Hot Rods I hate wiring ,chapter III. Sending unit.

Setting in 59 ElCamino working on another issue. cut on key, gas gauge pegs to full. I know it only has about 1/4 of tank. HMMMM. crawl under see that ground wire has broken. ok easy fix. It is a new tank and sending unit. The ground wire was broke at the unit so i don't know if the ground wire made contact with"hot " wire to S U. Any way, i replaced the ground wire, gauge still pegged??? I had put a new HEI dist in , had to take out ballast . took wires lose and taped them up. not together.Light brown hot, 2 light green out. I did make mistake on hot wire to supply dist. car won't start until you let off key. I know i need to move HEI supply wire to solenoid. But cant figure out S.U. pegged. I did not wire car, but it is pretty much all new wiring harness.Only time the gauge would move was when you cut switch off.I was under car, would unplug wire to gauge from fuse panel. would not move?Open for suggestions. Did i mention I HATE WIRING??

 

Are the gauge and the sending unit a matched set? Have you checked to see if the wire to the sending unit is grounded somewhere? And the fact that removing the supposed fuse to the gauge has no effect isn't good; you may be in for some serious wire tracing.

 

I don’t believe there is a hot wire going to the sending unit, usually it is the ground side of the circuit. Sounds like something is “shorted” to ground. As in it is finding a ground somewhere other then through the sending unit. Find a wiring diagram.

My disclaimer is I have never worked on one of those cars but they all basically work the same.


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The sending unit to gauge wire is not hot. Supply’s a ground signal through the pot on the s.u.

Is the gauge matched to the s.u? Some s.u’s go High resistance fill to low resistance empty and some opposite.
Fuel gauge needs to match the s.u.
Or possibly your sending unit wire is grounded out between the gauge and sender.

Strange one. Is the s.u an aftermarket replacement or something different and also with the gauge?
Did the Po wire it himself or was it s witing kit?

 

Gauge and S.U., stock gauge, el camino have E.C. specific sending unit.So it is supposed to be matched to gauge. I called the wire to sending unit (hot) because ....... I really didn't know what to call it.

 

Sorry guys, the wire to the sending unit is hot; if it wasn't, you wouldn't have a circuit. You will read voltage to ground on that wire ahead of the sending unit if everything is good. The voltage comes into the gauge, goes out, and the sending unit supplies the variable ground so the gauge reads right.

 

Has it ever worked the way you have it wired up?

 

If the gauge itself is working properly, then it will read past full when the sending unit wire is broken, the sender resistor inside the sending unit is broken, or the ground is not connected properly.

An ohm meter could help you find the problem...or you can short the sending wire to ground and see if the gauge shows empty like it should.

The problem could be anywhere, almost...at least we know the sender wire probably is not shorted to ground.

btw if someone accidentally wired "hot" power to the sending unit it, it will burn out the resistor, and cause it to behave how it is behaving now, after wiring it properly.

 

Question then Squirrel, on my Dodges if you ground the wire to the sending unit the gauge goes over full. I’m not overly familiar with gm stuff though. How are they different?


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It depends on the 'range' the gauge is designed for. If you have one that reads empty at 90 ohms and full at 30 ohms, it will read over-full at zero ohms (direct ground). There's a half-dozen 'common' gauge ranges depending on make/year.

 

Yup. I just remembered, I do have an after market Gauge rated to 33 ohms,(not in car) i bought it for another 59.EC i have .I could give it a try .

 

On most Ford and Chrysler products low resistance makes the gauge read full and higher resistance makes the gauge read empty. A 59 Chevy works the opposite way with stock parts.

 

Awesome thanks guys, that makes sense.


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GM senders are zero ohms (shorted to ground) at empty, and 30 ohms or 90 ohms when full (they changed in the mid 1960s).