New guy, M 103 A3 to Unimog 404 wiring

polvoson

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Hi guys
My name is Paul, I am new here. I have been around on the Mog mail list for a few years now and they recommended this place for great info.
I recently bought a US aArmy M 103 A3 trailer. It has a 12 pin NATO plug. My French 404 also has the 12 pin plug. So far so good.
When I first got it I noticed that the male(trailer) end of the plug is
keyed, the female(truck) end is also keyed. The two will only mate one way.
The holes in the female end were slightly misalinged so that the two could
not mate. I took the truck end apart and slightly rotated the plug in the
housing so that the pins and holes now line up. Both plugs have each pin and
hole identified with a letter. I made sure that the pins and holes were
correctly lined up so the letters match. Upon turning on the lights this is
what I found. Truck lights all work as before. Trailer lights: running
lights work. no brake lights, turn signals when activated do very strange
things. Pretty much everything blinks.
The trailer lights have 4 bulbs. Some of them are for blackout operations.
There are three small bulbs and one full size automotive type bulb with twin
elements. All the bulbs have MIL designations , and no Volt or Watt info,
although I know the trailer is 24V.

I am confused as to why if both vehicles have "standard Nato" plugs, the
systems are not properly communicating.
Should I identify each hole in the truck plug first and then do the same for
the trailer and switch the pins around? Or am I missing something?
Thanks
Paul
 

Recovry4x4

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Paul, check under the trailer. In the front of the left frame rail is where the plug harness connects to the trailer harness. There is a ground wire there. Chck it real close. It needs a good ground to operate. The everything flashing situation is indicative of feedback brom bad grounding. Iven if the wire is still attached take it apart and clean the surfaces. Also check the trailer lights themselves. The lights are grounded through both bolts that hold the lights. Triple check those grounds too. I think once the grounding issues are pu to rest, all will work fine. Temp test, run a wire from a goood ground on the truck to the bolts on the trailer lights to see what happens.
 

polvoson

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Hmmm
More questions
I thought I would start with figuring out if the 12 pins on my truck are correctly wired for the corresponding pins on the trailer. I checked the pins on the truck , and have found that when the ingnition is on, or the lights are on, most pins are showing -24.4 V. WHy is this hapening? SHould they only show when the actual pin for the circuit is acvitated by the switch, like one pin for running, one for left turn, one for brakes etc?
 

Recovry4x4

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Hmm, interesting! Instead of using the meter try using a test light. Most test lights have a simple bulb inside that is exactly like the dash light bulb in M series trucks. I've probably got some if you need one. Swapping the test light bulb gives you a useful 28V test light that works on 12V also just dimmer. Go to http://www.olive-drab.com/od_nato_trailer_connector.php3 to see the actual pinout numbers. I would find a suitable cleam metal frame ground and ground the light then start poking connectors to see what happens. Perhaps the bad gound is in the truck itself. If grounding outside the plug works then once you establish which circuits are hot for what, you can leave the testlight in a hot circuit and clip the ground on a piece of wire and search for the ground. Let me know if that helps.
Kenny
 

Recovry4x4

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Oh, by the way, the turn signal and brake feed are the same. In other words by simply testing left turn and right turn you are checking the brake light circuit too. Not true in civy cars that use te 5 wire setup with yellow signals, but this is how the Mog works.
 
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