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Fuel gauge inaccuracy

2K views 6 replies 5 participants last post by  Ivy66GT 
#1 ·
Gas gauge inaccuracy

Hi there, I have a 1965 mustang coupe and i am having trouble with the gas gauge. I just recently replaced the sending unit on the car so I know that that isnt the issue but when I start the car i look at my gas gauge and it goes from empty to way beyond the full line. I have no clue how to fix it and I would appreciate your input and direction if anyone has any ideas. Thanks!!
 
#2 ·
My first comment would be that you are much too trusting. You seem to think that a 'new' sender will be usable as it comes out of the box. In fact, with the repro gauge senders sold today I doubt that will EVER be true. Even the ones that can be made to sort of work right will often (always?) come out of the box with the adjustment so screwed up that they are nearly worthless when installed in a tank. My rule is that if its an original sender and its still working; keep using it since any replacement will not work as well.

To check out any of these gauges and senders you need a meter. If you don't have one Harbor Freight gives them away with a newspaper coupon. However you do it, you need a meter and you need to know how to use it mostly to measure Ohms resistance.

The CVR puts (pulsating) voltage on one side of the gauge head while the sending unit is connected between ground and the other connection on the gauge head. Surprsingly, for a Ford gauge it doesn't really make any difference what polarity you use to hook the wires up to the gauge. From an earlier thread of mine the resistance of the gauge sender vs. where the needle points is something close to:

78 lower side of E
65 E
39 1/4
24 1/2
15 3/4
10 F

A quick way to check your gauge itself is to connect the sender wire to ground bypassing the sender. That is less than 10 Ohms so the needle should peg to the right well beyond the letter 'F'. In order for your sender to give somewhat accurate readings the float arm has to be adjusted so that you get resistances close to those given above. You can check out the gauge itself by using resistors to ground to see where the needle points. Although you can use anything in the range of 10 to 80 Ohms, the two most useful resistors would be: 10 ~ Full, 24 ~ the center gauge mark (1/2 tank).

What kind of a sender did you buy? What was the max and min resistances possible from your new sender? What resistance does it give you when the tank is full, half full, empty, etc? Only by measuring those numbers can anyone tell you what your problem might be or how to fix it.
 
#3 ·
suggestion

Try bending the arm for these values

Resistance When Empty (Ohm): 75 Ohm
Resistance When Full (Ohm): 10 Ohm

These are approximate but should work fine. Do this with the wires disconnected from sending unit. these are the resistance in ohms for the sending unit with out any voltage.

Disconnect battery before testing to be on safe side.

Joe
 
#4 ·
OK sounds good. Thanks for your input I will definitely look into replacing the sending unit. I had just recently bough the newer sending unit about 6 months ago from CJ Pony Parts and everything was working great till about 2 days ago when I didnt know how much was in my gas tank at all. Thanks again I will look into those suggestions you gave. Thanks!!
 
#5 ·
Well, first you should have told us up front the info you put in the last post. You made it sound like the new sender never worked. The other posts took that position.

Since it did work for awhile lets say it is good until proven bad. With the gauge going to max full, that indicates a short to ground in the sending wire. Disconnect it from the sender at the tank and then turn the key to 'On'. No need to start the engine. Does it still go to max full? It should do nothing.
 
#7 ·
The Scott Drake repo senders are NOT a linear device as that link suggests. They aren't the same non-linear as the originals but they are definitely non-linear. At the half-tank point the SD sender is really pretty close to correct provided the arm is bent correctly.

They do require the float arm to be bent correctly which is rarely how they come from the box.
 
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