NHTSA ID Number: 10180377
Manufacturer Communication Number: TechTalk7673
TSB/Document Date: 2020-10-01
Summary
Tech talk discussion on 2021 M2 BHM/CHM issue. Left tail light goes out when brake lights come on.
2021 M2 BHM/CHM issue?
Ericka Natali 34 posts since May 14, 2015
2021 M2 BHM/CHM issue? Sep 8, 2020 12:56 PM
Good Morning All!
2021 M2 Vin ME6099 Customer complaint - Left tail light goes out when brake lights come on.
Lots of previously active BHM codes
524247/7 BHM/CHM Ignition Mismatch 126 Ct
879/6 Left Turn Lamp Circuit 67 Ct
Wiring inspection - no damage. Wire testing shows power is good at all points
Anyone else run into this? Could the issue be internal to the CHM?
Landover Laptops 1 posts since Feb 21, 2019
Re: 2021 M2 BHM/CHM issue? Sep 8, 2020 3:00 PM
Sounds like a bad ground, or possibly a bulb not install correctly
Kyle Siebert 3,908 posts since Nov 14, 2014
Re: 2021 M2 BHM/CHM issue? Sep 8, 2020 3:19 PM
On the left rear, are there two red lamp assembly’s? Both work for marker lamps I take it? But
only one works for the brake lamp?
It sounds like it may be miswired at the body builder connecton. You could back probe at the CHM for both
rear marker lamp circuits to see if they both are on with brake lamps on.
Robert Cadell Jr 2,590 posts since Nov 9, 2014
Re: 2021 M2 BHM/CHM issue? Sep 8, 2020 10:14 PM
does this unit pull a trailer? how was the unit built, meaning single rear tail lights and customer added the
option for dual lights? Had a similar unit it pulled a trailer and needed a parameter change for that option.
Just some thoughts.
Robert Cadell Jr 2,590 posts since Nov 9, 2014
Re: 2021 M2 BHM/CHM issue? Sep 8, 2020 10:17 PM
879
6
Front left turn
lamp — current
The BHM measures the current on this cir-
38L
CHM C3, N
1
2021 M2 BHM/CHM issue?
above expected
value
cuit when the
lamp is commanded ON.
This fault indicates that the
lamps connected to this circuit
are drawing more current than
the circuit is designed to supply, or there is a
wiring fault shorting this circuit
to ground. from
the m2 manual
54.12.410 Bulkhead Module
Fault Code Information
Michael Palumbo 1,371 posts since Nov 13, 2014
Re: 2021 M2 BHM/CHM issue? Sep 9, 2020 11:05 AM
If it is the factory Grote light box from BOM 294, it could be as simple as the 1157 bulb.
bulbs in the 1157 socket behave like this.
I have seen 1156
Talbert Simonian 1,741 posts since Nov 21, 2014
Re: 2021 M2 BHM/CHM issue? Sep 11, 2020 12:17 AM
Yes it could be a issue with the CHM. I think Robert Cadell Jr is leading the correct direction. I would unplug
the harness and load test it. Then check it to Ground.
Robert Cadell Jr 2,590 posts since Nov 9, 2014
Re: 2021 M2 BHM/CHM issue? Sep 11, 2020 12:21 AM
And to add on Talbert Simonian comment cross ohm the circuits its a me thing make sure they arent shorted
together.
Kyle Siebert 3,908 posts since Nov 14, 2014
Re: 2021 M2 BHM/CHM issue? Sep 11, 2020 1:58 AM
If it was a parameter issue, it would be both sides acting up though.
Jon Cecil 783 posts since Nov 25, 2014
Re: 2021 M2 BHM/CHM issue? Sep 11, 2020 3:39 AM
2
2021 M2 BHM/CHM issue?
Looking through schematics makes it seem it is a body builder issue. Power for the left tail light is
shared with the right tail light. Like Kyle Siebert said with a parameter, any issue with circuit 23
should be affecting both tail lights. Power leaves BHM C1-K to CHM C3-P. Power then continues
out of CHM C2-G, C1-D, C1-E, and C1-F. There shouldn’t be any wires populated in C1-E or C1F. C2-G goes to the trailer tail relay in the trailer PDM. If only one tail light is having an issue, It
really seems to be a body builder issue. I wouldn’t be surprised if the issue lies in the 7-way.
Ericka Natali 34 posts since May 14, 2015
Re: 2021 M2 BHM/CHM issue? Sep 11, 2020 12:25 PM
* Update *
Cleared codes and customer took truck for body inspection.
Checked codes on return - 12 previously active BHM codes. Left turn lamp is working.
1-4 counts for missing smart switch, ignition switch, vehicle speed, wiper park mismatch, data error, cab dome
light, cigar lighter, starter relay, wiper motor speed & back-up light and alarm horn.
27 Ct - 524247/7 BHM/CHM Ignition Mismatch
3 Ct - 879/6 Left Turn Lamp Circuit
Road tested, hit a bump, light out. On the driver side found chassis harness pinched on top of hose shield
12-22261-000 under the cab causing short. Opened up and found 4 bare wires. Road tested, light works, no
codes came back.
Thank you all for your feedback!
Robert Cadell Jr 2,590 posts since Nov 9, 2014
Re: 2021 M2 BHM/CHM issue? Sep 11, 2020 12:32 PM
Great Job, Good Hunting my friend
Rob “Doc” Cadell
DTNA Master Tech/Elgin Lead Technician
Trans Chicago Freightliner
BFWD
Shorewood,IL
(803)-917-5397
3
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TSB/Document ID: TechTalk7673
Replacement Service Bulletin Number:
MFR Communication Date: 2020-09-11
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Communication Type: Service Bulletin/Repair Instructions
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