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Fuel Guage Calibration 39 Plymouth


Dennis Detweiler

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When calibrating my fuel guage, will the length of wire have an effect on the outcome? In other words, calibrating the guage with the sender using short alligator clip connections to a battery on a workbench vs connecting the guage on the front seat to the sending unit in the trunk? Will the accuracy of the guage be different with short wires vs long wires (added resistance)?

 

I have a new poly tank 6 1/4 deep. I cut a cardboard box 6 1/4 deep and mounted the new sending unit in it. I adjusted the guage on the bench replicating empty and full with the float arm. I put the sending unit in the tank which has 10 gallons of gasoline in it (16 gallon tank). The sender is grounded properly.  I installed the guage in the dash. The guage  shows under 1/4 full? I reversed the wires and the guage shows empty.  The guage should be showing over half full.

 

Is the longer wires in the car causing enough added resistance to mess up my bench tested calibration?

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the gauge of that wire and the installed length should not have any effect on resistance....loose poor connection however will..use of resistor wire will...you need now to troubleshoot the wiring in the car with the ohm meter and verify that they are not grounding or that there is also no open from the gauge to the sender....as your gauge has a built in voltage regulator, it also has to be grounded properly for the correct voltage to be divided across the wiper of the sender to ground.

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I spent most of the summer rewiring the whole car (new wires). 39 fuel sender and guage is a two wire system only pertinent to that one year. So, I have two new wires running through plastic wire loom to the rear. I checked for continuity to be sure and and it is good. The ground wire is new and I used a grinder to clean the rust from the frame at the rear before installing a connection. The car body and frame are ground to the battery using braided ground wire connections and all shows good continuity. All other circuits in the car test good. The fuel guage is the last detail to finishing the electrical system and making it all functional.

I suppose I could try running a different ground to the sender?  Maybe run another wire from the sender all the way to the firewall and to the new fuse panel ground?  There would be no doubts about the ground contact that way.

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Two things come to my mind:

1. might it be that the float is leaky ? Added weight from fuel would prevent that it floats on the proper level (I once had a float which was leaky although it was new)

2. I do not know if / how much influence your provisional cabling might have. So you eventually could use different lengths / sizes of cables to verify if / how much difference there is in reading ?

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The reason I questioned the length of wire effecting the calibration, I am a musician and know that the quality of wire, length and thickness effects resistance in a guitar cord and changes the tone. If the fuel guage is very sensitive, it may change the calibration setting?

 

I'll try pulling the sender out and set up my cardboard box in the trunk drop the guage down from the dash and recalibrate using the car's wires. Also, try another ground if I don't get results from recalibrating. I can check the plastic float to see if there's fuel in it

 

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Dennis,

   I see where you’re coming from, but the correlation between a guitar string, and an electrical wire, isn’t a direct one. The guitar string’s length, and diameter, directly affect its frequency resultant to its vibration when strummed, which yields the auditory sensation which we recognize as pitch – the musical note.

   Electricity flows on the surface of the strands of wire, and not thru the interior of the strand. Wire can have one large strand, that being the gauge of the wire in question, or many smaller diameter strands, the sum of which make up the gauge of the wire in question. The length of the wire will have no effect on the reading of the gauge. Corrosion on any connection in the gauge’s wiring, however, will have an effect on the reading of the gauge.

   Also critical to any electrical component’s proper functioning is a good ground. The engine must be grounded to the frame, as must be the body. All connections to ground must also be free from corrosion, and especially paint. I know it’s sometimes painful to scrape brand new paint to bare metal, but it’s got to be done. Lastly, use star-washers on your electrical connections – they scrape the metal to ensure a good contact – rather than regular lock-washers.

    Best regards . . . . .

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I'll start with putting a ground wire direct from the sender to the fuse block ground. The engine and body are both grounded to the fuse block ground with braided ground wire. I cleaned each point of contact with sandpaper and/or grinder and used new bolts and sanded the new nuts. However, I didn't use star washers on the large connections. I ran continuity tests from the body to the fuse block and other areas and the needle showed positive, but maybe it's not enough.

