Plymouth: The First Decade
Disclaimer: While an effort has been made to assure a reasonable repair procedure, no guarantees are made. We are not responsible for any damage or injuries that may occur as a result of following these instructions. Applicability to your vehicle is for you to decide.
This page describes fitting a modern universal replacement fuel level sending unit to the early 1930s Plymouths. For fitting a modern one wire universal replacement fuel level sending unit to 1949 and early 1950s Plymouths see the other fuel sender page. If you have a two wire sending unit (1937-48 Plymouths) you will have to look elsewhere for help.
Early 1930s Fuel Sending Unit
The parts book shows a different sending unit for each year and, of course, a different dash unit. However comparing notes with a number of other early Plymouth owners indicates that the resistance values for empty and full might be common across the years 1930 through 1935.
The reader is urged to make careful measurements of their specific car’s characteristics. The dash unit can be characterized by disconnecting the wire from the sending unit then connecting various resistors between the terminal for the sending unit and a good ground.
Physical Characteristics
Mechanical
The 1933 PD fuel tank is roughly oval in cross-section with a height of about 8 inches, front to back measurement of about 15 inches and a width of about 37 inches. The shape is not exactly oval and the interior width is less than the exterior by a bit because of the overlap seam used to attach the ends to the body. The owners manual lists the capacity as 15 US gallons or 56.7 liters. Computing volume of an oval tank, it would seem the best guess for the interior dimensions would be 8"x15"x36.8" which calculates out to 15.0 US gallons or 56.8 liters.
Digression: On a sedan it is not clear to me that the tank can be filled completely as the lower edge of the fill tube is at just about the same height as the top of the tank. I personally have never been able to get 15 gallons into the tank, even when filling it after being totally drained. The coupe bodied cars have a longer fill tube and it should be easier to fully fill the tank.
The mounting flange for the fuel sending unit conforms to the five hole SAE standard that modern universal replacement fuel level sending units expect with regards to mounting holes but the holes are threaded for a smaller screw than comes with the new style universal sending units.
Electrical
The 1930 to 1935
Plymouths used a single wire sending unit. The dash unit on
my car seems to want the following values:
| Reading | Resistance |
|---|---|
| Empty | 120 Ω |
| 1⁄8 | 94 Ω |
| 1⁄4 | 60.5 Ω |
| 1⁄3 | 47 Ω |
| 1⁄2 | 37 Ω |
| 3⁄4 | 23.5 Ω |
| 7⁄8 | 16 Ω |
| Full | 8 Ω |
From correspondence with the owner of a 1935 PJ, I
understand that his fuel level sending unit has 3Ω
resistance for full and 128Ω resistance for empty.
Given errors in measurement and the age of the components it
seems a reasonable guess that the early 1930s Plymouths used
a common range of resistance values.
Survey the possibilities
A number of antique specialty suppliers claim to have appropriate sending units but they are expensive ($85 and up) and the illustrations and range of models they purport to fit (at least for the $85 one I found) does not match the years that my service manuals indicate reasonable. There is also a source for “duplicate of original” sending units but they are even more expensive.
Inexpensive universal sending units are available in the following resistance ranges:
| Intended Application | Resistance Range |
|---|---|
| Stewart-Warner Gauges | 240Ω to 33Ω |
| Chrysler and Ford Gauges | 78Ω to 10Ω |
| General Motors Gauges | 0Ω to 90Ω |
| VDO Gauges | 10Ω to 180Ω |
Note that these universal units can be assembled with the resistor unit either way up so the empty/full resistance can be easily swapped. Unfortunately none of the units I have found is an exact match to the range needed.
Digression: All these universal replacement units are basically the same: A float moves an arm that runs a wiper over a circuit board that has a set of contacts connected via thick or thin film screen printed resistive material. The design is more reliable than the old style with a resistive wire winding as the wear is on a contact specifically designed for it while the resistance material is separate from the contacts. On old style senders the wire would wear unevenly causing a number of issues. An Internet search turns up the fact that the circuit element is called a “fuel resistor card” and there are companies that make these to your specifications. So, in theory, we could simply have a batch of units made up with the appropriate “curve” for our old cars. However the sites I found require minimum orders in the range of 5,000 units and up.
Mechanical Compensation
It is possible to
adjust the universal sending units such that the float
cannot move over its entire designed range. This will reduce
the range of resistances. Looking over the possible units
the VDO one might work: Adjust the unit such that the float
rests against the bottom of the tank before it reaches the
end of its swing (at a point where it reports 120 Ω)
but such that it can float to the top of its arc to report
10 Ω at full.
Electrical Compensation
Active electronics to compensate for an incorrect sensor range is beyond my ability to design. Dealing with a unregulated and very noisy supply line and trying to feed a dash unit that is expecting a simple resistive load would take some actual engineering.
However it is easy to shift the range of values by adding a resistor in series. Or to shift and decrease the range of reported values by adding a resistor in parallel. Looking at the universal sending units again we see:
| Intended Application | Designed Resistance Range | Resistor | ||
|---|---|---|---|---|
| Value | Connected | New Range | ||
| Stewart-Warner Gauges | 33Ω to 240Ω | 120Ω | Parallel | 26Ω to 120Ω |
| General Motors Gauges | 0Ω to 90Ω | 10Ω | Series | 10Ω to 100Ω |
| VDO Gauges | 10Ω to 180Ω | 360Ω | Parallel | 9.7Ω to 120Ω |
The GM version of the gauge is reasonably close but, again, the VDO version is the best fit from among the available universal replacement sending units.
Plan of Action
The universal unit with the 10Ω to 180Ω range looks like it can be limited in its float arm swing range to give readings in the proper range for the dash unit. It also looks like that same unit can be connected in parallel with a 360±5%Ω resistor to achieve the correct electrical output using the full swing range of the float. Given that there are two different possible ways of making the VDO calibrated unit work, it appears to be the best unit to experiment with.
First Cut — VDO Sending Unit
Add 360Ω Resistor in Parallel With VDO Sending Unit
Using a VDO compatible sending unit a 360Ω resister was placed in parallel and it was observed that the empty and full positions indicated correctly on the dash unit. However when the float was set to ½ full the dash unit indicated around ¼ tank. This indicates that the response curve of the sending unit is enough different from the response curve of the dash unit to make this simple modification undesirable.
Mechanical Compensation of VDO Sending Unit
The VDO compatible sending unit was then mounted on the bench and constrained to operator through a smaller arc than designed to limit is output to the range of the dash unit. It was also found that the dash unit indication did not follow a nice linear change with float height. Again because of the difference in “response curves” between the sending unit and the dash unit. In addition the dash needle was jumpy because the discreet transitions between contacts on the sending unit’s “card” became significant with the reduce arc of the wiping contact arm.