1969 Dodge Charger Fuel Gauge Problem

[MJC1001]

Hi there. This is my first post.

I am the new owner of a 1969 Dodge Charger. I believe this car has been through several owners and somewhere along the line the engine was upgraded from a 318 to a 440. Needless to say, that is not the problem. What is the problem is th sun pro after-market fuel gauge that someone installed below the dash.

All of the original gauges in this car have been replaced with after market. The original fuel gauge does not have a reading arm on it, therefore it looks like that is the reason the after market was installed.

The after market gauge will do down below E when the car is off, which tells me it's not stuck anywhere. When I had the car filled, it would go up to F, but would not go down past 3/4 of a tank. I had to replace my sending unit anyway (original one was leaking terribly) so the other day I dropped the tank and replaced it with a new one. Now, the tank reads 1/2 a tank and will not go below that reading. In order to change the sending unit I removed approx 11 gallons of fuel (I believe about 4 to 5 gallons was left)

Since then I drove the car around and the reading never went past 1/2 a tank I grounded the wire going from the sending unit to the chassis of the car and the gauge went back to F which tells me the wiring is okay I assume. I used a multi tester and got a Ohms reading on the sending unit of 76 (which tells me that there is approx 1/4 or less in the tank after doing some online research) if that is right. I went to my local auto parts store and bought two screw clamps and a grounding wire. I clamped one side to the sending unit outlet (above the rubber fuel line) and ran the strap over to one of the J bolts that holds the tank on and clamped the other side to that to ground it out. (I have new gas tank straps on the car which I thought would be great conductivity in addition to the J bolts going to the trunk). After that, my gauge is still reading 1/2 a tank. Just for safe measure I tested it again with an alligator clip going from the outlet to the body. still 1/2 a tank.

At this point I'm not sure where to go. Should I try a new after market fuel gauge. Should I pull out the cluster and look at the old (non active one) to see if the wiring is problem? voltage (I did notice that the fuel gauge spikes back and forth a bit with directional use). The wiring behind the after market gauge seem to be clean.

Also, I'm curious as to why the new sending unit would change where the gauge is getting "stuck" from 3/4 to 1/2 tank. Any help would be greatly appreciated.

 

[q-ship]

From the description of your problem, I suggest you pull up the trunk floor covering and look at the wire that goes from the tank toward the front.

Since it just sits on the trunk floor, it is commonly pinched, smashed, or damaged due to various stuff that gets thrown in the trunk over the years.

Make sure it is not grounding to the trunk floor, or has split insulation with corroded wire from decades of trapped moisture.

Even if it looks intact, try moving around and see if the Gauge changes, as I've seen them that have been stretched enough to break the copper, while not tearing the insulation (due to throwing a flat tire in the trunk, or whatever).

hope this helps.

 

[Meep-Meep]

The factory gauges have a voltage limiter that controls them. It's the little silver box on the PC board. Basically it takes 12V and reduces it to about 5V by making and breaking a contact - kind of like a relay. If this is bad your fuel and temp gauge and most likely the oil pressure gauge won't work. You can always get a new limiter (or build one using a 5V series regulator from Radio Shack) and hook everything up like original and see what happens.

The aftermarket gauge probably just gets a 12V hookup and has a built in series regulator that reduces the voltage to 5V. I'm pretty sure the resistance of the OEM float should be OK with the aftermarket gauge but maybe not. Something to look into. I'd take the sending unit out and physically move the float while observing the gauge reading. It's also possible your float is hanging up mechanically.