Question re: "Aux Power Circuit Relay" and trailer plug pin "k"

rchalmers3

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Hey guys,

According to the schematic, the deuce trailer plug pin "K" has 24 volt aux power whenever the main switch is on. The current is supplied by a dedicated "aux power circuit breaker" via circuit #37 to the trailer plug.

Can someone confirm this, and does anyone know what the circuit breaker amperage limit is?

Rick
 

steelandcanvas

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If you follow that wire #37 back from the trailer receptacle (from right to left), you'll see it marked "37 Auxiliary Power (Tractor Only)" I once tested the "K" pin in the receptacle on my M35A2C and found no power. Do you have that circuit breaker on the firewall with a #37 wire on it? Sounds like they made a different harness for tractors.
 

rchalmers3

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Yessir, I saw that note. I'm nowhere near my cargo truck so I can't peek under the dash to confirm the existence of the circuit. I thought that hopefully, since the circuit was drawn, then it had to be there.....

I'm considering what it will take to equip the M35A2 for towing a trailer with electric brakes. There are numerous threads explaining many different ways to go about doing this, but of course I wanna do it my way. I'd like to perform minimal changes and additions to the truck, or trailer. My current idea is to fabricate a detachable box located at the front of the trailer that will perform the light and brake interface.

For my purposes I'll need two additional circuits added to the truck trailer plug: a brake electrical signal and the mentioned 24v aux power. The trailer will require light bulbs that operate on either 12v or 24v. I understand those are readily available.

I saw this brake controller offered in Australia that seems to eliminate the expense and complexity of adding a 24v-12v converter:
http://www.ebay.com.au/itm/REDARC-T...AILER-BRAKE-CONTROLLER-EBRH-ACC-/221456455447

I have a query in to the company to confirm the controller can operate three axles, as the specs say.

Rick
 

steelandcanvas

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Yessir, I saw that note. I'm nowhere near my cargo truck so I can't peek under the dash to confirm the existence of the circuit. I thought that hopefully, since the circuit was drawn, then it had to be there.....

Rick
We have the same reasons for trying to locate the mysterious #37 wire. I was also looking for a wire already in the trailer receptacle to use as a trailer brake circuit. I'm wondering if that wire just isn't folding back inside the taped harness, possibly right on the firewall where the circuit breaker would have been installed, since it was not used on cargo trucks. I think I'll put a wire tracer signal into the "K" hole in the receptacle, and see if I can locate it in that firewall wiring. Good catch finding that 24 volt brake controller in Australia. Is the trailer you're going to tow, a camping type trailer? I found an issue with using a straight 24 volts though...camping trailers almost always have onboard batteries that are usually charged through the trailer plug from the towing vehicle while traveling. While this is not a concern with a box, boat or flat trailer, it may become an issue with a camping trailer. I have always liked the ability to charge my trailer batteries on the way to my "dry campsite".

Jonathan
 

rchalmers3

Half a mile from the Broad River
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Jonathon,

By reading your replies, it occurs to me that you may have already inspected the trailer plug and found a wire entering the trailer plug at position "k"? If so, please let me know what you find when you do that trace.

Saturn Surplus has a variety of breakers for the purpose of providing aux power to the plug, with ratings from 15 to 60 amps. Prices start at $20, plus shipping. However, if you found there is already a wire, it's probably 14 gauge like the others, and you are then limited to about 15-20 amps, given the length of the wire run. That should work for your purposes of charging house batteries at 12v, if you don't push the batteries that hard. Is it proper to calculate 20amps at 24v being about equal to 40 amps at 12v? If so, 40amps available sounds plenty sufficient.


I am preparing to tow an equipment trailer, so I have no need for charging batteries. According to the instruction manual on the above referenced brake controller, I am good to go with no additional electrical components required, other than the the two additional truck circuits.

I believe there vendors on this forum who are selling LED brake light bulbs or lamp assemblies that operate well on either voltage. So the trailer mods are apparently easily solved, hopefully without too heavy an expense.

Cheers,
Rick
 
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steelandcanvas

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Rick,
No such luck having that spare wire. I counted the wires coming out of the trailer receptacle...there were eight. I didn't do the trace after a visual of the rear of the plug. No #37 coming out of the "K" hole in the back of the receptacle, everything was accounted for according to the schematic. Bummer, seems as though they (per the schematic), made three different harnesses, one for the tractor (M275A2), one for the cargo truck (M35XXX) and another one for the Pipeline Construction Truck (M756). Wouldn't it have been more cost effective to just produce one with all the wires?
 
Last edited:

MWMULES

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Nope the tractors have two places to plug a trailer in to one is at the back on the cross member like a cargo and the other is on the front of the cat walk. not sure what is different on the pipeline to rate it's own harness.
 

rchalmers3

Half a mile from the Broad River
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Thanks guys. I'll be near my vehicles in a few weeks. In the meantime I'll order some wire and connectors to create those two circuits.

Another question: is there a sexy way to create a dual connection at the brake light switch? I'd like to avoid cutting or splicing the existing brake signal wire. For example, sometimes you can find a connector that converts a single female contact into two male connectors. Just wondering?

Rick
 
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