[dgb100]

Started up the old 70 TA yesterday, and the the fuel gauge needed moved from 1/4 tank the whole way past F to being flat down horizontal at the bottom of the gauge window. Any one have any ideas what the problem might be?

Guess I'm going to start getting gas every 50 miles now. 93 octane, 12 gallons, yesterday... anyone, Bueller...?
$35.88! Lovely! This .30 increase for 93 is BS. Can anyone reccomend an octane booster? I figure getting 87 then adding the booster has got to be cheaper than paying an extra $5 a tank for premium.

[giles]

dgb100, My '71 did the same thing several years ago. The fuel gauge gets its input from the sender which is a variable resistor mounted in the tank. The sender has two wires; 1 goes to the instrument cluster and the other goes to a ground strap on the tail panel near the trailing edge of the tank. I bet that the gound wire broke or some corrosion built up on the connector. I cleaned that connector and mating surface real well and it works great.

As far as octane booster is concerned most are not worth a darn IMO, just a little pint of ethanol that might raise 10 gallons of gas by 0.1 point. Bite the bullet and pay the $5. They price the "per gallon" differential at 20 cents a gallon because they know that you can't buy 4 or 5 octane points per gallon any cheaper.

[The Boss]

Common problem - sometimes mine works, most of the time it doesn't. Maybe I'll fix it tomorrow.

[dgb100]

Thanks Giles, I'll check it out. I was hoping the problem wasn't in the tank itself.

[PetesPonies]

true a sending unit is a variable resistor. When the resistance is low the needle moves towards full. It sounds like your wire has shorted to ground,virtually no resistance, thus the full deflection of the needle.

[superdutybob]

I tend to agree with the diagnosis above IF you drive the car all the time.

When I bought my car it had been sitting for a long time with very little fuel in the tank. The entire sending unit was covered in rust including the resistor board. I couldn't buy a new one so the only option was to take it all apart and thoroughly clean it. It now works perfectly.

[Brewster]

Mine works its that dam clock that doesnt !

[superdutybob]

Brew:

That's an easy fix also. Someday I'll share my secrets with you.

[dgb100]

I've been driving it at least once a week for the last month and half, but the underside of the car was just recently cleaned to the point where you could eat off of it. The bottom looks better than the top.

I don't think I ever had a car where a dial clock worked properly.
If the weather is good this weekend I'll see if I can find the problem.

[PetesPonies]

check continuity between your wire, (take it off the sending unit ) and body ground.

[tdavidl]

Originally Posted by PetesPonies

true a sending unit is a variable resistor. When the resistance is low the needle moves towards full. It sounds like your wire has shorted to ground,virtually no resistance, thus the full deflection of the needle.

any idea where the wire runs from the tank to the gauge ? I think my wire may have been damaged when the rear brakes were being worked on and i'm not seeing where the wire goes from the tank to the dash

[77 Canamman]

When there is an open circuit, thus infinite resistance, the needle will go past full. If the circuit was shorted to ground, the needle will read empty. You have an open circuit, meaning a bad connection, tank unit or cut wire. The resistance range is 0-90 ohms

[FrankieT/A]

Originally Posted by 77 Canamman

When there is an open circuit, thus infinite resistance, the needle will go past full. If the circuit was shorted to ground, the needle will read empty. You have an open circuit, meaning a bad connection, tank unit or cut wire. The resistance range is 0-90 ohms

Negatory Big Ben

[77 Canamman]

Originally Posted by FrankieT/A

Negatory Big Ben

How much do you wanna bet???

[77 Canamman]

0 ohms=empty reading on gauge

90 ohms=full reading on gauge

[necdb3]

Agreed, that's how GM and Chrysler worked, Ford was the opposite.

Originally Posted by 77 Canamman

0 ohms=empty reading on gauge

90 ohms=full reading on gauge

[tdavidl]

Originally Posted by 77 Canamman

When there is an open circuit, thus infinite resistance, the needle will go past full. If the circuit was shorted to ground, the needle will read empty. You have an open circuit, meaning a bad connection, tank unit or cut wire. The resistance range is 0-90 ohms

thx, that's what i'm thinking, gauge was working fine, had car in for brake service and just as I got home noticed the needle pegged past full so now i'm wondering where to look for the wire.....they wouldn't have been under the dash, they were working under the car around the tires\brakes and the MC\proportion valve....just not sure where to look for cut wire or lost connection...

[Dens71TA]

Originally Posted by tdavidl

any idea where the wire runs from the tank to the gauge ? I think my wire may have been damaged when the rear brakes were being worked on and i'm not seeing where the wire goes from the tank to the dash

Fuel gauge sending wire is part of the trunk harness, rubber grommet through the trunk floor behind the license plate/fuel filler tube and on top of the gas tank. Nowhere near the brakes.

[tdavidl]

Thx....

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