Doing some re-wiring; have a few questions

dago_red · Post by **dago_red** » Fri Nov 25, 2011 1:07 pm

54 M38A1; PO did a 12v conversion and I'm re-habbing it, mostly the dash wiring. The firewall harness regulator plug was removed and is in the spare parts box. After researching the TMs and using Wes' wiring diagrams for the 12v conversion, I have some questions:

1. in stock form does the jeep use any headlight relay? My lights seem fine; also, horn is loud (horn is a 12v stock-type)
2. did the jeep ever use fusible links?
3.

in my TMs and in the diagrams, I see no mention of fuses or circuit breakers, etc. The jeep's 12v conversion included a fuse block but it's use was not well thought out.

I plan on using another fuse block under the dash and maybe a fusible link between the alternator and post at the starter (though that's only about a 16" run of wire). If a relay for the headlight is a practical idea, I'll install a 60amp one near the driver's side headlight bucket. Also I'll be putting in a 12v voltmeter. And the alternator will be put back in service as a 3-wire set up instead of the current one wire.

I put a shutoff switch from Pete D (from an M-series truck) in the heater port, easily accessed from cab ( I fabricated a plate- couldn't bear to drill even a part like that). The original wiring is still intact and functional, though I'll eventually be going through and closely inspecting all of it. The only bad area was the back of the igniton switch where I chipped off the waterproofing putty , exposing the wires . I will remove the 4 old wires and solder on new ones.

Also, one other thing: I pulled out the dash lights, cleaned them and replaced the 24v bulbs with 12v. The glass cups in the housings are red- is that standard?

TIA

PS Wes' wiring diagram was a big help

Post by **wesk** » Fri Nov 25, 2011 9:29 pm

1. in stock form does the jeep use any headlight relay? My lights seem fine; also, horn is loud (horn is a 12v stock-type)

No. But that was because the amp load at 24 volts was no where near as high as the amp load with 12 volts. 12 volt light setup should have used a relay. The GI 24 volt system doesn't need it. also the 24 volt light switch has an internal auto-resetting circuit breaker (except very early M38 & M37)

2. did the jeep ever use fusible links?

GI 24 volt No.

3. in my TMs and in the diagrams, I see no mention of fuses or circuit breakers, etc. The jeep's 12v conversion included a fuse block but it's use was not well thought out.

24 volt GI used only the built in main light switch circuit breaker (except very early M38 & M37) (however add on circuits like the heaters added their own independent circuit breaker)

4. If a relay for the headlight is a practical idea, I'll install a 60amp one near the driver's side headlight bucket.

It's a practical idea but I would determine the exact amp rating needed. 60 is quite high)

5. The glass cups in the housings are red- is that standard?

Yes, red is standard for GI night vision)

dago_red · Post by **dago_red** » Sat Nov 26, 2011 12:58 pm

Thanks Wes. Hadn't considered that the 24v system saw the lighter amp loading thus affecting the system design.

I'll go ahead and figure out the use of fuses, fusible links, relays, etc.

In your opinion, is there a real risk, with a 12v conversion, of fire due to shorts? After all, there are several places running higher amp loads now and not protected by the light switch breaker (gas gauge, horn, alternator, etc)? The PO at least used some fuses in the system, but there were still a couple places I'd have put a fuse in.

Post by **wesk** » Sat Nov 26, 2011 2:11 pm

If you have all the original military wiring then you're in luck since the military used much heavier gauge wiring then the 24 volt system needed. Even the original military battery cables are larger gauge then needed by 12 volts. The only real problem is the age of the heavier wiring may counter the heavier gauge due to deterioration of the insulator.

Many people unknowingly assume that less volts is less amps/load but one way or the other they soon learn their error.

I would fuse any circuit that is always hot and any circuit that has tired wiring.

dago_red · Post by **dago_red** » Sat Nov 26, 2011 5:06 pm

There are a few spots where the original wiring has been replaced; let's see, fuel sending unit and wiring associated with the original water temp and oil pressure gauges. I will remove the aftermarket amp gauge and replace with voltmeter, so that means those (original) wires will be gone. But all the lighting wire is there along with horn, stop light switch, dimmer, dash lights, trailer receptacle and batt cables. Wiring is about 95% original if you don't include the loss of the original gen and reg.

There is some tired insulation near the headlight buckets (and maybe a bit by the tail lights). That might be because of the increased UV light whenever the hood is opened. Engine compartment temps might have something to do with it also. I'll protect the system with both fuses and some fusible links. I'll put a couple relays for the high/low beams in and incorporate fuses or fusible links there as well (even though the switch has the circuit breaker). Eventually, after inspection of the balance of the wiring, I'll wrap all but fusible links with friction tape.

One of the areas I am still not sure about is the draw on various components (lights, horn, gauges mostly) and therefore the proper amp fuses to install in the fuse block. But that's a few days down the road....