P15 fuel gauge conversion idea

[47heaven]

After the temp gauge conversion that was just done, I was wondering if I could do that same thing with the fuel gauge. When I was at Norm's, he showed me a '53 Plymouth fuel gauge and I think with a little bit of switching the mounting plates and swapping the face plates, I can make a '47 fuel gauge that will run on one wire. There is a '51 and '53-'54 gauge on Ebay. I have a a J.C Whitney fuel sender unit that is sitting in the box, still, which I think will hook up to it. I really would ike to get away from the two wire system because it's been nothing but a total dissapointment. I just want to wash my hands of it and go to a one wire system because I'm tired of having to keep topping off the tank and guesstamating how much gas is in it. I think I read on here that Plymouth fuel guages went to a one wire system in 1949. Any feedback or suggestions?

[Norm's Coupe]

Darin,

I thought about doing a similar thing using a 49 gage before I finally got my gage working. To change over to a single wire unit you also have to change the wiring under the dash to go along with it. There are only a couple of reasons your original 2 wire unit doesn't work. It's either a bad sending unit, bad dash gage, or you have a poor ground or hookup somewhere in the wiring. I spent many hours trying to trace my problem down and many of those were wasted. However, as soon as I replaced the wires from the trunk to dash the gage started working again.

In all that fiddling around I collected two extra gages, plus a new sending unit. I still have the new sending unit in the box in a cabinet. The only thing I used the new sending unit for was to test my three gages. All the gages worked on the bench but not in the car, until I changed the wiring. The old sending unit that was in the car was working all the time, it was the wires that were bad.

Your problem may not be the wires. However, you really can't do a good test until you have a known good sending unit to test the gage on a bench. So, I would pick up a new sending unit and run the test. If it turns out that you have a good sending unit in the car, the worse that happens is that you now have a spare sending unit, and you've found your problem.

[47heaven]

Darin' date='

I thought about doing a similar thing using a 49 gage before I finally got my gage working. To change over to a single wire unit you also have to change the wiring under the dash to go along with it. There are only a couple of reasons your original 2 wire unit doesn't work. It's either a bad sending unit, bad dash gage, or you have a poor ground or hookup somewhere in the wiring. I spent many hours trying to trace my problem down and many of those were wasted. However, as soon as I replaced the wires from the trunk to dash the gage started working again.

In all that fiddling around I collected two extra gages, plus a new sending unit. I still have the new sending unit in the box in a cabinet. The only thing I used the new sending unit for was to test my three gages. All the gages worked on the bench but not in the car, until I changed the wiring. The old sending unit that was in the car was working all the time, it was the wires that were bad.

Your problem may not be the wires. However, you really can't do a good test until you have a known good sending unit to test the gage on a bench. So, I would pick up a new sending unit and run the test. If it turns out that you have a good sending unit in the car, the worse that happens is that you now have a spare sending unit, and you've found your problem.[/quote']

Norm...I assume that the sending unit works because I had Tim rejuvinate it right after I bought my car. The fuel gauge is NOS out of the box and the wires are new. So I can't see those things being the problem. It has to be a grounding issue because when I turn the ignition on, it shoots past F and stays there until the ignition is turned off.

[TodFitch]

Norm...I assume that the sending unit works because I had Tim rejuvinate it right after I bought my car. The fuel gauge is NOS out of the box and the wires are new. So I can't see those things being the problem. It has to be a grounding issue because when I turn the ignition on' date=' it shoots past F and stays there until the ignition is turned off.[/quote']

If it shoots past full, look for an open on wire 1 and a short to ground on wire 2.

The wires go to either side of a resistance coil that is grounded by a wiper run by the float arm. When full, wire 2 has low resistance to ground while wire 1 has high resistance. When empty the reverse is true.

To get a full (or past full reading) wire 2 is basically grounded and wire 1 has high resistance to ground.

I have to scan my old Dyke's Automobile Encyclopedia page that describes the actual operation of this type of unit. The factory service manual leaves out the theory of operation and only gives you a step-by-step method of diagnosing. I like to know what is happening, then I can figure out my own way of doing things if I don't have the factory tools and equipment.

[Don Coatney]

I believe this has been posted before.

