Vannin' Home Forums Dodge How To (Dodge) HELP PLEASE with fuel gauge/sending unit

OK..I need some help. I have a 77 B100 with a 318. Lat weekend I installed a new fuel tank and sending unit (along with everything else from the carb back (lines, filters, fuel pump). The van runs great but the gauge isn't working.
There is not a lot of gas in the tank (3-4 gallons) but that should be enough to read on the gauge. I checked the ground wire on the sending unit and it is good and getting a good ground. I also know the other wire (from the sending unit to the gauge) is good because when I ground that out the gauge will rise.
I am inclined to put another 5 gallons in to see if that will register but I really don't want to have to pump it all back out if I have to drop the tank again. It could be that the new sending unit is bad but I wouldn't suspect that but is there a way to check it when it is in the tank.
One other issue: I pulled the instrument panel to get to the back of the fuel gauge to see if I was getting a connection and when I check it with a test light I don't get a steady light but a pulsing circuit almost as if a blinker would work. What causes that?
Any advice or help I can get would be GREATLY appreciated....I really don't want to have to drop the tank again!!!
Thank you!

When you first turn the key to 'ON' does the needle move at all? My theory is that these aftermarket sending units are not of the same quality as OEM, so there may be some ohm range variances. Worst case is put your old sending unit back in, if it worked.


Does the pulsing occur with the engine running? If so, that could be an alternator/voltage regulator issue. The lights on my 72 would pulse at idle. I ended up replacing the alternator and regulator (I ran a dedicated ground to the regulator as well). That fixed the issue.

Like Nate recommends, I would put the old sending unit back in. However, while you have both sending units out, I would use an ohm meter to check that each sending unit (a) has the same resistance value and (b) the value changes smoothly as the float lever arm is moved.

You could have a faulty instrument cluster voltage regulator. There are new replacements that re a vast improvement over the old stock regulator. Get the new and improved instrument cluster voltage regulators HERE and HERE or you can build your own as described HERE.

Unfortunately, I don't have the old sending unit anymore. When I dropped the tank to check out the inside of the tank and the sending unit I found them to be way worse than expected. The tank had a hole in it where it had rusted through and the tank was rusted very badly inside and had about 3/4 inch of rust and "gunk" in the bottom of the tank. The sending unit was in bad shape, the wire had corroded through and wasn't attached, the entire sending unit was caked in about 1/2 of rust and gunk and it was so rusted that the float lever wouldn't even move.
I am slowly narrowing down the problem (I think). I know the gauge will work because if I ground out the connection at the sending unit it goes to showing full. I know the ground is good because I even ran a ground right from the battery to check it. I know the wire from the sending unit is good because the gauge will move if I ground it out at the sending unit. So....that leaves either a bad sending unit (even though it is brand new) or a bad gauge cluster voltage limiter. It is my understanding that if I put an ohm meter to the sending unit it will give me a reading of 90-95 if the tank is empty or 5-10 on an full tank. I have about 4 gallons in it so I expected the ohm meter to go to 70-80. When I put an ohm meter to it, it read 76 ohms which is about what I would expect so it appears that the sending unit is ok. The gauge just stays on empty. So I am guessing (hoping) it is the gauge cluster voltage limiter. I ordered one today and can pick it up tomorrow. I hope that's it....I REALLY don't want to have to empty and drop the tank again!!!

It'll take more gas than that to make the float "float". Think about the surface area of the bottom of the tank and how thick the diameter of the float is. Most vehicles have a "reserve" when the gauge shows empty because there has to be at least a couple inches of gas to get that float off the bottom.


In the event that you ever need to drain the tank again, simply jack up the rear axle just to the driver's side of the pumpkin a bit, and disconnect the line at the fuel pump. Gravity will drain it dry.

This is true. My 83 Dodge has the plastic 36 gallon tank. When the gauge says empty I actually have about ten gallons of gas left. So that means I can drive about 100 miles on "E."

What Boot said. With all of what you checked, it has to need more fuel to get the unit to move from just above the reserve fuel.

Yes..what Scotty, Dr. McCoy, and Mr. Spock said. LOL!