuel gauge, it stays on empty and doesn't move. With the key on I removed the plug at the tank and using a test light I saw no pulse of light, yet when I ground the plug the gauge moves to full. I am not sure if this means the wire to the tank is good or bad.
I removed the instrument cluster and the test light lighted on one terminal of the IVR and the light pulsed on the other terminal of the IVR. On the fuel gauge terminals the test light pulsed on one terminal but the second terminal there was no light and I am not sure what I should see on the second terminal.
I would think the fuel send unit is probably bad having been sitting in gas for 16 years but I am not confident the wire going to the tank isn't a problem. Thanks for any input.
 
#2 ·
If you are a regular reader of these pages you should have seen many posts on this subject. You need to read Veronica's blog for the gauges. Look under March 2008.

The water, fuel, and oil gauges all work the same and get power from the small voltage regulator on the back of the cluster. It is the thing that rapidly opens and closes contacts to effectively lower the usable voltage to 6 volts. Hence the pulsing. The power goes thru the gauge along the wire to the sending unit. The sending unit changes the resistance, in the fuel system, by way of a float on an arm. With max resistance the gauge says empty, with min it says full.

The common tests are: a) remove the wire from the sending unit and ground it. Go inside and turn the key to on and the gauge should swing to full. Turn key off, do not let it stay on more than a few seconds. b) with the cluster pulled back, test for any voltage coming from that small VR.

If the car has been sitting for that many years you should replace the entire fuel system anyway.
 
#3 ·
Thanks for the reply and I have read much on here and elsewhere about this subject and I did everything you suggested already. Did you read my post? LOL
My question shortened is, when grounding this wire the gauge goes to full( good) but using a test light I see no pulse or light at all on this same wire. This doesn't make sense to me.
I am replacing many parts, I am also trying to learn from something that makes no sense from what I have read on here and elsewhere.

This video may explain the reason for my question.
https://youtu.be/EnB3tlMMjKs?t=162
 
#4 ·
Hello Ismellelephant,
Great video and thanks for that

I stink at electrical but am learning LOL
I am almost certain that there is a little issue with the video
and your test.
If you ground the sending unit wire and it pegs the gauge yes that
is a great thing = it tells you the gauge works and the circuit is
good - -thus the issue is the sending unit or perhaps a bad ground.

Before I finish my thought on that however - - I think the reason you get
no pulse of light is that you test light is not grounded. The screwdriver
type of tester used in the video has a wire that must be founded - then
you touch the sending unit wire with the tip of the screwdriver light.

Now - -there is a slight chance your gas tank is not making a good ground.
It is possible that the tank edges need to be clean to make a good ground to
the body of the car. I THINK - -you could run a small ground wire on the
face of the sending unit and a clean ground and the gauge may move.
It is far more likely that the sending unit is bad

Keep in mind - -I and really not that good at electrical but I honestly think
I am pretty certain that you are not grounding your little test light so there is
not a complete circuit

Print Dad
 
#5 ·
I just watched the video and you have to look closely to see the ground wire on his tester. He does not mention that ground. The rest of the video is pretty good, except I would either have a helper watch the gauge move or leave the key off while hooking up the ground jumper and then turn the key on to check for needle movement.
 
#6 ·
Hello All,
I would like to be clear on my thoughts on the video
1. - I LOVE the video and spend many hours watching and learning from Doc
Great videos and very solid advice

2. - To Driveway and others - -yes I see the ground wire at the end of
the test light - -but - -I felt as though elephant and others may not notice it

Again I only pointed this out because I think for some they may not know
that the light needs to be grounded

Ironically I watched 3 more videos last night and they are GREAT.
This guy really knows Mustangs and how to deal with issues.
I advise everyone to check them out

Print Dad
 
#7 ·
Thanks for all of the responses. I did have the test light grounded and it was to the same ground used when I tested to make the gauge go to full. I will recheck this just to make sure. I am sure after 16 years the fuel send is bad, I'd just like to make sense of my tests so far.
This is the car I am working on.


 
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