Gas Guage not working
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Senior User
Joined: Jan 2006
Posts: 566
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From: Mobile, Alabama
Gas Guage not working
OK Guys, did a search on inop Gas Guage and tried all the fixes. I ohm the sending unit at full tank and get 74-75 ohms. I've used about 1/8 tank of gas and it ohms at about 62 ohms so I think the sending unit is good. I short the gauge wire to ground at the gas tank and get a full needle so that tells me the guage is working. I've checked for ground continuity at the tank and that checks ok. I checked continunity on the wires coming out of the sending unit through the conector going forward to the guage and still ok. I must be missing something since the sending unit ohms out and the guage pegs at full short to ground and wiring seems to be continuous. The needle goes up to the E line and thats where it stays. Any sugestions? Thanks
Posting Legend
Joined: Sep 2001
Posts: 30,255
Likes: 37
Did someone put in a chevy sender?
You don't give the year but these are the values you should see on a 78:
For calibration on a 78 fuel gage use:
----------------------------
33-ohm 5W resistor for FULL
100-ohm 2W resistor for 1/2 tank
270-ohm 1/2W resistor for Empty
You don't give the year but these are the values you should see on a 78:
For calibration on a 78 fuel gage use:
----------------------------
33-ohm 5W resistor for FULL
100-ohm 2W resistor for 1/2 tank
270-ohm 1/2W resistor for Empty
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Senior User
Joined: Jan 2006
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From: Mobile, Alabama
Thanks Torque1st for the reply. I should have metioned the year is a 1978 F100 and I would be surprised, knowing the originality of this truck, if the PO had installed a Chevy sending unit. Everything worked great until a week ago when the Guage suddenly stopped working. I appreciate the info on the Calibration but I think the problem exists elsewhere and I can't quite put my finger on it.
Posting Legend
Joined: Sep 2001
Posts: 30,255
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Try those resistors to see if the gage is right. You can get them at any Radio Shack store. You should also measure those resistances at those tank levels. Check your ground at the tank. Since it was operating correctly you may just have a bad sender. Check the Read First:FAQ thread at the top of the 73-9 forum for instrument panel repair info.
Ford Parts Specialist
Joined: Nov 2006
Posts: 88,826
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From: Simi Valley, CA
Originally Posted by redlightning93
OK Guys, did a search on inop Gas Guage and tried all the fixes. I ohm the sending unit at full tank and get 74-75 ohms. I've used about 1/8 tank of gas and it ohms at about 62 ohms so I think the sending unit is good. I short the gauge wire to ground at the gas tank and get a full needle so that tells me the guage is working. I've checked for ground continuity at the tank and that checks ok. I checked continunity on the wires coming out of the sending unit through the conector going forward to the guage and still ok. I must be missing something since the sending unit ohms out and the guage pegs at full short to ground and wiring seems to be continuous. The needle goes up to the E line and thats where it stays. Any sugestions? Thanks
C0AZ-9202-A .. Fuel Sender Unit Float
C0AF-9276-A .. Fuel Sender Gasket
D1AZ-9A011-A .. Filter Screen
Posting Legend
Joined: Sep 2001
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If the float was compromised enuf to read empty his resistance would read 250+ ohms , and it only reads around 70 ohms. He has an electrical fault, bad ground, bad, sender, bad connection.
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Senior User
Joined: Jan 2006
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From: Mobile, Alabama
Thanks Guys for all the input. This is why I read this forum daily to get all the helpful info that keeps my old truck going. After reading the posts and rethinking, I am leaning more toward a bad sender or float or a combination of these items. Torque1st, I am going to do the resistor checks and see how that looks and go from there. NumberDummy, thanks for your info on the parts as well. I will report back after I do these things. Thanks again!
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There I go...... neglected again 
The bayonet SU ring if rusted can prevent the grounding of it to be at a minimum causing a mis-reading.
Make sure you use a BRASS punch for any R & R of that ring.. Spark + Fuel vapour = Kaboom
The bayonet SU ring if rusted can prevent the grounding of it to be at a minimum causing a mis-reading.
Make sure you use a BRASS punch for any R & R of that ring.. Spark + Fuel vapour = Kaboom
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Senior User
Joined: Jan 2006
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From: Mobile, Alabama
LOL, good advise Million and sure didn't mean to leave you out. Was hoping I would not have to drop the tank but looks like I probably will need to. So, I'm off to find that brass punch!
Senior User
Joined: Apr 2007
Posts: 167
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From: Topeka, Kansas
You did get 2 different ohm readings though, maybe you do have a drowning float but different ohm readings would change the gauge reading a little. Check and see if the ohm reading at the gauge corresponds with the tank reading.
