Automatic Retarder Control (ARC)
The Automatic Retarder Control system (ARC) is designed to modulate the brakes of a truck during the descent of a long grade. The ARC modulates the brake system in order to maintain a constant engine speed. The ARC system is a pneumatic system on many models except for the 784C, 785C, 785C HAA, 789C, and 793D. On these models, the ARC system is a hydraulic system. The basic function of both types of ARC systems are the same. Some components have been changed on the hydraulic configuration. These components are covered in the following text.
Automatic Retarder Control Air System
Hydraulic ARC system for the 784C, 785C, 785C HAA, 789C and 793D
The ARC will be activated when the following conditions exist:
• The ON/OFF switch is in the ON position.
• The throttle pedal is NOT depressed.
• The parking brakes are in the RELEASED position.
The ARC is not connected to the service brakes and the manual retarder. If the ARC is ENGAGED, air (oil) will flow from the valve of the ARC to a separate relay valve. The relay valve is located near the brake master cylinders.
Air activated ARC valve components located in the compartment in front of the cab
(1) Supply solenoid valve
(2) Control solenoid valve
(3) Auto retarder pressure switch
(4) Retarder pressure switch
Hydraulic activated ARC valve components located on the left frame rail close to the differential
(1) Supply solenoid valve
(2) Control solenoid valve
(5) Purge solenoid valve
(6) ARC accumulator
In order to activate the ARC system, the Integrated Brake ECM sends electrical current to the supply solenoid (1). This will allow pilot pressure to shift the spool in the ARC valve which will result in high pressure air (oil) to flow to the control solenoid valve (2). The ECM will send a proportional current to the control solenoid valve in order to control the amount and duration of pressure that is applied to the brake cylinders.
On machines with the hydraulic ARC system, the ECM will energize a front brake cooling diverter solenoid when the retarder is activated in order to cool the front brakes.
On machines with the hydraulic ARC system, the steering bleed module will control the purge solenoid (5). When the key switch is turned to the OFF position, a timer in the steering bleed control will activate the purge solenoid in order to purge the ARC accumulator.
The ARC is set at the factory in order to maintain a constant engine speed. An engine speed of 1900 rpm +/- 50 rpm should be maintained for the 3500 engines. An engine speed of 2230 rpm +/- 70 rpm should be maintained for the 3400 engines. The engine speed setting is programmable. The engine speed may oscillate beyond +/- 50 rpm of the target when the ARC initiates control of the retarder process. The engine speed should stabilize within a few seconds.
The operator can activate the system by using the ARC ON/OFF switch in order for the ARC to operate properly. The operator should then select the correct gear for the grade, the load, and the ground conditions. The ARC is designed to permit the transmission to upshift into the selected gear of the shift lever. The ARC will apply the retarder after the transmission shifts into the selected gear and the engine speed exceeds 1850 - 1950 rpm for the 3500 engine and 2300 rpm for 3400 engine. The ARC applies the retarder in order to maintain a constant engine speed.
The ARC system also provides Engine Overspeed protection. The ARC will engage the brakes if the following conditions exist:
• An unsafe engine speed is reached.
At 2100 rpm for the 3500 engine and 2475 rpm for the 3400 engine, an overspeed condition will activate a horn and the ARC will be activated. The retarding indicator lamp will illuminate. The ARC will activate the retarder at 2180 rpm for the 3500 engine and 2560 rpm for the 3400 engine.
If the ARC is unable to reduce the engine RPM, the transmission/chassis ECM will upshift one gear. The ECM will only upshift one gear above the position of the shift lever. If the shift lever is in the top gear, the ECM will unlock the torque converter at 2300 rpm for the 3500 engine and 2675 rpm for the 3400 engine.
The ARC also provides service personnel with enhanced diagnostic capabilities through the use of onboard memory. The onboard memory stores diagnostic codes that can be viewed by using Cat ET.
The ECM receives signals from several switches and sensors. The ECM analyzes the various input signals and the ECM sends signals to the output components. The output components are two solenoids and a retarding indicator lamp.
Programmable Speed Adjustment For The Automatic Retarder Control
The control speed of the Automatic Retarder Control (ARC) for the 3500 engines can be set between 1850 rpm and 1950 rpm. The control speed for the 3400 engines can be set between 2160 rpm and 2300 rpm. The control speed can only be programmed when the engine speed is below 1450 rpm.
Note: Some older controls require the engine speed to be less than 1000 rpm and the ARC On/Off Switch should be in the OFF position.
