12 volt Conversion

I converted my truck to 12 volts when I installed a Cummins 4BT out of a Chevy P30 van (bread truck). I figured that it was less expensive and a better match with other folks on the road to go 12 volts with the truck instead of converting the engine to 24 volts…


Converting a M37 from one operating voltage to another isn't as hard as it might seem since the electrical system is pretty basic compared to a lot of other vehicles on the road.  Basically, you have to change the regulator, generator, starter, lights, and any other electrical items such as gauges or wiper and fan motors.   In some cases, you may find that an item can be left at 12 volts even though it was designed for 24 volts.  This is the case with the heater fan, at 12 volts, it just puts out half as much air as it did before...

Conversion broke down into five major groups: lights, wipers, gauges, alternator, and fuses.


For the lights  I replaced the light bulbs with equivalent 12 volt versions. Jeff Smith at www.mvpc.com has a nice chart showing 6, 12, 24 volt bulb cross references with identification drawings.  A little dialectric grease was smeared on the base of the bulbs helps to prevent them from corroding and sticking to the bases in order to ease future bulb replacement. 

Bulbs used in the conversion are:

Application 12v Bulb Number 24v Bulb Number (what I had before converting)
Small bulb in front marker lights and tailights, dash lights 67 or a 97 (both work) 1251
Large bulb in tailights 1156 1683
Headlight 55-60 watt H4 70-75 watt H4
Turn signal indicator (in switch housing) 1445 313

I then converted the flasher unit from the military 24 volt solid state unit that I had been using to a generic 12 volt two pin flasher unit (got it at Pep Boys). You ignore the ground pin on the flasher harness in the truck and just take the other two pins on the cannon plug and connect each one to a pin in the civilian flasher relay. Eventually, I plan to install the flasher in a waterproof box with a cannon plug but it  currently is just taped to the top of the steering column out of sight


This was pretty easy as United Technologies makes the wiper motors that I was interested in using to replace the original vacuum units with in both 12 and 24 volt versions.  I ordered the 12 volt versions and installed them as described on the Electric Wiper page. 


This was another easy one. I replaced the entire set of guages with new Datcon units using the 12 volt versions.  The truck had a mixture of electrically operated guages such as the fuel level and voltmeter (both worked) and mechanical ones such as the speedometer and the water temperature guage (both worked, but I kept breaking the speedometer drive cable at a hard bend near the transfer case).  The guages are described on the Datcon Guages page.


The Cummins came with a Delco 10SI already attached to it, so all I had to do was figure out how and where to connect the three wires coming off the alternator, and what parts of the original setup to remove.  I disconnected the wiring harness plug from the original voltage regulator and removed the regulator from the truck leaving the bracket in place.  I then cut wire #10 a few inches back from the plug leaving enough wire that I could splice a line onto the plug for possible future use.   I then took a 8 guage wire and ran it from the battery box to wire #10 in order to provide power to the existing electrical systems in the truck.  I then connected the large lead from the alternator to the starter post where the positive lead from the battery and slave connector are connected to the starter using a 8 guage wire.  I ran a 12 guage wire from pin 2 (marked "F") of the alternator to the original ignition switch in the truck in order to provide the voltage signal to the alternator's internal voltage regulator.   I then ran a 12 guage lead from pin 1 (marked "R") of the alternator to the tachometer as described on the Datcon Guages page.


More information on wiring up the Delco 10SI alternator can be found on the oldengine website.



I decided that to seperate the new electrical items from the stock system in order to have an easier installation and to avoid overloading any of the circuit breakers.  I purchased a FH-6HL ATO fuse holder that had a common power bus and dust cover from Terminal Town.  The fuse holder is rated for six circuits at 30 amps per circuit.  I mounted the unit on the bracket where the original voltage regulator had been bolted.  I ran a 8 guage wire from the fuse holder to the battery box.   Circuits on this fuse block are the starter, windshield wipers, and the engine cooling fan.


Headlight relay (installed after I purchased the M101 trailer)

When I purchased my M101 3/4 ton trailer, I changed the light bulbs in the trailer tailights from 24 volts to 12 volts for the ride home.  A couple days later, after having reworked the wiring on the back of the M37 at the trailer plug in order to enable the turn signals on the trailer (trailer has newer style pin assignments on its plug than the outlet on the truck had, a description of the rewire is located on the M101 trailer page) I went went for a nighttime drive.  As soon as I hit the highway, all of my lights went out.  I went 1/4 mile down the road and the lights came back on as the circuit breaker cycled.  Before I could make the loop around the block to my house, the circuit breaker had cycled on and off several times.  In reviewing the situation, it became apparent that the addition of the tailights on the trailer had raised the current level through the circuit breaker above it maximum.  Since I had replaced all of the bulbs in the truck and trailer with units of the same wattage as the original bulbs, but had converted to 12 volts, the current had doubled, and was now overloading the system. 


After thinking about the problem for a while, and talking the situation over with a friend, I decided to isolate the headlight's current flow from the rest of the lighting circuit.  Calculating the current draw from all of the lights showed that about a third of the current was being used by the headlights.  Removing the headlight draw therefore would leave the circuit breaker handling rear and trailer lights, turn signals, parking, and dash lights (seven internally lit guages plus high beam indicator). 


I had a Hella relay sitting in a tool box that I had never installed in another vehicle, so I dug it out and installed it on the bracket where the voltage regulator used to be mounted.  It has a hole in a tab for a mounting screw that I opened up using a "G" drill bit to match the hole I drilled in the front face of the bracket.  I then ran wires from the relay to the headlight dimmer switch and to a convenient power supply. 


The relay is wired as follows:
Vehicle circuit 16 from three lever light switch to pin 86 of the relay (disconnected existing wire 16 from the headlight dimmer switch, plugged a new wire onto it and ran the new wire to former voltage regulator bracket where the relay is mounted).
Pin 85 of the relay to ground (used ring terminal on the wire to connect to the relay mounting screw).
Pin 30 of the relay to 12 volt positive (ran a line to the fuse block mounted on former voltage regulator bracket even though relay has built in fuse holder as it was a really short wire run...).
Pin 87 of the relay to the plug on headlight dimmer switch where circuit 16 had been plugged in.

I used Hella relay 4RA 003 510-361 (according to the inked stamp on box) or 87105 (according to sticker on box).


Selected wire lengths

Battery cables:
The battery to cut off switch is 60" and is routed from the negative post on the primary battery over to the switch under the driver's seat on the outside of the seat box. The wire from the battery cutoff switch to the frame is 33" long and it uses the brake return spring bolt that also has the slave cable ground attached to it on the frame beneath the driver's side of the cab as a grounding point.  The battery positive to starter wire is 74" running down from the battery box to the frame and up to the right front of the engine.  I am using a pair of new surplus inter battery cables to hook up pair of 12 volt batteries in parallel since I like the extra security of all those extra amps.

Smaller lines:
The fuse block 10guage wire from the battery positive to the block that is mounted on the old voltage regulator bracket is 117" long.  The feed from the fuse block to the windshield wiper switch in the dash is a 14 gauge wire 45" long.  The feed from the fuse block to the starter push button switch mounted in the dash is a 14 gauge wire 45" long.