Mack Trucks Issues Liftable Axle Installation Guidelines

Mack Trucks Issues Liftable Axle Installation Guidelines
December 17, 1996

Liftable Axle Installation Recomendation

This is criteria that must be met, but may not be limited to the following.

1. The chassis must be equipped with the proper amount of air volume.

A. FMVSS121 section 5.1.2 describes the air volume requirements for the brake system.
B. The various axle manufacturers also have air volume requirements for the up and down function of the axle.
C. All liftable axle functions for braking, and up and down must be supplied with air from the expansion air reservoir.

2. The added axle must have its own service brake relay valve added to the chassis.

A. FMVSS 121 section 5.3.3 describes the brake actuation timing requirement.
B. For straight service brake function, the Bendix R-12 relay valve P/N 103009 matches the performance of Mack installed valves.
C. Use 1/2” plastic air brake tubing from the expansion air reservoir to the supply port of the lift axle relay.
D. install 3/8” plastic air brake tubing from the "T” fitting in the control port of the rear drive axle service brake relay valve to the control port of the lift axle relay.
E. Install 1/2” supply hoses from the delivery ports of the lift axle relay to the lift axle brake chambers.

3. The liftable axle brake dynamometer certification rating must match or exceed the GAWR that lift axle witl be certified to.

4. The liftable axle must have the proper combination of mounting ride height and tire/wheel equipment so that is it capable of carrying the GAWR that the lift axle is certified to.

5. The chassis must have the complete vehicle documentation sticker affixed in the required location. This sticker must state the GAWR of each axle, and the GVWR of the vehicle. See Mack Trucks Service Bulletin #SB 91 G001.

NOTE: FMVSS1 21 is constantly evolving with new requirements. it is the responsibility of axle installer to keep pace with these requirements, making sure the components they add to the chassis comply with FMVSS 121 regulations in effect at the time of installation.

Affect of Incorrect Plumbing

1. Violation of Federal Regulation FMVSS 121.

2. Can jeopardize the Mack Trucks air system and knock it out of compliance with FMVSS 121.

3. The chassis does not have enough air volume to operate the brakes and the up and down function of the axle.

4. Creates an air imbalance between the front drive axle brakes, and the rear drive axle brakes.

NOTE: When a Mack chassis leaves our manufacturing facility, it is in compliance with all FMVSS121 regulations in effect at the time of manufacture. This compliance does not cover the requirements of additional axles that are added, unless it is specified at the time the chassis is ordered.

Air System




MR688S-7783, ordered with standard air capacity arrangement.

Air reservoir arrangement - 22QEA41101 P38 Consists of: 
1 - 22QE2404P12 - 2100 capacity tank.
1 - 22QE2404Pl4 - 4100 capacity tank.
                            6200 total volume.

Brake chambers:

Front steering axle - 19QE452 - 74.0 x 12 x 2 = 1776
Rear drives - 19QE498 - 87.5 x 12 x 4 = 4200

1776 + 4206 = 5976 minimum to meet FMVSS 121 section 5.1.2.

6200 - 5976 = 224 left above chamber requirement.

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