Jimmy, as you press the pedal down it will engage the transmission brake. It may not activate unless the throttle is a 0%. The transmission brake works well BUT they will heat up the transmission quickly so you need to watch the transmission temperature gauge closely when using it going down a long hill. The Allison transmission retarder is hydraulic. The retarders were switched as off or on but on some units, a variable amount of retarding power could be controlled by a lever or pedal. You likely have the HT-748 transmission rather than the HT-740 that most of us have. Under normal operation, the HT-7xx series runs 180-220 degrees F. The max temperature from the converter output should not exceed 300 degrees without the retarder or 330 degress with the retarder. The 748 used an output retarder. The output retarder has a multiplate clutch pack and a vaned rotating element. The control valve releases fluid to fill the retarder cavity and applies the retarder clutch. This absorbs energy and converts it to heat.
According to Allison:
Quote:
CAUTIONS: Observe the following cautions when driving a vehicle equipped with an input or output retarder.
APPLY AND OPERATE THE RETARDER WITH ENGINE AT CLOSED THROTTLE ONLY.
DO NOT USE TH ERETARDER WHEN ROAD SURFACES ARE SLIPPERY. De-energize the system at the master control switch.
OBSERVE TRANSMISSION AND ENGINE TEMPERATURE LIMITS AT ALL TIMES. Select the lowest possible transmission range to increase the cooling system capacity and total retardation available.
In the event of OVERHEATING, DECREASE VEHICLE SLPEED TO REDUCE RETARDATION POWER REQUIREMENTS.
OBSERVE THE RETARED "ALERT LIGHT" to ensure that the vehicle control system is functioning properly.
DO NOT OPERATE THE INPUT OR OUTPUT RETARDER SIMULTANEOUSLY WITH AN ENGINE EXHAUST BRAKE. Extreme torque loads can be produced in the range section, damaging the transmission.
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Quote:
R - RETARDER NOT FUNCTIONING PROPERLY (Models With Output Retarder)
Excessive Retarder Response Time
1. Priority valve sticking - Clean valve
2. Pressure regulator valve sticking - Clean valve
3. Charging valve sticking - Clean valve
4. Air lines leaking or incorrect size - Replace air line
5. Clutch piston seals leaking - Replace seals
6. Accumulator piston seals leaking - Replace seals
No Retarder Operation at Low Speed
7. No air supply - Check and repair air supply
8. Regulator charging valve sticking - Clean or replace valve
9. Pressure regulator valve sticking - Clean valve
10. Charging valve sticking - Clean valve
11. Check valve malfunctioning, omitted, or installed backwards - Replace or install check valve correctly
12. Clutch piston seals leaking - Replace seals
13. Retarder clutch worn - Replace clutch
No Retarder Operation at High Speed
14. Air supply too low - Check air supply, correct as necessary
15. Air line to retarder control valve ruptured - Replace air line
16. Air control defective - Repair or replace control
17. Retarder control valve sticking - Clean or replace valve
Low Main and/or Retarder Pressure
18. Charging valve sticking - Clean or replace valve
19. Pressure regulator valve sticking - Clean or replace valve
20. Rotor hub seals leaking - Replace seals
21. Accumulator piston seals leaking - Replace seals
22. Clutch piston seals leaking - Replace seals
No Retarder Output
23. Throttle (fuel) interlock/wiring defective - Repair interlock/wiring. (Refer to vehicle service manual.)
24. Master switch malfunction - Repair master switch. (Refer to vehicle service manual.)
25. Air actuator piston sealrings damaged - Repair air actuator piston sealrings, retarder valve body
26. Retarder valve sticking - Repair retarder valve body.
Harsh Retarder Apply
27. Plug valve installed incorrectly - Install valve so that dimple on valve is toward springs
28. Pressure regulator valve spring incorrect of broken - Replace spring
Retarder Will Not Release
29. Brake pedal assembly or manual apply valve malfunctioning - Repair brake pedal assembly or manual apply valve. (Refer to vehicle service manual.)
30. Air actuator piston sealrings damaged - Repair air actuator piston sealrings retarder valve body
31. Retarder valve sticking - Repair retarder valve body.
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The transmission retarder should be used the same way as the jake brake except the transmission temperature should be monitored much more closely.