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Discussion Starter · #1 ·
Hey guys,

What's a good way I can test my gas guage on my 70 Chevelle? The entire dash is out of the car now so I have clear access to it. It was acting a little funny last month before I pulled it out. The guage would only go up to about a 1/4 tank when the tank was full. Once I put the marker lights on, it went to empty.

I want to make sure the guage is working correctly as I'm having a rollcage installed through the dash an will not be able to get to it any more in a few weeks. Any help is greatly appreciated!!
 

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it could be 3 things that I know of or can think of,1) bad sending unit,2) bad guage, or 3)short in wire going to sending unit, id check the fire going to sending unit with lights on, with a test light, check the fuse block wire and back at sending unit on my chevelle its a light brown wire. check at sending unit to find color and trace through the harness to fuse block,if u have fire then id pull sending unit and clean good or replace. if thats good then the guage has to be bad or sticking. start with small stuff first and work to main source.Might also wanna check the ground at sending unit. hope my 2 cents helps ya
 

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Discussion Starter · #4 ·
Thanks guys, I appreciate the help but the entire assembly is out of the car..........I really need to test the gauge because all the wiring is out of the car too and the tank will be replaced with a cell........anyone got any ideas??
 

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The back of the fuel gauge has three connections. I don't remember which is power and which goes to the tank resistor. You can follow the trace on the circuit card to determine that. One post gets power, its the pink wire in the cluster connector and is shared with the temp gauge and the oil press light. The bottom post is grounded to the metal back of the cluster. The third post is connected to the tank resistor, tan wire in the cluster connector. Sensing lead.

From a 12 volt battery connect power, battery +, to the gauge, post one, and ground, battery -, to the cluster back. Connect a 90 ohm resistor from the sensing post, third post, to ground. The gauge should read F. Replace the 90 ohm resistor with a 45 ohm resistor, the gauge should read 1/2 scale. Ground the third post, wire from the post to the cluster back, the gauge should read E.
 

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Discussion Starter · #6 ·
Thanks guys, that's exactly what I was looking for!!
 

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Discussion Starter · #7 ·
Elree,

I pulled my gauge cluster and checked the fuel guage with some interesting results. Even though it doesn't make any difference, I'm working with the large round gauge which is easier to see.

I connected it to a battery I had on hand that read 12 volts. When I first connected it, the gauge went two lines past full with nothing on the sensing post, as I figured it would. When I grounded it, it went to empty as you had said in your reply.

I bought some resistors at radio shack which were rated at 47 and 100 ohms respectively. They are rated at a 1/2 watt with 5% tolerance if that makes a difference. Unfortunately I am not real familiar with electrical connections and such.

When I connected the 47 ohm resistor which should have given me 1/2 tank, I got a little below a 1/4. When I used the 100 ohm piece, I got a little above a 1/4. Just trying them together I had a 1/2 tank.

My question is, the gauge seems to work properly even though it's off. Could this be because there is some resistance in the wire since the sending unit streches all the way to the back of the car? This is all I can figure or is it possible I am using the wrong resistors since they are only rated at a 1/2 watt? I would think that if the gauge is bad I would get something different in the form of readings.

Any help is greatly appreciated since I do not want to install a non working gauge in my car. Thank you in advance!!
 

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The wattage isn't hurting you. Those ohm values are just fine for the test. Does the gage have a white ceramic piece attached between 2 of the terminals on the rear? Maybe about 1 inch long with 2 holes in it and a green or red stripe area on it. That's a resistor and almost sounds like that is missing or bad.
 

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Discussion Starter · #9 ·
John,

I remember the back of the gauge was white, but I don't remember seeing the resistor. I'm at work and will check when I get home.

If that is the problem, where can I get this resistor?

Would anyone have a picture of it?

Thanks in advance!!
 

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Delaware..cold..snow. Was going to suggest getting one off the back of a 70s/80s vehicle at the junk yard. They are all the same and I think they used the same ones on the gas gage until the mid 80s.
Had a site that sold them in my favorites but someone said they don't respond. Maybe someone has an old one on the back of a fuel gage and can send it off. Gave away the last one I had off an 80s Olds gage.
It looks like a rectangular ceramic piece about 1 inch by 1/2 inch with 2 holes in it.
 

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Discussion Starter · #11 ·
Yes it's cold here but better than it has been...............normally it isn't too bad but they say we had the coldest January in 110 years!! :eek:

I'll bet it's great to be able to drive your car all year round!!

If that's all I'm missing I'll be a happy man!! Would make my life alot easier!! I'll let you know my findings tonight when I get home from work!!
 

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Discussion Starter · #12 ·
John,

Ok, I'm home and found the ceramic piece is still on the back. About 10 years ago I redid the guage cluster and I didn't remember this piece until you had mentioned it. One side is plain ceramic and the other side has two metal half circles with a square blue resistor in it. Do you knwo how many ohms this is supposed to be so I can test and more importantly, what direction should it face?

I think we are nearing the end of the problem!!
 

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Discussion Starter · #13 ·
John,

I figured out what the issue was........I had the little ceramic resistor on backwards.


The gauge is working perfectly now.......thank you once again for your help!!

 

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You have fun Elree. The 1/2 watts are small but do work for a quick test. Not my fingers on them anyway.
Beer break I guess is in order for this thread.
 

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Discussion Starter · #16 ·
Elree and John,

Thanks again for all your help!! I do appreciate it.........I would have most likely bought a new guage if I didn't have your help which would have cost me at least a $100.00!!

Thanks again and yes, my fingers are burnt......I'll know better next time!!
 
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