gas tank sending unit

[Guest leftwing302]

I have a 37 plymouth that I bought a new sending unit for.When I spoke to Roberts I told them I had a 6 v. system with a positive ground.They said this unit would work.It works backwards.Up is empty,down is full.I can see no way to reverse the float.I will call them in the morning,but does anybody know if there are different units for positive or negative ground systems? This will drive me crazy tonight,anybody with knowledge on this subject?Any help will be appreciated.

Lew

[critterpainter]

The "in tank" unit is just a variable resister. It does not care if the car is pos or neg ground, nor does it care if the car is 12v or 6 volt.

It sounds like the guage and the sender are NOT a match for each other.

Bill

[Guest Jim_Edwards]

I have a 37 plymouth that I bought a new sending unit for.When I spoke to Roberts I told them I had a 6 v. system with a positive ground.They said this unit would work.It works backwards.Up is empty,down is full.I can see no way to reverse the float.I will call them in the morning,but does anybody know if there are different units for positive or negative ground systems? This will drive me crazy tonight,anybody with knowledge on this subject?Any help will be appreciated.

Lew

Sounds like a case of wires being crossed assuming there are two wires going to the sending unit involved.

[ply33]

The "in tank" unit is just a variable resister. It does not care if the car is pos or neg ground, nor does it care if the car is 12v or 6 volt.

It sounds like the gauge and the sender are NOT a match for each other.

Bill

That vintage Chrysler built car most likely has a two wire sending unit so things are a bit more complicated than just a wrong sender.

Sounds like a case of wires being crossed assuming there are two wires going to the sending unit involved.

Ditto Jim_Edwards comment. There should be two wires connecting the sending unit to the dash unit. I'd have to double check the service manual but I believe that swapping the wires would make it read backwards. Other strange behavior can result if one or the other wire is broken or shorted out and the factory service manual lists the symptoms.

[Guest leftwing302]

Thanks everybody for all the info.I should have mentioned this is a one wire unit,and I made sure of a good ground by making a new ground connection from the tank to the frame.If Roberts Motors can tell me whats wrong I will post it on this forum.

Thanks Lew

[Guest GeorgeCagle]

I had a similar problem on my 37 Dictator. The closest sending unit that I could find to fit my car was from JC Whitney and it was a 0-90 ohm unit. This unit is universal and fully adjustable. As the unit is shipped, 0 ohm would register full on my gauge, but the float was in the empty position. What I really needed was a 90-0 ohm sending unit. The variable resistor on the sending unit is held on by two screws, so it was a simple matter to remove the resistor and turn it 180 degrees and reattach it. I installed it in 2003 and it is still working. Good luck.

[ply33]

You are right, looks like they started their two wire thermostatic fuel gauge setup in 1939. If it helps any the samples I've looked at for 1931 through 1935 seem to use a range of approximately 3Ω to 4Ω for full and 106Ω to 117Ω for empty. (Lets see if those unicode Ohm symbols come through.) That does not match up to any modern universal sender.

I've been too cheap to pay for the official replacements so I've futzed around with modifying a cheap universal sender. See: Plymouth: First Decade - Early 1930s Fuel Sending Unit

[padgett]

Difference between a one wire and a two wire (plus ground) sender as used by GM is that the differential (two wire) type is insensitive to battery voltage changes.

Mfrs have used both 0-90 and 90-0 as well as several other types in the past.

Cars that have been ignored for a while often have the sender fail due to gum formation on the open windings. Five minutes of cleaning usually restores. Only difficulty is that you usually have to drain and drop the tank to get to it. If you have an in-tank fuel pump this s a good time to change along with any rubber hoses and O'rings.

[Guest notnow]

On the Gas Tank Gauge ,You might want to check at MAC'S antique auto parts.. He list one that can be used on positive or negative ground .. I bought one and installed it on a 41 lincoln ..After adjusting the length of the arm it works fine ..hope this helps ..

[Guest leftwing302]

I thank everybody who has responded to my post.I called Roberts Motor Parts and they told me that a few sending units have had their resistors mounted upside down,lucky me I got one.I drilled out the rivets reversed the resistor.Now down is empty and up is full.You had the right idea George.Thanks again everybody,this is a great place to solve problems.

Lew