If you don’t own a WW2 HMV, you may want to skip this article. If you don’t own a WW2 HMV with directionals and using a 12 volt battery, you may want to pass, finally, if you don’t own a larger WW2, HMV with directionals and tow HMV trailers with electric brakes, this may not be for you.
If you meet the criteria above, you have an interesting problem making the electrical connection to the trailer. The standard WW2 HMV trailer connector has only 4 electrical connections. You need: 1 - running lights, 2 – RH Directional/Stop, 3 – LH directional/stop, and 4 - Electric Brakes. Oh, I almost forgot, you do need a ground – 5
That is the problem: You need one more connection that the WW2 HMV trailer connector provides.
Having both a CCKW and a Searchlight trailer, and an M6 Bomb Truck and M5 Bomb trailers, I have been dealing with this problem for a while. Originally I had a separate connector plug for the electric brakes. Unfortunately, if either that connector or the main connector came out, I would loose the electric brake capability on the trailer. Besides, it was not original looking.
Being an Electrical Engineer, after analyzing the problem, I came up with a simple converter circuit so that you can use the 4 pin WW2 HMV trailer connector and have electric brakes.
Basically what I do is to use diodes which have two characteristics that I need here: One is that they reduce the voltage by about .6 volts every time you pass though one of them and that is, unlike a resistor regardless of the load (current) that they are being asked to carry and Two they will only let the electrical current pass through them in one direction.
What the converter does is to eliminate the parking/running/tail light connection. Now as you legally need rear tail lights, we need to substitute some other way to accomplish this function. What the converter does is to just rely upon the bright (stop/directional) filament in the tail lights for both functions. This is accomplished by using a series of diodes with a loss of about .6 volts per diode to reduce the voltage to the bright filament. In this case, I used 7 diodes so that I loose about 4.2 volts. This loss makes the filament glow about the same as the normal (other filament) tail light. There is another diode, one per side that bypassed this diode string when the stop or directional function is needed. Yes, it does also reduce the brightness slightly (.6 volts worth), and because the diode will only pass current in one direction, it does not affect the normal tail light function. This has now freed up one connection, the one needed for the electric brakes. Even if you have a trailer without electric brakes, this will still work as described here.