Although I am unable to work on the actual fuel sender in my car as the car is in storage, I decided to do some bench testing with what I have available. I have the instrument cluster from my ’65 2000 and reproduction senders from two different manufacturers. I thought it would be interesting to see their respective accuracy. I believe that one of the senders is the same as the one installed in my car. The other one is from a different manufacturer. Although the voltage regulator on the '65's instrument cluster seems to be operating properly, I decided to bypass it and us a regulated 10-volt supply to eliminate any possible secondary issues and the annoying fluctuations as the voltage regulator switched on and off. When I do the work on my '68 TC in the spring, I will replace the mechanical voltage regulator with a solid-state one I have procured from Moss Motors.
The comparison between the two senders was interesting.
- They had slightly different vertical measuring range. One has a range of 21 cm and the other 25 cm. The one with the 21 cm range needs to have the arm bent significantly as the float at the empty position sits well below the top of the main fuel inlet tube. The other one seems to be set up properly. When I have the sender out of my tank, I will have to measure the distance between the sender flange and the top of the tank to see which range better matches the height of the tank.
- Both of them have similarly looking rheostats and wiring. The instrument connection on the sender is connected to one end of the rheostat and the other end is connected to the earth/chassis connection. The wiper arm attached to the float arm progressively shorts the windings to earth as the float moves to the “full” position. Both senders measured a resistance of approximately 285 ohms when at the empty position.
- I set up the senders against a vertical scale and measured the resistance of the sender and the resulting reading on the fuel gauge for positions corresponding to 0%, 25%, 50%, 75% and 100% of the respective senders range. The positions do not correspond directly to the volume that would be in the fuel tank due to the irregular shape of the tank, but close enough. The results were as follows:
Sender 1 (21 cm range)
Position---------gauge reading-------Ohms
Full-----------------Full------------------27
3/4-----------------5/8------------------60
1/2----------------<1/4-----------------133
1/4---------------<1/16-----------------208
0---------------------0-------------------286
Sender 2 (25 cm range)
Position--------Gauge reading---------Ohms
Full-----------------Full------------------19.2
¾--------------------¾------------------52
½------------------>1/2-----------------74
¼------------------<1/4-----------------165
0----------------------0------------------284
Where I use the < and > symbols, it means that the reading was a needle width below above the indicated mark, respectively.
Although neither sender is perfect, there is considerable difference between the two units. Based on the inaccuracy I am seeing in my car, I expect it is the same as Sender 1 in the test. When I get my car out of storage in the spring, I will check how much vertical distance is available in the tank, bend the float arm if necessary and install sender 2 and the solid-state voltage regulator and see if the accuracy improves.