my 70 Cutlass fuel gauge is pegged, it looks like past the normal Full line. Mechanic says sender unit is ok. How do I check the gauge? I've had it out and tapped on it in hopes of it becoming "unstuck" to no avail.

 

There are about 2 million threads on this site dealing with fuel gauge issues.

A gauge pegged past the Full mark indicates an open circuit. The two most common causes of this are a bad ground connection from the sending unit to the chassis of the car, or the sending unit itself. The ground connection is usually up under the car in front of the gas tank. It can often become dirty and corroded over the decades. Did your mechanic check this?

As far as the sending unit, how did your mechanic test it? It's not easy to test while it's in the tank and the tank is on the car because you have to vary the height of the float and measure the resistance. Did he measure the resistance across the two wires coming from the gauge (he would have to have disconnected the ground wire as well as the hot wire coming from the sending unit) while varying the level of fuel in the tank? The resistance should be close to zero when the tank is empty and about 90 ohms when the tank is full.

You can test the dash gauge before taking it out by disconnecting the wire coming from the front of the car from the wire coming from the "hot" side of the sending unit. This connector is usually inside the truck near the latch or up under the rear of the car just behind the rear bumper. Pull this connector apart and ground the side coming from the gauge. Then turn the ignition to ON. The gauge should go to E as a short circuit is seen by the gauge as zero ohms. Then remove the wire from ground and watch the gauge. It should go past F as this causes the gauge to see infinite resistance (which is way more than 90), so that's why the gauge goes past F and doesn't stop on it.

If the gauge passes these two tests, it's fine. and you can turn your attention to the sending unit and its wiring.

In spite of what your mechanic says, the problem in about 90-95% of situations like this is the sending unit. The other 5-10% are a bad ground. Problems with the dash gauge are rare. Remember, the sending unit in your car, if original, is now 50 years old. That's way past its original design life, so it's reasonable that you might need a new one by now.

 

There is also the possibility that the gauge wire connection to the sending unit is loose or corroded. Essentially the same problem as the ground wire side.

 

Mine will do that if the dash sits in the hot sun for hours. If I put a sun shade between the windshield and dash it doesn't happen. I'm guessing my issue is a bad connection/ground somewhere under the dash.

 

Do what?

Is all of this with the ignition on? When the key is off, there is no power to the fuel gauge system, and where the gauge needle comes to rest has no meaning.

 

Most of the time I've seen this, it's NOT the sending unit--it's the wire between the gauge and the sending unit. Generally, cut and then corroded.

When it was my '88 K1500, I had to splice in several feet of wire to get to copper-colored wire on either side of the break. The wire near the break was blackish-green.

As said, this can be a wiring "open" anywhere between the gauge and the ground.

 

Mine past both tests!
I need to buy a sender.
one or two line sender dose it matter?
mine was a two line i thunk it has three at the pump fuel delivery a return and a breather?

 

Your gauge passed both tests?
You need to test the wiring.

Breather??? Pump inlet (suction), pump outlet (pressure) and vapor return.

 

I did test my wiring

Breather return same crap and yes I did test my wiring?

 

I would think it does matter. Why wouldn't you just buy a unit that matches what's on the car now?

 

because the only lines hooked up are the feed to the carb and the line to the tank

 

I don't understand. When you say the "only" lines, you mean there are more lines on your sending unit than are connected to something?

I also don't understand what you mean when you say "line to the tank." There's usually 1, 2, or 3 pipes coming up OUT of the sending unit. One of them reaches to the bottom of the tank, and that's the one that draws fuel from the tank and connects to the line going to the fuel pump and carb. All sending units have at least this one line.

If there's a return line from the fuel pump, which there can be on cars with air-conditioning, that line will be connected to another of the pipes coming out of the sending unit. If there's a third pipe coming from the sending unit, that could be a vent or perhaps a connection to the charcoal canister, but I don't know for sure.

In any event, you shouldn't just guess. You should look at the service manual for your car, get the sending unit that is proper for your car, and connect the lines the way they're supposed to be connected. If there's any reason to doubt that the sending unit currently in the car is correct, you shouldn't just replace it with the same thing without checking first.


I had to replace the sending unit on the '67 Delta 88 I owned until earlier this year. That car had no A/C, and the sending unit had only one pipe (that connected to the fuel pump). The only replacement sending units available had two pipes, so I just capped the second pipe as it was not needed.

 

You'd be nuts to NOT connect the vapor return plumbing.

One of my cars, that didn't have vapor return originally, got vapor return when I updated the fuel system.

 

what?

 

huh care to share why?

 

Reduces chance of vapor lock.

 

i dont mean to hijack the op's thread here but while on the subject why would a fuel gauge only go a little past 1/4 tank when the tank is full of gas ?

 

For practical purposes, the fuel gauge is a specialized ohmmeter. It's not reading fuel level. It's reading the resistance in the fuel gauge circuit. Anything that screws with the circuit resistance, screws with the gauge reading.

Typical is 0 ohms = empty, 90 ohms = full. An open circuit is more than 90 ohms, so the needle indicates over-full.

If the wire is grounded, you've got very low ohms, and the needle indicates empty.

PARTIALLY grounded, the needle indicates towards empty, depending on the resistance of the ground.

Your diagnosis for this is to find out WHY there's low resistance in the fuel gauge circuit--a bare wire? A stuck sending unit? A sending unit float that's gone heavy?

Perhaps a stuck gauge--not an electrical problem at all--a mechanical problem.

 

Thanks for the clarification i believe my issue is a partially grounded sender wire.

 

Found where it goes just need some 5/16 hose

 

Typically, the vapor return is 1/4", but I suppose there could be exceptions.


;

 

mine is 1/4 on the pump only and 5/16 on the line returning to the tank so yes there are exceptions that you supposed incorrectly

 

Thanks for the info. I will proceed with the circuit test. My tank also needs replaced so this helps.