M813- Wire #40

[Ajax MD]

I have a small 24v-12v stepdown converter that I'd like to use.

I'd like to use an un-amplified, bone stock CB radio and cell phone chargers on this circuit. I have no plans for high current draw stuff like electric tools or air compressors.
Wire #40 feeds the gauge illuminators from the headlight combo switch. There is a convenient, spare plug tapped into that circuit behind the dash.

I'll wire in a fuse for protection on the supply wire between the 24-12 converter to protect the wiring. Is there any reason why I shouldn't do this? I'm not a fan of tapping directly to the batteries.

 

[M35A2-AZ]

I think wire#40 is for the panel light. Should be ok, but you may have to have your panel lights on for that to work.

 

[Ajax MD]

I think wire#40 is for the panel light. Should be ok, but you may have to have your panel lights on for that to work.

Gauge illuminators/panel light. Po-tay-to/Po-tah-to. Thanks.

 

[US6x4]

This is just a guess, but I'm thinking that circuit #40 might output 2 different voltages based on whether the light switch is on Panel Bright vs Panel Dim. Not sure what your converter would do if it were receiving 12 volts, say, instead of 24 V. A volt meter in that free socket would confirm what actually happens on Panel Dim...

 

[US6x4]

...I'd like to use an un-amplified, bone stock CB radio and cell phone chargers on this circuit.

I've been wanting to add a 12V charging port for cell phones on my M813 also and I've been entertaining the idea of splicing in to the "glow plug" circuit #569 with a wye connector just before the switch since it already carries 12V to the dash. Not sure if it's a sound idea but it would be rather simple.

 

[Ajax MD]

Sure that'll work. I just hate cutting into the wiring harness at all so I used that free connector.

I just finished hooking everything up and it works fine. I just need to mount the adapter plug.

 

[US6x4]

Sweet! Any chance you measured voltage for bright vs dim?

I don't plan on cutting anything. I'm hoping to unplug 569 from the pre-heater switch, plug it into a wye, then add a short section of wire from the wye back to the switch and then another section of wire to a USB port.

 

[Ajax MD]

I didn't measure the voltage. I'll check it out.

 

[Ajax MD]

Sweet! Any chance you measured voltage for bright vs dim?

I don't plan on cutting anything. I'm hoping to unplug 569 from the pre-heater switch, plug it into a wye, then add a short section of wire from the wye back to the switch and then another section of wire to a USB port.

Ha, you're a genius. I only set the switch on "dim" and plugged my CB in. The CB is *very* old and has been sitting in my garage for 11 years. It wasn't working properly in the truck (dim lights, low output TX) and I attributed that to it being damaged from sitting in the garage for over a decade. Turns out it was because the power to the converter wasn't high enough. I flipped dash switch to max brightness and the CB lit up fully and works perfectly.

I found another dangler under the dash, a female connector on an octopus. The metal tag is smashed and unreadable. I'm going to put a meter on it and see what it does. It may also be part of wire #40, I don't know.

 

[US6x4]

There is also a #27 spider harness which is the power to the electric gauges and is keyed hot by the battery switch.

 

[Ajax MD]

There is also a #27 spider harness which is the power to the electric gauges and is keyed hot by the battery switch.

View attachment 858245

View attachment 858247

That's it, #27. I was able to see a "7" on the tag.

I moved the step down converter over to that wire. It's live all the time when the battery switch is energized. It's on a thermal breaker, I've also wired a fuse on the 24v supply to the converter and the CB and the cell phone cord also have fuses.

What page of what manual are you showing? I'm viewing the giant foldout diagram in .pdf format on a laptop.

 

[US6x4]

What page of what manual are you showing?

This is pages 2-113 & 2-114 from printed TM-9-2320-260-20 July 1972 in the electrical troubleshooting section.