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Discussion Starter · #1 ·
Ok, here's the setup:
66 Mustang, stock fuel tank (16 gal), repro fuel sending unit (new), autometer fuel level gauge p/n #2515 (tried two of them, both with exact same results). This gauge says on the box that it is for most ford vehicles before 1988, 10 ohms full, 73 ohms empty. So everything sounds like it should work to me.

Problem is...gauge works fine when the tank is empty, and when it is full. Rest of the time, it's off very badly. Right now, I have 26.6 ohms at the sending unit (without the gauge connected), and 26.6 ohms at the wires that connect to the gauge under the dash (again, with the gauge disconnected). I pulled the filler hose and looked into the tank, there's at least 3/4's of a tank in there, which would agree with the sending unit reading of 26.6 ohms. However, the gauge reads only 1/4 of a tank!! I tried it hooked into the dash wiring (all new, car has a painless wiring chassis harness on it), and then tried hooking it directly to the sending unit under the car with jumper wires. Both tests were exactly the same, 1/4 of a tank. I bought another gauge today, same part number, and it gave the exact same results as the older gauge I have in all cases.

I'm totally stumped by this thing! It seems like it should work, but the gauge doesn't read correctly. I'm thinking it has something to do with the gauge, because the sending unit is reading 26.6 ohms, which is far closer to 10 ohms (full for the gauge) than it is to 73 ohms (empty for the gauge), so it makes no sense for the gauge to read 1/4 of a tank! This thing is
!

Any help would be greatly appreciated!!
 

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I have an autometer ultra lite fuel gauge and have a similar problem. It doesn't read higher than 3/4 tank even if I slowly fill the tank until I see the filler neck full of fuel. I run out of gas just below the 1/4 tank. I swapped the sending unit out and get the same results.

I think its just the gauge.
 

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I've got an ultra lite electric 1/4 sweep gauge in the Ranger and it works perfectly... do all Ford sending units work on the same 73-10 ohm (or something like that), or are the older ones different?
 

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Probably the angle of the float on the sender arm.

My Autometer does the same thing, but it reads about 1/4 past full as full, and 1/8 past empty as empty. 1/4 is about half a tank.

I think the arm just needs to be bent downward, to read a higher level of fuel at mid tank.
 

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Can you guys test the gauges using a 10 ohm and ~73 ohm resistor to ground to check the full/empty readings?
 

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Discussion Starter · #7 ·
I have tested it using resistors. I tried 10 and 73, which worked ok. The Full and Empty part is fine..it's the area in between that's messed up. Right now for instance, it's reading 1/4 tank. There's 3/4 of a tank of gas in the car. The sending unit is reading 26.6 ohms..which, as I said, is closer to full (10 ohms) than empty (73 ohms) That would mean that the first 3/4's of the gauge work over only 16 ohms, and the remaining 1/4 of the gauge is over 46 ohms, which makes no sense to me at all.
 

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I always thought it was due to the change in position of the sending unit after lowering the rear of the car. Maybe that doesn't make sense but I can see the float reacting to the fuel differently with the rear of the tank lower than the front in relation to the stock position. Maybe, if I shim the rear part of the tank I could see a change in how the sender/gauge react to the fuel level? I do not know how to test the resistance of the sender/gauge. Is it as simple as hooking up an ohm meter?

Bart
 

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Lowering your car's rear wouldn't make THAT big of a difference... I could see maybe 1/8th of a tank tops, not 1/2! if it were that sensitive, going up/down hills would send it flying all over the place.
 

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Discussion Starter · #10 ·
OK, I talked to autometer about the issue today. I also spoke with the service manager and owner of the mustang store I work at, and this is essentially what I've learned about the problem... The autometer fuel gauge is non-linear. The fuel sending unit that I have has a linear response. The guy at autometer said that a stock ford fuel sender will have the matching non-linear response of the gauge, but I tested a Ford sending unit (actual unit made by ford) and it was the same as the repro I have. I'm assuming that the later Ford sending units may have a non-linear response.

The owner/service manager at the mustang place I work at told me that the gauge is the problem. He says to his customers that have autometer gauges that the last 1/4 tank of gas will just last forever. The full and empty still work, so as long as I get gas when the gauge reads empty, I'll be just fine.

Personally, that bothers me, because I want the gauge to work, so I'm going to try to make a circuit to translate the ohm readings from the sending unit to match the resistance the gauge expects. I'll think about it and try to whip up a prototype circuit..then I'll let everyone know how it turns out.
 
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