Partial 12v Conversion

[tankboy74]

Greetings and glad to be here.

I have an M37 that had a hack job of a 12v conversion done to use a Chevy small block and a new 24v wiring harness that I also acquired. I want to use this new mil-spec wiring harness to replace the rats nest of miscellaneous wiring and get all the switches, dials etc back to being as close to original as possible.

My question is when I use this harness can I simply use Douglas Connectors to modify the 12v connections and integrate them into the harness?
If I do are there any concerns that I should be aware of?

[Cal_Gary]

I run a Painless Performance harness on mine so I'm no expert of the military setup other than to say the wiring doesn't care if you're 12V or 24V. Your circuit breakers, bulbs and gauges will need attention however. Perhaps some Forum members will have additional insights.
Gary

[just me]

You are opening a can of worms that is really impossible to answer easily over the Internet.
The short answer is run it ALL either 12v or 24v. Center tapping batteries to get 12v on a 24v system will damage batteries. There is a file floating around the internet on how to convert an M37 to dual voltage. But it adds a second generator to do it correctly.
The wiring does care the voltage. The 24 v harness is lighter gauge wiring as it doesn't have to carry the current it would on a lower voltage. (Ohms Law)
Fuel, oil anf temp sensors don't care. They are just resistors. The gauges care.
There is a minor problem with the dash lights if you run the system on 12v.
There are various buck and boost devices to change voltage that work with varying levels of success.
Every situation where the vehicle has been modified is different and the details can't be adequately described online.

[John Mc]

It doesn't sound to me as though it's a hybrid 12/24V system, just a poorly done 12V conversion.

Two concerns I see:

1. Do the ends of the wires on the 24V harness come out where you need them to to reach things on the small block Chevy engine
2. As just me mentioned, some of the higher amperage circuits might be drawing a bit much for the 14 gauge wire that is likely in the 24V wiring harness.

The starter and battery is not a part of the wiring harness: it has its own heavier gauge wire. I'm not sure what sot of accessories you have on your truck, but on a stock truck, two of the heavier loads were the headlights and the horn (also consider the wire supplying power to the 3-lever light switch, since that feeds all of the lights. I have never calculated he draw of those circuits. Figure the current for that light switch feed wire with all of the lights turned on (though if you are switching to LED headlights, this should not be a concern.) The horn is such a brief intermittent load that even if it's close to the limit for 14 gauge wire at 12 V, I would not be worried as far as overheating the wiring. Voltage drop might be a consideration depending on the length of the run.

Here is a table of 12V automotive wiring showing the voltage drop for certain amperage draws given the length of the run. If you are under the recommended voltage drop, you are also well within the safe load carrying capacity of the wire (i.e. no overheating).
https://jascoautomotive.com/automotive- ... ity-chart/

[tankboy74]

Thank you all this is helpful.

The title partial in retrospect is a bit inaccurate. I am not looking to run 24v, just use an original wire harness mostly I want as much original type equipment as possible (or a reasonable 12v facsimile). Definitely not dual voltage as that just screams nightmare.

Finding our more about the truck itself as it appears to already be an M152 CDN converted to be an M37 by combining at least 2-3 vehicles. It was essentially rigged up to be a farm work truck and on property snow plow and the gas tank is currently mounted on the passenger bench to I have plenty of electrical and fuel system work to sort out.

It starts, stops, steers but nothing on the dash or the lights work and the battery doesn't even charge (the snow plow worked....but that is gone now) Since I have to do it all over to get it on the road the ~$600 for a wire harness seemed the most straightforward since I want to get it registered and at least driving on the road by the end of the summer or sooner.

Currently most of the wires had to be just straight up removed and they were far below standard with residential connections.

@just me hit it on the head with my concern:
- "The wiring does care the voltage. The 24 v harness is lighter gauge wiring as it doesn't have to carry the current it would on a lower voltage. (Ohms Law)"

I guess the most basic question is if there is any harm in running a 12v system on the original harness using aftermarket 12v gauges and lights? If no clear safety issue keeping it mil-spec will help me get it to a place where the wiring is logical and runs where it is supposed to with minimal effort.

[just me]

One of my trucks is 12v converted. I used a brand new (not NOS) stock wiring harness from VWofM.
I've used the stock instruments and senders by installing a 12v to 24v 30A (overkill, but makes it run cool) boost converter in series with the spider.
Battery, generator, regulator, starter wiring has all been increased in diameter to handle the added current.
I've added a handful of additional loads and just added Douglas connector devices and military wire. Ive even added the metal wire tags with correct numbers to the wires.
I plan taking this truck back to 24V when I repower with a correct T245 engine in the next year.

[Cal_Gary]

With respects, amperage is the concern with damaging wiring, not voltage. That's why you can test your wiring circuits with a charger set to 2 amps versus 10 amps or more. Low amps won't fry the harness. I verified that with Painless Performance when I installed my harness.
Gary

[John Mc]

I've used the stock instruments and senders by installing a 12v to 24v 30A (overkill, but makes it run cool) boost converter in series with the spider.

One problem with using a 12 to 24V converter to run the gauges is that your voltmeter will read 24V all the time, even if your 12V battery is down to 10V. You'll get no indication of an electrical problem until the voltage drops below the minimum threshold for the converter to operate (usually around 9V).

Another suggestion if using a converter: go with a 12-28V converter, as the 28 V is closer to the voltage you get from a "24V" electrical system when the engine is running. Gauges will be more accurate.

[just me]

I did add a voltmeter and ammeter under the dash for the 12v.

[PistonPulse]

Greetings and glad to be here.

I have an M37 that had a hack job of a 12v conversion done to use a Chevy small block and a new 24v wiring harness that I also acquired. I want to use this new mil-spec wiring harness to replace the rats nest of miscellaneous wiring and get all the switches, dials etc back to being as close to original as possible.

My question is when I use this harness can I simply use Douglas Connectors to modify the 12v connections and integrate them into the harness?
If I do are there any concerns that I should be aware of?

Using a mil-spec wiring harness with Douglas Connectors to clean up the 12V conversion on your M37 is a great idea, especially if you're looking to restore it closer to its original setup. You can definitely modify the 12V connections to integrate them into the new harness using Douglas Connectors, but there are a few things to keep in mind. First, make sure the connectors and wires you’re using are rated for the 12V system to handle the current properly, as mil-spec harnesses are typically designed for 24V systems and may have different wire gauge specifications. Also, double-check the grounding points and connections since mixing voltages can sometimes lead to unexpected grounding issues or potential shorts if not done carefully. As long as you take these precautions, you should be able to create a reliable and tidy wiring setup!