Gas Gauge

[1987cp]

All my cars have carbs, actually. Hm ... kid's taking a nap break right now, this would be a good time to test 87cv's claim.

[1987cp]

Back from the garage. Big surprise, the potentiometer behaves like a potentiometer. Fuel gauge reads the same given the same armature position when wires are reversed, provided that the body of the sender is not grounded. When the body of the sender is grounded, the gauge reads correctly when connected correctly, and reads Full regardless of armature position if connected backwards.

Fact: Sender plug consists of two wires, one signal wire and one ground.

Fact: Fuel gauge reads over Full if signal wire is connected directly to ground.

Observation: on at least some older Ford fuel level senders, only the signal wire was used. This is because that in most installations, the steel tank is effectively grounded to the steel chassis (such as in first-gen Mustangs where the tank bolts directly to the trunk floor), and the steel sender body is in turn grounded to the tank. Hence, no theoretical need for a redundant ground wire going directly to the sender.

Postulate: The ground wire was added to the normal sender circuitry to make the fuel gauge more reliable, in the event that a tank attached by straps (such as ours) happened to be inadequately grounded or there happened to be an inadequate electrical contact between the sender body and the tank. It's simpler to connect a ground wire directly to what needs to be grounded, rather than worry about whether the sender is adequately grounded via its retaining ring.

Prediction: If your sending unit happens to be adequately grounded to the vehicle chassis, reversing the wires to your fuel level sender will cause your fuel gauge to read Full regardless of how much fuel is in the tank. If your sending unit happens to be not grounded at all, your fuel gauge will read Empty. In some cases, the resistance between the sender body and chassis ground could conceivably be between 73 and 10 Ohms, and in such cases the gauge will read somewhere between Full and Empty. There is NO correlation between gauge reading with the plug reversed and the position of the armature, and this procedure does not test the function of the sending unit.

[Brown_Muscle]

nice write up! Allright well thats what i needed to know, at the very least i can test to make sure my fuel gauge will go all the way to full. and from there I can begin taking the sending unit out. Thanks to both of you!

[87gtVIC]

very nice 1987cp

[1987cp]

lol - thanks, but it was really more of a rant than a writeup. I get bugged when someone has misinformation about something basic.

[86VickyLX]

Well you saved him the trouble of reversing the wires to see if the float was bad, since it doesn't work that way. Phil, when you're ready to do the job, and you need any help, give me a call mang.

[87_crown_vic]

if anything, it would let him know if there was a bad wire or gauge was bad... when reversing it it will at least show that things still work and narrows it to the sending unit

[gadget73]

Back from the garage. ....

Bingo. Reversing the plug just sends the guage straight to ground and will peg it provided that the guage and it's wiring is good. Its a useful check of the guage and the wiring circuit though. And yes, the ground is there to ensure that the tank is truly at ground potential. Steel isn't the best conductor, add to that years of rust, dirt, etc between the straps and the tank, and between the straps and the body and it creates the potential for really messed up readings.

[cld783]

if anything, it would let him know if there was a bad wire or gauge was bad... when reversing it it will at least show that things still work and narrows it to the sending unit

thats exactly what it does and is hardly misinformation.. I had this problem before and used this procedure to find out i just had a bad fuse.

[Brown_Muscle]

Well I was driving around yesterday and I looked down and my fuel gauge is working all of a sudden....so im now guessing its wiring, I allready bought the float and stuff, i wonder if i can return it. I'll have to check it out, haha. Opps

[87gtVIC]

keep it..you may never know when it will actually shit the bed....but it is sure nice to have money in the pocket.

[86VickyLX]

Like I mentioned there should be some orange box under the glove box that should work with the gauge, or maybe it's just the light. If it's only for the light, which you don't have a fuel light, that box won't be there. Could someone verify this for me??

[gadget73]

The box is for the light. It should actually say Low Fuel Module or something to that effect. Mine is actually black, and lives next to the gas pedal though.

[86VickyLX]

Maybe it's different placement for a towny?

[88MercWagon]

Must be different on a Grand Marquis too because I looked behind the dash and couldn't find it. Does anyone know where it is on an '88 GM?