Fuel gauge reading wrong

[KillerBee1972]

I installed an aftermarket fuel gauge on my 64 sport fury I created a new ground from the sending unit to the body. Kept the original sending unit gauge wire, but wired it to the new gauge. I have power going to the gauge and grounded the gauge. The gauge works, as though I thought, once the gauge reads just below 3/4 of a tank. it really is empty. now this car was an old drag car, none of the factory gauges worked when I got the car. I finally got all of the lights to work it had a aftermarket fuel gauge in it I replaced it with a new aftermarket gauge. I plan on trying to get the factory gauges to work, but I have no clue where to start on that because the lights for the factory gauges don't even work. But anyway with the fuel gauge issue would it be a faulty sending unit? That's what im leaning towards but want another opinion.

Thanks
Scott Pausch

[MaximRecoil]

Your aftermarket fuel gauge is most likely designed for a different ohm range than your factory sending unit uses. Some aftermarket fuel gauges can be set to different ohm-ranges to allow them to match up with various factory sending units, so you should find out if yours has that functionality. The ohm-range you need is 73 to 10 ohms if I remember right, with 73 ohms being empty and 10 ohms being full.

Another thing that can complicate matters is that the Mopar factory sending units don't have linear resistance across the range, and aftermarket gauges designed with linear resistance in mind will be inaccurate even if they use the correct ohm range.  

[Pete in NH]

HI,

Maxim is right when he said the sending unit in the tank and gauge need to be designed to work together. This really applies to all gauges just not the fuel gauge. The ohm ranges of 73 to 10 are standard for all types of Chrysler gauges of the period. One thing got reversed though, it should be 10 ohms full, 73 ohms empty. I don't have any wiring diagrams for a 64 Fury but if you have the instrument cluster out of the car and can post some pictures I think we can figure out what is in there. They were fairly simple systems, the gauges were powered by a 12 to 5 volt electromechanical regulator and the other end of the gauge when to the sending unit whether it was oil, temperature or fuel.

[KillerBee1972]

The cluster is not out of the car but could be east to get out. It is very simple to work on seeing there is hardly anything back there I was very curious and wondering if the voltage regulator is indeed faulty and that's why I have no power to the factory gauges. I have a wiring diagram for the fury's but the colors they say in the diagram do not match up to what I have in the car. For instance the diagram says the sending unit wire is blue. But in my car its actually tan. So its a little hard to work on it but I can post some pics later on hopefully it will help

Thanks
Scott

[MaximRecoil]

One thing got reversed though, it should be 10 ohms full, 73 ohms empty.

Thanks. I corrected that part of my post.

I was very curious and wondering if the voltage regulator is indeed faulty and that's why I have no power to the factory gauges.

Are you sure there is no power to the gauges? The gauges' nichrome wires could also be burned out. You can do a continuity/resistance check across the terminals of each gauge. If you get no continuity at all, the gauge is dead, and the only way to fix it is to replace its internal nichrome wire (a tedious job). A working gauge will give a reading of about 20 ohms across the terminals.

[Pete in NH]

Scott,

Yes, it is possible that the gauge voltage regulator is bad and not supplying voltage to the gauges. Unfortunately, it's more likely that if the regulator failed, it failed in a closed position and put 12 volts on all the gauges and burned them out. That is the most common failure mode.

I asked for the pictures because I am not familiar with the back of the instrument panel on your 64 and Chrysler was known to have built the voltage regulator into the fuel gauge on some models. If they did you will have three mounting bolt connections on the fuel gauge. Other wise the regulator is in a separate little metal can.

The Chrysler test set for gauges is the C3826 test set and if you do a search on this site and the internet on this test set you will find a lot of informantion on building a test set from Radio Shack parts and gauge repair. The gauges can be repaired if burnt out but as Maxim said it's not a fun job. You might also be able to come up with New Old Stock replacement gauges or a used instrument cluster.

[KillerBee1972]

Today I got a auto meter gauge that had the correct ohm range for Chrysler's and it is reading correctly so that fixed that issur . But I will try to tackle fixing the gauges later on I appreciate all the help.

Scott