1934 Plymouth - How does fuel sending unit work

[happychris]

Working on a 1934 Plymouth Coupe - Gas Gauge only reads 1/2 full when totally full. Dropped tank and pulled sending unit. It goes from approximately 200 ohms in empty position to 33 ohms when fully up ( reads about 1/2 full on gauge ). Seems like this vehicle requires a sending unit that is approximately 180 ohms in the empty position and 0 ohms in the full position ( if I short the sending unit wire to ground, the gauge reads full ). Can not find a sending unit with these specifications anywhere. Closest thing is a 0 empty to 180 full gauge for a GM vehicle, which would be reverse of what is required. Does anyone know how the 34 Plymouth gauge originally worked? Is it correct to require high resistance ( relative ) for a empty reading, and zero for a full indication? Is it possible to just reverse the wiring of the gauge to get ti to read correctly using the ) to 180 ohm sending unit? Does anyone know where to get a correct sending unit for this vintage vehicle. I am of course assuming that the sending unit is original, but there is a good possibility it was changed over the years.

Directions to any information on how the 34 Plymouth Fuel Sending Unit worked would be appreciated, as would an opinion on if reversing the gauge wiring would allow use of a GM type 180 to 0 sending unit. A source for a original 180 to 0 sending unit would be greatly appreciated. Need to ge this great old coupe bak on the road!

BST RGDS
GARY ( happychris ) :confused:

[Oldcar_Mechanic]

It sounds to me like you either have a poor connection or a defective sending unit. Have you tried asking someone like: Instument Services in Roscoe Illinois, 1-800-588-2674 or Scott Young in Califonia, 1-415-472-5126. Both places do restoration of gages and maybe can shed light on your problem.

Have you tried reversing the wires and useing a G.M. unit?

Ron

[happychris]

Oldcar Mechanic,

Tried reversing the wiring to the gauge - did not effect the reading on the gauge as it works off of resistance and resistance is resistance regardless of the polarity. To change to a GM type sending unit would require replacement of the gauge with a GM type gauge. I don't want to do this as I am trying to continue to use the original 1934 Plymouth Gas Gauge in the dash. One thing that does come to mind is that the 1934 gauge originally ran off of 6V and the car has now been coverted to 12V. I will check to see if reducing the voltage to the gauge from 12V down to 6V solves the issue with the original gauge. If not, will try the numbers you provided - THANKS

BST RGDS
GARY ( happychris )