Troubleshooting M101A1 lights

[McGuyver]

Hello all,

I hooked up my trailer to the recepticle on my M880 tonight to see if the lights worked. The running lights work okay in black out mode and normal running mode, and the single brake light works in black out mode, but there seems to be no "brake on" illumination or turn signals in either mode. Is this normal? I was disappointed to see this. I hope it's not by design, would like to have turn signals and brake lights on the trailer, and not just running lights.

Thanks,

McG

 

[CGarbee]

You have a problem with the wiring, lights, or bulbs... Check (in order) your grounds, check the pins (plug and receptical) for cleanliness and contact, check the connections all the way from the plug through the lights, check to see that your truck's plug actually is wired right...
The trailer lights provide the same functions as the towing vehicle assuming that you have lights with all the functions (ie, I have some early M101 that didn't have running lights on both sides, but niether did the early M37's...). Current trailers with compositis (or ones with three bulb round lenses) should have full service and blackout functions on both sides.
If the TM page with the cable pin connection diagram is not on the TM section of this site, check on www.olive-drab.com as I know it is on that site...
Good luck.

 

[McGuyver]

On the 24 VDC to 12 VDC conversion, is this just as simple as taking the 24 VDC bulb out and going to the parts store to find a 12 VDC equivalent socket style? Do you know of a P/N for the 12 VDC bulb?

On the turn signals and lights, I think it might be an issue with the trailer plug on my truck, because when I took a multimeter to it and tested the plug sockets, I never found any that were giving me a flashing reading when the turn signal, or hot just when the brake was applied. I will have to do some further investigation.

 

[McGuyver]

Question: Are the Black out lights supposed to have functioning turnsignals, or just brake lights? (This is a question for the tow vehicle as well as the trailer.)

 

[CGarbee]

Blackout has turn incorporated into the stop function...
Bulb cross reference for 12-24volt is on my website at:
http://www.garbee.net/~cabell/12voltconversion.htm ( I even tell you by size/function...).
Converting the lights from one voltage to the other is as simple as you think, swap the 12v in for the 24v...
Good luck.

 

[McGuyver]

Thanks a lot CGarbee for the link.

when you said:

Blackout has turn incorporated into the stop function...

I'm not sure what you mean here. My M880 has the civilian taillights and separate B/O marker lights. Brakes illuminate on both modes, but turn signal only in normal running mode on the civy lights. Trailer has marker lights for normal and B/O mode, and brake light for B/O mode but no turn signal for either mode, and no trailer brake lights for normal operation. Is the trailer supposed to have all functions for both modes? Is the truck supposed to have turn signals on B/O mode as well?

 

[McGuyver]

Just looked up the cross reference numbers from the www.mvpc.com web site you listed. Bulbs that are currently in the rear lights are 1683, and 623. Looks like this crosses over to 1156 and 1073 respectively. Thanks for the tip on this. I'm assuming these are for equivalent wattage bulbs, so the current running through them would be double that of the 24 VDC bulbs. Is this going to be a problem for the trailer wiring, or is it sufficiently large enough gage wire to still be safe?

edit: I think I screwed up. the 623 bulb (1 7/16") should probably be replaced by a 67 bulb (also 1 7/16")

 

[McGuyver]

Well, my passenger side B/O marker light and my driver side service turn and stop light are not working now. However I got the passenger side service stop and turn light to work! Seems like a bad connection problem. At least I know it's got turn and stop lights now. Now to just figure out why they aren't all working...

 

[CGarbee]

Check the grounds out (including the one on the towing vehicle (if your harness is like the ones on the M37, M38, M35's that have a short gournd line included) making sure to check that both mounting bolts on any given light assembly are grounding...
The wiring will hold up to the current using 12volts... I've got two M101 trailers running 12v bulbs w/o problems...