During the final stages of my WC-52 renovation, I became aware that very little information was available for the wiring between a 6 volt and 12 volt electrical system for a WC-52. In particular, the voltage converter mounted at the rear left bottom of the cargo box drew my attention. I replaced all the wiring harnesses with new reproduction harnesses. The new harnesses fit very well and were well made. All was fine until I tried to connect the voltage converter in the electrical system to supply 6 volts to the trailer connector. The harness supplying the voltage to the converter had three wires bundled together to go to the voltage converter box but it had only two terminals on the inner (input) side. The converter did have three terminals on the outward side from which a short three wire harness went to the trailer connector.
Looking at the available literature, such as the TM 9-808, all voltage references are 6 volt which is not surprising since 12 volt electric systems did not come out until later in WW II. It was important to maintain the 6 volt supply at the trailer connector for backward compatibility. In a 6 volt system, the three wires that go to the trailer connector are for the stop light (green/black), tail light (blue) and ground (green).
The harness bundle which feeds the converter box was taped together with all of the wires having the same free length. This would be fine if it was going directly to the trailer connector. However the inner side terminals on the voltage converter box were stamped ST and TL with no natural, physical way to connect the ground wire. The outward side terminals on the converter box were stamped with ST, TL and GND.
Being an electrical engineer, I examined the voltage converter box. I found it to consist of two simple voltage divider networks, each comprised of two wire wound, power resistors. The resistors in each network were of the same value which made sense since the desired final voltage (6v) was to be half of the input voltage (12v). However, the resistance values between the two networks were slightly different (as well as their physical size) which is not surprising since each are supplying a slightly different load.
Figure 1 illustrates the electrical schematic of the voltage converter box. Note that the two voltage divider networks are not electrically connected the same. The voltage divider ground for the stop light is directly grounded to the box of the converter, while the voltage divider ground for the tail light is connected to the ground terminal on the outward side of the voltage converter. It is interesting to realize that the two voltage divider networks derive their ground differently. One is through the box of the voltage converter (and hence through the left side of the cargo box) and the other one is through the wiring harness. Note that it is important to properly ground the cargo box in a 12 volt system. Each left and right side has its own grounding strap. Additionally, the ground for the tail light voltage divider is achieved by bringing the ground from the trailer connecter to the voltage converter.
Figure 1. The electrical schematic for the voltage converter box.
With this information, I contacted the vendor of the reproduction harness to inquire about this discrepancy. He was aware of the 6 and 12 volt electrical systems but never came across this situation. His template, acquired from another source, did not accommodate any difference between the two voltage configurations. He made a comment from his experience that the voltage converters were seldom wired in.
From the above information, I concluded that the ground wire should not have been bundled (taped) with the stop and tail light wires as far as it was. I confirmed this conclusion through correspondence with Raymond Shumate who has a WC-51 with a 12 volt electrical system. His WC is still in its original state (no renovation). Additionally, this was also confirmed in a picture of the voltage converter box on page 202 in the book by Emile Becker and Guy Dentzer entitled Dodge VC-1 - WC-64KD, 1940-1945.
A picture of the final installation in my vehicle is shown in Figure 2. I hope this information can assist other people too.
Figure 2. Illustration of wiring for the voltage converter box.