I'll try for a better ground before I re-calibrate.

I have two gauges. One is a NOS. I tested resistance of the coil resistors on each of the two bi-metal blades and both gauges don't show a short. I initially had trouble with both gauges needles shifting up and back (unstable) until I ran some used 400 wet/dry paper between the 4 sets of points on each guage. That solved the instability problem. I sanded the guage wire connection surfaces (nuts).

I'll work on it today and reply with results. Thanks for the input.

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Dennis,

   I forgot to ask – is your fuel gauge a 1-wire, or a 2-wire, arrangement??? Ours is a 2-wire set-up, and it’s the first one I’ve ever encountered. When I rewired the car, using a Ron Francis harness, the fuel gauge didn’t work properly – it registered “full” when the tank was practically empty. So, I switched the wires at the sending unit, since I wasn’t in the mood to be a contortionist again by “dash-board diving”, and now the gauge works just fine.

   I made 3 “common ground” points in the car – 1 under the hood, 1 inside, and 1 in the trunk. It made fining a good ground a “non-issue” for me. They’re interconnected by a large diameter wire, and the 1 under the hood’s connected to the hot-lug of the fender-mounted starter solenoid, which has a battery cable connected to it from the positive post. Yep, the car’s wired 6v neg ground, but that’s the way it was when we got it, and I saw no need to change it back to pos ground.

   It sounds like you’ve got the ground issue addressed, so I’m confident you’ll get this resolved in short order. Warmest regards . .. .  Thx.

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I have a '39 Plymouth. It's a 2 wire setup. Both the sending unit and dash gauge are stock. I had a calibration issue with the dash gauge. I replaced it with an NOS unit and it now reads correctly at all fuel levels. I replaced the float with a nitrile one and also added a ground wire between the sending unit and the sheet metal.

 

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I just came inside to check the forum. I ran a ground wire from the sender to the panel fuse block ground and got the same results. So, it's not a ground issue. I'm going to pull the original fuel guage which I calibrated earlier and I'll calibrate the other one (NOS) which I also cleaned up all 4 contacts and wire terminals. I pulled the new correct 39 two wire sender out of the tank again, mounted it on my cardboard box to simulate the tank depth and have it ready to set at empty and full with the float arm to calibrate the guage which I'll set on the front seat and connect to the wires under the dash. I'll make sure the sender and guage are grounded. Then, I'll reply back with results again. The final electrical process to completing this rewire job and it has to be the biggest pain in the @$$!

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Dennis. This is rich Hartung I sent you a PM about the gas gage.    How did you calibrate the fuel gage. Did you use the service manual to calibrate it correctly?

 

rich Hartung

Desoto1939@aol.com

cell 484-431-8157

home 610-630-9188

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Dennis there is a reproduction 1937-39 All Steel gas tank on ebay for $299 plus shipping and from what I can tell it is spot on and this is a great price for a repro tank NOS one are over the top.

 

Call me 484-431-8157 about the Ply Gas Guage service bulliten that I have.

 

rich

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I'll give you a call tomorrow. It's too late in the evening now.

Since the new tank is 1 inch shallower than the orginal, the non-adjustable float arm can't travel it's full distance which means I can calibrate the guage to show empty and 3/4 full or full and 1/4  from empty. I already cut the float arm to allow it to make full range empty to full. I will recalibrate it tomorrow. I think this will solve the problem.

The old tank was rusted out on the bottom in a few places. It looked like someone had repaired it with some kind of goop at some point in time. There used to be a few places around here that reconditioned and repaired gas tanks and radiators, but they aren't in business anymore. This is the generation of "throw it out and get a new one."  I bought a poly tank for the 39.

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I shortened the float arm to accommodate a 6 1/4 depth (original tank is 7 1/4 deep) fuel tank which now gives the sender full travel. I calibrated the original fuel guage and a NOS guage with the same results. For some reason, I can only calibrate the guage to register 3/4 tank. So I set it to read 3/4 full on a full tank which makes it accurate on empty. I put this in the "makes no sense file."

 

 

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