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I bought a 1951 M37 ("Brutus") a couple of months ago. The previous owner had a hybrid 12/24 volt system: two 12 V batteries in series. He had a 720 watt 24 to 12 v converter in the battery box which he ran most accessories off of (lights, ignition, horn, etc). The starter is still 24V.

I'm planning on switching most of this back to 24v. The complication is that I want my trailer connection to put out 12V for the trailer lights and brakes. (I run some trailers in the woods on which I really prefer to have functioning brakes when coming down some of our hills with a load of logs, and it just seems it would save ear and tear on the M37 brakes when using a trailer.)

I'm pretty sure it makes sense to run the brake controller off the main 12V converter he has in the battery box - I'll just run the brake power wire back from the controller to the trailer connector.

The trailer lights are another matter. Do switch all the truck lighting back to 24V, keeping the truck itself as simple and "stock" as possible? I think this would require putting a 24V to 12V buck converter on each trailer lighting circuit (blinkers, running lights and brake lights), so I would have 12V at the trailer connector. In this case it probably makes sense to mount the buck converters somewhere near the rear of the truck (potted, waterproof 24v to 12v buck converters of the appropriate amperage rating are about $10 each, so it won't brake the bank)

The alternative would be to leave the truck lights on 12V circuits (I think they all are 12V now). This makes the truck itself a bit of an odd-ball, hybrid system, but means everything going back to the trailer connector is already 12V.

I'm sure lots of you have already dealt with this. I'd really appreciate your thoughts on the best way to go.

All my trailers are military, so the lighting is 24V. I built a light bar with 24V lighting for use with borrowed trailers without. Handy for towing cars and such, too.

PoW

Put 24V bulbs in the trailer.
If you use the trailer with other vehicles, add a second set of lights for 24V.
Change the trailer brakes to hydraulic.
Restore the truck to 24V
Problem solved

I guess I’m another one in favor of going all 24 volt, although for powering accessories it’s easy enough to tap off of just 12 volts where need be.
Most of my trailers both for on and off road have hydraulic “surge” brakes and they seem to be simple and dependable.
As far as lights go, you can find led replacements that will work with a spectrum of voltages so when you change it back to full 24 volt, your trailer lights will work with your 12 volt rigs too.
Happy trails.

pulling twelve off one battery will case the other battery to get overcharged.

I've got several trailers that are set up for 12V, and I also occasionally pull trailers owned by others, so converting all the trailers it will ever pull to 24V is just not an option. Likewise with hydraulic surge brakes: all of my trailers that have brakes are already set up with electric brakes.

I do agree that pulling 12V off of one of the batteries is not a great solution. That's why the previous owner had it set up with a DC to DC converter to drop the 24V down to 12V, rather than pull off of one battery. The wiring on the truck was basically just enough to get it going. When I return things to 24V, I still need a way to get it to work with 12V trailers.

Some folks on the DPW and Steel Soldiers forum gave me some leads on some 24 to 12v trailer converters that should work. I think I've settled on one of these: Military to Civilian Trailer Lighting Converters. They can be installed without interfering with the function of the 24v trailer plug, if the truck already has one of those.

And I just found a brake controller that that can be powered by a 24v system (and be triggered by a 24v brake light switch) and output appropriate for 12v trailer brakes. It will run in either proportional mode (which is best for on-road use) or "User control mode" (a.k.a. "Time Delay" controller) which is better for off-road/rough-terrain applications. The bonus is that the main controller mounts remotely, so the only thing on the dash is a single control knob.

Redarc Tow-Pro Elite Trailer Brake Controller

I put it on my Christmas list. A little elf told me that "Mrs Claus" has ordered one for me. Between that and the 24v-to-12v trailer light converter cable linked in the previous post, I guess I've got my trailer figured out, and can proceed to returning the rest of the truck to it's original 24v set up.

Let us know your thoughts once it's installed and in use, John.
Thanks!
Gary

Cal_Gary wrote:Let us know your thoughts once it's installed and in use, John.
Thanks!
Gary

Will do. It may be a bit until I get to it

The RedArc controller seem to be very highly thought of. While a 24v controller is somewhat rare, they also work on 12v systems, so have a fair amount of use in other applications.

I have not laid hands on either that or the trailer light conversion yet. I'll let folks know how it works out. I'm looking forward to posting a picture of "Brutus" (my 1951 M37) in action in the woods with my log forwarding trailer.