ALLISON-OM1334EN

Hydraulic retarders are available on some models of the AT 500, MT 600, and HT 700

Series transmissions. The AT 500 retarder and the MT 643R input retarder are located

between the torque converter and the transmission gearing. The MT 600 output

retarder is installed on the rear of the transmission in place of the output housing. The

HT 700 retarder is installed between the torque converter and the transmission

gearing. The function of the retarder is to provide auxiliary retardation.

The AT 500 Series retarder is available on models without lockup or PTO provision.

The MT 643R retarder is available on models with lockup but without PTO provision.

The retarder is an integral part of the transmission which slows the vehicle by

applying a hydraulic braking force. Retardation and cooling are greatest at high input

(engine) speeds. Maintain high input speed by preselecting a lower range. The

external retarder controls (OEM-furnished) consist of a master switch and typical

options shown in Table 1. External retarder controls apply the retarder at the

50 percent or at the 100 percent level. External controls are also required to

automatically turn off the retarder in first range or when the vehicle anti-lock brake

system is activated. External controls also provide a signal to the modulator control.

The signal causes downshifts to occur at a higher speed when the retarder is operating

than would normally occur during closed throttle operation.

The MT 600 Series output retarder is mounted on the rear of the transmission as an

integral part of the transmission and provides your vehicle with an auxiliary

retardation system to slow the vehicle. The unit combines hydraulic and friction clutch

retardation capabilities. In many applications, the retarder is applied in conjunction

with the service brakes.

Table 1. Typical AT/MT Input Retarder Controls

Type

Description

Amount Of Application

Manual

Manual 3-Position Switch

0%, 50%, Or 100%

Automatic

12V or 24V Signal When Closed Throttle Is Sensed

50% Or 100%

12V or 24V Signal When Brake Pedal Is Applied

50% Or 100%

Combination Manual 2-Position Switch and Closed Throttle

Sensor

50% From One Device,

100% From The Second

Device

Manual 2-Position Switch And Brake Pedal Apply

50% From Switch,

100% From Brake Pedal

Closed Throttle Sensor Plus Brake Apply Sensor

50% From Closed Throttle,

100% When Brake Applied

ALLISON AT, MT, HT SERIES ON-HIGHWAY TRANSMISSIONS

OPERATOR'S MANUAL

2000