Ground Precautionary Messages (GPM) (cont)
Mr. Terry Callahan, M939 ABS Program Quality Assurance, DSN 786-5932, Commercial (586) 574-5932, E-Mail:
Mr. Mohan Khabra, M939 Lead Engineer, DSN 786-7833, Commercial (586) 574-7833, E-Mail: firstname.lastname@example.org.
Mr. Reginald Dionne, MWO Coordinator, E-Mail: email@example.com.
Mr. Ken Yoesting, Headquarters USAREUR/7A ODCSLOG, DSN 370-8282, Commercial 01149-(0)6221-57-8282, E-Mail:
Description. Inspection of USAREUR M939 series trucks retrofitted with the anti-lock brake system (ABS) modification
discovered the fact that contractor teams mistakenly applied an air line and its fittings at the location of the primary air tank
which should have been connected to the secondary air tank. At the same time, application teams generally left the existing air
connections to the secondary air tank untouched. The proper ABS MWO procedure calls for disconnecting an existing red tube
attached to the secondary air tank and reconnecting that line to the primary air tank as its source of air. Into its place, the
MWO calls for installation of a new line coming from the ABS "double-check valve # 7" to be connected to the secondary air
tank. The mistake basically reverses those two connections. It installed the "new line" to the primary air tank instead of the
secondary air tank. It then left the secondary air line as found. However, the M939 brake system still functions safely despite
these reversed connections. M939 trucks accidentally mis-plumbed during ABS retrofit reportedly passed all QA inspections.
Post-application driving and braking tests also failed to reveal any operational problems with the truck's braking system. Only
a total, catastrophic loss of system pressure from the secondary air tank exclusively could result in an unsafe braking
condition. System engineers and safety technicians deem the probability of such an occurrence happening as "improbable" or
"unlikely to occur but possible". Trucks affected do not revert back to M939 FOV's 40 mph speed restrictions per safety of use
messages SOUM 98-07, "Safe Operating Speeds for M939 Family of Vehicles". Trucks affected also remain operationally ready
as is. Repaired trucks do not require re-testing of the vehicle's brake system as brakes themselves are not affected by these air
Expected results if failure occurs.
(1) Total loss of air from the primary air tank (only) causes M939's spring brakes to apply, once air pressure drops
below 60 psi. Front axle brakes continue to work normally.