Need help troubleshhooting MBT wiring

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ctbea
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Need help troubleshhooting MBT wiring

Post by ctbea » Tue Nov 06, 2018 10:45 am

I’m trying to verify operation of the MBT trailer lights before anchoring/clipping all the wiring. I used my 6V battery and clipped the battery POS to the TL lips at the jeep end of the pigtail and battery NEG to the GRND lips of the pigtail…no lights in SVC or BO switch position. Same with the SL lips of the pigtail…no lights.
Then I started checking continuity. I’m finding from the SL lips I have continuity at the switch (svc position) with the SL (of course), SS, ST and TL contacts. Setting the switch to BO, I have SL continuity with SL and BS.
From the TL lips of the pigtail (svc position) I have continuity at the switch (svc position), with TL (of course), SS, ST, and SL. Setting the switch to BO, I have TL continuity with TL and BT.
Ground continuity from the pigtail lips to the rear light connectors is good.
Does it make sense that there is continuity at the switch between SL and TL when the switch is in the svc position? Could be that I am misunderstanding the reality of what SL and TL are, and how they work together? Is my testing with the battery hooked to only one wire of the trailer faulty logic?
Any help would be appreciated. Thanks, Darryl


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Chuck Lutz
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Re: Need help troubleshhooting MBT wiring

Post by Chuck Lutz » Tue Nov 06, 2018 12:17 pm

Look at it this way.....the trailer harness from light switch to trailer socket is always in the non-BO position. It is the BO switch on the trailer that switches the lights on the trailer to a BO configuration.

The first thing to always check is if the trailer socket is wired correctly.
Next, with the trailer plugged in and the BO switch turned to the RIGHT, away from the "BO" marking, check again for taillight and brake light functions.

Note...at this point setting the jeep light switch to the "BO" function, the trailer is not showing BO lights. You have to turn the switch to the LEFT to have BO light functions on the trailer.

Another possible place for error is wiring up the switch itself. Since you have some light functions it seems that a good GROUND on the trailer socket on the jeep and the wire attached to the mounting face of the trailer are not the issue.
Chuck Lutz

GPW 17963 4/24/42 Chester, PA. USA 20113473 (USA est./Tom W.)
Bantam T3-C 1947

ctbea
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Re: Need help troubleshhooting MBT wiring

Post by ctbea » Sun Nov 11, 2018 3:25 pm

Thanks for the reply Chuck. I rechecked the switch wiring and all the connections are correctly made. I'll need a better look with a magnifier to see if there are any observable bare wire jumps between contacts. I have not connected the trailer to the jeep...not currently accessible...so I am using my battery connected to the TL then SL slots at the pigtail where it would typically connect at the Jeep. I checked the continuity on the pigtail and its fine...no cross connects. Is there any reason with power to the SL contacts on the pigtail that the switch would show power at BOTH the SL AND TL contacts in the switch...and vice versa, power to TL and I see power at both TL AND SL contacts ?? Very confusing. There is no cross connection at the pigtail contacts (jeep end).
I appreciate your help. Apologies for delayed reply. I've been working at getting lighting into the trailer shed so I can continue working on it late and get a small heater in there too.
Darryl

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Chuck Lutz
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Re: Need help troubleshhooting MBT wiring

Post by Chuck Lutz » Mon Nov 12, 2018 3:35 pm

Darryl....

Check the position of the switch first...and be sure it is not in the BO position. It seems that either you have shorted the attaching wires when you connected them to the switch or the switch itself is defective. Since the switch is not yet installed and you are positive you have wired the trailer plug to the harness correctly (you checked continuity before attaching to the switch after the plug was attached though, right?), those would seem to be the two most likely causes.

Did you happen to check the continuity of the switch before attaching any wiring with the switch in BOTH positions?
Chuck Lutz

GPW 17963 4/24/42 Chester, PA. USA 20113473 (USA est./Tom W.)
Bantam T3-C 1947


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