1966 fuel gauge problem

[66blackbeauty]

replaced the fuel tank and fuel pump in my '66 hardtop, 390 engine, and the fuel gauge drops all the way to the left (empty) when i turn the engine off, filled the tank once and the gauge goes past full, all the way to the right. But i have been driving the car for a while and the fuel gauge still goes all the way to the right, past full, when i start the engine.also replaced the constant voltage regulator with a new one. any thoughts?

[sseebart]

Well, unless you converted the car to electric somehow . . .

Sounds like the new CVR is either defective or not properly grounded. How do the oil and temp gauges read? If they are also off, then it's the CVR. If they read fine, I'd suspect that there's an issue at the sender--a stuck float or bad electronics back there. Last resort would be the gauge. They break but not often.

~Steve

[66blackbeauty]

the new cvr is a new electronic unit. previous owner added a set of aftermarket gauges so none of the other factory gauges work. if i unplug the fuel gauge wiring under the gas tank there is no reading on the gauge, reads as empty. how does the new cvr ground?

[paulr]

If the CVR is mounted properly, it grounds through the body of the speedometer.

[RAVEN]

Think you have a pinched wire from dash through to the sender, giving you a full ground condition and a full deflection of the gage. Trace the gage wire.

[Craneman16]

I have a 1966 Convertible with a 428 engine "Q" code. My gauge goes to full when the tank is full and as I drive, it normally goes down to empty. The only problem is when I fill up the tank, it only takes 14 gallons of gas in a 22 to gallon tank. So I replaced the sending unit with one from "Pat Wilson Thunderbirds" whom I found to sell quality products. However after I installed it, I still have the same problem. It still not showing the 8 gallons left in the tank. Any suggestions?

[sseebart]

I have a 1966 Convertible with a 428 engine "Q" code. My gauge goes to full when the tank is full and as I drive, it normally goes down to empty. The only problem is when I fill up the tank, it only takes 14 gallons of gas in a 22 to gallon tank. So I replaced the sending unit with one from "Pat Wilson Thunderbirds" whom I found to sell quality products. However after I installed it, I still have the same problem. It still not showing the 8 gallons left in the tank. Any suggestions?

I thought for sure you were going to say "it takes 14 gallons of gas and I've only driven about 100 miles!"

I can't explain why you'd have the same problem with two different senders, but I've heard that some have to bend the wire at the sending unit to get accurate readings. It's a trial and error process, so hard to say if the effort is worth it.

~Steve

[Terry64HT]

Steve, over the years I have purchased two senders, one from a Tbird supplier and the other a "no-name". Both read low when first installed and I had to take them out and bend the float higher to get proper readings. I think all these are now universal from a jobber and are not "tuned" to our cars although I'm sure there is a difference in quality.
Terry
64HT

[66blackbeauty]

update: I removed the sending unit, moved float In both directions and the fuel gauge will change from full to empty. if I insert the sender into the half full tank, the fuel gauge reads half full. if I mount the sender to the tank, making contact with the tank, the fuel gauge reads past full again. The sender is new, the tank is new, the electronic constant voltage regulator is new. so- what should I do next?

[paulr]

update: I removed the sending unit, moved float In both directions and the fuel gauge will change from full to empty. if I insert the sender into the half full tank, the fuel gauge reads half full. if I mount the sender to the tank, making contact with the tank, the fuel gauge reads past full again. The sender is new, the tank is new, the electronic constant voltage regulator is new. so- what should I do next?

Just to be clear, you're using the sender gasket, right?
Since we have no way of tracing the manufacturing tracks of these reproduction parts, experience is what we're left with. Terry's experience is my experience, too, and I think this is dead on. Plus, going on anecdote, the quality among different users on this forum swings wildly. You may have a defective piece.

I think all these are now universal from a jobber and are not "tuned" to our cars although I'm sure there is a difference in quality.
Terry
64HT

[66blackbeauty]

tell me about the sender gasket- there is no gasket between the sender flange and the tank when I removed it. just looked in my Larrys Thunderbird catalog and they sell a gasket. Is the gasket to go between the sender flange and the outside of the tank? I did not do the original install (tank and sender) so did not know about the round gasket. So is my sender touching the tank, without a gasket, causing the ground issue??

[jtschug]

There should be a gasket, but the missing gasket should not cause it to ground out. The bayonet ring grounds it to the tank on the other side.

[RAVEN]

If you look at the sending unit, you will find that there is either 2 or 3 prongs. Two prongs is a standard sender and the three prong is for a low fuel level indicator as part of the Convience Package.
On either sender there is a distinct GROUND prong which is riveted to the sender body, the others are one for float level and the other for the Thermactor (low fuel indicator). The plug body from the car has the 2 or 3 receptacle and has 2 or 3 wires in the harness, of which one of the wires is grounded in the trunk area, A) under the spare or B) under the hydraulic pump depending on the car body style, and the others for their respective use of gauge or low fuel sender.
Indirectly the tank is grounded as Jtschug has stated thru the sender captive ring, but has really no effect to the sender ground.
If you bought a new tank, normally there is a gasket enclosed with it. Spectra tanks, unless things have changed sent a gasket in the box. Any I have bought had one.
You stated that the sender would work properly in your hand when plugged into the harness; this indicates that the car wiring is OK and the ground lead is conductive to ground(car body).
BUT you state that when installed in the tank fully the gauge moves to full even though tank is only half full, this leads me to think the float lever is incorrectly bent and is sending a false signal. The ground has been proven ok previously, and regardless of ground continuity has no effect.
Did you meter the sender to see if the ohms resistance was smooth and consistent thru full range of float movement?

Just my thoughts, Good luck

W

[66blackbeauty]

thanks for the continuing replies fellas.

raven- with the sender out of the tank and plugged into the wiring harness, the gas gauge goes from empty to full when I move the sender up and down. When I insert the sender into the tank, just did not fully contact the flange against the gas tank, holding it steady n my hand, the gauge read between 1/4 and 1/2 full, indicating the accurate amount of gas in the tank at that time. But, once I seated the sender into the tank, the flange making contact against the tank body, then the gauge reads past full. So I am confused as to what to look at next. I will attempt to check the ohms of the gauge and make another update.

[cacockrum]

My understanding of the fuel gauge is as follows: Current flows from the ignition switch, to the constant voltage regulator, to the gauge, to the rheostat in the tank sending unit (the position of which is controlled by the float), and finally to ground. The position of the gauge is determined by the current passing through it, which is determined by the resistance in the circuit, and that is determined by the position of the wiper on the rheostat. As Raven points out, one of the two connections to the sending unit is tied directly to the flange while the other goes to the rheostat, one end of which is also tied to the flange. As the float rises in the tank the wiper moves toward the grounded end, reducing the overall resistance in the circuit and moving the gauge towards full. I measured the resistance of an old sending, and it went from 160 ohms in the down position (near empty) to 60 ohms in the up (full) position. I would expect the gauge to read well above full if the resistance in the up position was reduced to zero. Since your gauge is working correctly when the sending unit is not in the tank, but reads above full when it is in, it appears that putting it in the tank somehow connects the gauge to ground. You didn’t say if your car has a low fuel warning light. If you do not, it would be possible to have simply rotated the connector to the sending unit so that the contact that should go the rheostat wiper is actually going to ground. This would not matter if the sending unit is out of the tank, but would connect the gauge directly to ground when installed in the tank. If you do have the low fuel light option, perhaps you could make a set of jumper wires and try reversing their connections to the sending unit.

Let us know what you find out.
Art