[Norm's Coupe]

Norm...I assume that the sending unit works because I had Tim rejuvinate it right after I bought my car. The fuel gauge is NOS out of the box and the wires are new. So I can't see those things being the problem. It has to be a grounding issue because when I turn the ignition on' date=' it shoots past F and stays there until the ignition is turned off.[/quote']

Darin, if you hook up the wires backwards the gage will shoot to full and stay there like you said. I had the same problem once when I hooked mine up. Try reversing the wires on one end (under dash or the sender) and see what happens. Also remember these sending units are thermal and work slower than the electric ones do. So it takes longer for them to rise to the occasion:D . Sometimes several minutes. Doesn't mean anything is wrong because they are slow, it's just the nature of the beast. For example: my wifes 02 Kia must have a thermal sending unit in it. You can fill it up until it's running over the fill tube. Get in the car and it will only read about 1/2 or 3/4 tank when you start the car. Get about a half mile or a mile down the street and the gage says over full like it's supposed to be.

Also you could have built up a little rust in the arm lever of the sender that doesn't allow the arm to move as freely. If that's the case as you drive around and the gas sloshes it should break it free, then the gage will start working.

Not taking anything away from Tims work. However, how long ago was that when he worked on the sender? Remember, you can buy new electrical parts and have them go out in a short time, or they don't work when you put them in. So, I would still pick up a new sender and run the necessary test if you can't get the gage to work. Like I said before, doesn't hurt to have a spare. If you don't want to keep the spare new sender, I'm sure you wouldn't have any trouble selling it to get your money back after testing.

With the spare, do the testing on the bench, not in the car. If it works on the bench all is good and the problem is probably a short, poor ground or other wiring in the car, not the gage or sender.

[Plymouthy Adams]

should it have over currented due to improper connection, do not overlook the posibility that the internal voltage reulator contacts of the guage itself may be fused together..only thing going to ground on this set up is the wiper of the sender unit which is the body of the sender itself..the guage is push pull setup with the 30 ohms felt at all time proportional to the wiper position..if there is 10 ohms on one leg to ground the other leg will have 20

ohms...when connected and as stated earlier should be a slow rise and take about 3 cycles to heat the element when the tank is full or near full..if you do not see the needle pause..I would see why the internal regulator contacts are not opening up...

[47heaven]

Darin' date=' if you hook up the wires backwards the gage will shoot to full and stay there like you said. I had the same problem once when I hooked mine up. Try reversing the wires on one end (under dash or the sender) and see what happens. Also remember these sending units are thermal and work slower than the electric ones do. So it takes longer for them to rise to the occasion:D . Sometimes several minutes. Doesn't mean anything is wrong because they are slow, it's just the nature of the beast. For example: my wifes 02 Kia must have a thermal sending unit in it. You can fill it up until it's running over the fill tube. Get in the car and it will only read about 1/2 or 3/4 tank when you start the car. Get about a half mile or a mile down the street and the gage says over full like it's supposed to be.

Also you could have built up a little rust in the arm lever of the sender that doesn't allow the arm to move as freely. If that's the case as you drive around and the gas sloshes it should break it free, then the gage will start working.

Not taking anything away from Tims work. However, how long ago was that when he worked on the sender? Remember, you can buy new electrical parts and have them go out in a short time, or they don't work when you put them in. So, I would still pick up a new sender and run the necessary test if you can't get the gage to work. Like I said before, doesn't hurt to have a spare. If you don't want to keep the spare new sender, I'm sure you wouldn't have any trouble selling it to get your money back after testing.

With the spare, do the testing on the bench, not in the car. If it works on the bench all is good and the problem is probably a short, poor ground or other wiring in the car, not the gage or sender.[/quote']

Norm-

The needle literally wastes no time going to past full when the key is turned. There really is no waiting time. The guy who did my electrical installed a 12-6v reducer on the fuel gauge. He didn't check the guage because the tank was full of gas and he assumed the the gauge was registering correctly.

I'm just goin to take it all out and do the mentioned tests on it and see if that solves the problem. If not, I'm going to go to the one wire conversion plan because my patience level on this isn't has high as everyone else's.

[Normspeed]

Darin, how accurate was the guage before the 12 volt conversion?

Just be really careful about sparks when you have the sender out and the tank open. probably best to disconnect the battery ground cable before you work on the sender. You know, it wouldn't be all that difficult to simply run a fresh pair of wires from the dash to the sender.

[47heaven]

Darin, how accurate was the guage before the 12 volt conversion?

Just be really careful about sparks when you have the sender out and the tank open. probably best to disconnect the battery ground cable before you work on the sender. You know, it wouldn't be all that difficult to simply run a fresh pair of wires from the dash to the sender.

It wasn't accurate at all...never has been. Like Tim says...it's probably the way it's connected.

No, I would never test that thing near the gas tank. I'll give it a shot, and if not, maybe you and I can troubleshoot it, if you want. After all, you have all those neat testing tools...LOL!

[Normspeed]

Yup, any excuse to buy another gizmo.

Seems like if you can get the stock setup working you'll be better off than using an aftermarket sender, unless it's a really great replacement fit at the tank.