Cab Roof Harness Repair and Pass-Through Hole Modification
M-375-001
(March 2009)

Valid for
All MRU

Case description
On certain MRU model chassis, the possibility exists for the cab roof harness to rub against the edge of the passthrough hole where the harness transitions down the B-post on the left-hand side of the cab. This will result in an
eventual short to ground due to harness wire insulation rub-through, and become evident as various problems with the
gauge illumination lamps, clearance lamps, marker lamps, headlamps and/or the headlamp switch.
Area Where Harness Damage Can Occur

Note: Not applicable to Mack Trucks Australia.

Repair Procedure
1. Secure the chassis for service, apply the parking brakes and block the wheels to prevent the vehicle from
moving.
2. Disconnect the batteries by disconnecting the negative battery cable(s) first, and then the positive cable(s). If
equipped with a battery disconnect switch, turn the switch to the OFF position.
3. Completely drain the air system.
4. Remove the headliner from the cab as follows:
a. Remove the radio (if so equipped), and the radio box.
Remove Radio and Radio Box

b. Remove the camera mounting plate (if so equipped).
c. Remove the speakers.
Remove Speakers and Camera Mounting Plate

d. Remove the passenger-side dome light switch.
e. Remove both the left- and right-side sun visors.
f. On both sides of the cab, pull the door seals from the door opening, starting at the top to
approximately half-way down the opening.
Pull Door Seals from Door Opening

g. Remove all accessible head liner retainers.
h. Remove the screw that secures the air horn lanyard to the cab ceiling.
Remove the Screw Securing the Air Horn Lanyard

i. Remove any remaining head liner retainers, and then remove the head liner from the cab.
5. Remove the upper seat belt anchor bolt.
Remove Upper Seat Belt Anchor Bolts

6. Remove the left- and right-hand rear sidewall panels from inside the cab.
7. Locate the cab roof harness at the left-side B-post. Pull the harness upward through the pass through hole and
inspect for damage. If damage is evident, repair the harness by cutting the damaged sections of wire from the
harness, and then splice the wires together using solder-type splice connectors and adhesive-type heatshrinkable tubing to seal the splice(s).
8. Inspect the harness pass-through hole at the top of the B-post.
Inspect Pass-Through Hole at Top of B-Post

The hole should be completely open and round. If not, follow steps “a” through “o:”
a. Disconnect the supply and control air lines from the air horn valve. Pull the air lines back toward the
B-post, and then down through the pass-through hole.
Note: If equipped with a factory-installed rear-view camera, pull the camera harness along with the air
horn lines, back toward the B-post and down through the pass-through hole.
b. Disconnect the marker lamp, clearance lamp, dome lamp, radio, etc. connectors across the front of
the cab roof.
Disconnect Cab Roof Harness Connectors at Front of Cab Roof

c. Feed the roof harness down through the B-post pass-through hole until completely through the hole.
d. Using a die-grinder, completely open the pass-through hole, and then grind the edge of the hole until it
is round.
e. To prevent rust, use a vacuum cleaner to clean the metal particles created during the grinding
operation from the B-post.
f. Reinstall the cab roof harness by feeding it up through the pass-though hole, and then reconnect the
connectors across the front of the cab roof.
g. Pull the air horn line and rear view camera harness (if so equipped) back into place.

9. Install a length (approximately 61 mm [24 inches]) of 1/2- to 3/4-inch diameter protective conduit over the
repaired area, being sure to cover the repaired area with approximately 305 mm (12 inches) of conduit on each
side of the splice. Secure the protective conduit to the harness with electrical tape.
10. Using a hacksaw, slice through three grommets (part number 25085999) to allow installation over the harness.
Slice Grommet (Part No. 25085999) to Allow Installation Over Harness

11. Install one grommet around the roof harness at the pass-through hole in the B-post, and then insert the
grommet into the pass-through hole. Install the remaining grommets around the turn signal harnesses on both
the left- and right-hand sides of the cab, and then insert them into the pass-through holes in the cab structure.
Grommet Installation Locations

1. Grommet Location (B-Post pass-through Hole)
2. Grommet Location (Turn Signal Harness, LeftHand Side)

3. Grommet Location (Turn SIgnal Harness, RightHand Side)

12. Reinstall the cab back panels on both the left- and right-hand sides of the cab.
13. Reinstall the cab head liner, sun visors, radio box, radio, speakers and rear view camera mounting plate (if so
equipped).
14. Reinstall the seat belt upper anchors, air horn lanyard and the passenger-side dome lamp switch.
15. Reinstall the cab door seals.
16. Reconnect the batteries by reconnecting the positive battery cable(s) first, and then the negative cable(s). If
equipped with a battery disconnect switch, move the switch to the ON position.
17. Check operation of the clearance lamps, marker lamps, dome lamp, radio, etc. to ensure all connectors were
connected correctly.

Warranty information
Reimbursement
This Repair may be eligible for reimbursement if a product failure was experienced within the time and
mileage limits of the applicable warranty coverage. Reimbursement is obtained via normal claim
handling process.
Claim Type (used only when uploading from the Dealer Bus. Sys.)

01

Labor Code
Primary Labor Code

7246D 00 56 — 3.4 hrs., time
allowed to inspect cab roof
harness at B-post passthrough hole, repair harness
as required, repair B-post
pass-through hole and install
three grommets in passthrough holes at B-post and

left- and right-hand side cab
structure. Does not include
“take-charge” time.
Causal Part

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Issued by
Technical Service
Mack Trucks, Inc. engages in a continuous program of testing and evaluating to provide the best possible product.
Mack Trucks, Inc., however, is not committed to, or liable for updating existing chassis.