ROD-DOOR 2T3502 - Caterpillar

--------------- END SUPPLEMENT ---------------SUPPLEMENT: APRIL 6,1998Service Magazine SEPD0400, Article 28, Page 27, "New Water Valve Assembly And New Board Control Assembly That Improve Reliability Of HVAC System Are Available."Chart 1 "100-6925 Air Conditioner Group Parts Identification" is incorrect and should be replaced by the following:
--------------- END SUPPLEMENT ---------------A new water valve assembly (2) and a new board control assembly (module assembly) (1) improve the reliability of the HVAC system. The new water valve assembly (2) and new board control assembly (module assembly) (1) are direct replacements for the former water valve assembly and board control assembly.
The cab heater water valve is electronically controlled on these models. The setting of a potentiometer (temperature selector knob) (F) determines the position of the water valve in the heater. The board control assembly (module assembly) (1) is an electronic control that controls the position of the water valve assembly (2), according to an input from the potentiometer. The potentiometer is the temperature selector which the operator may adjust.
The water valve assembly (2) is a rotary water valve controlled by a DC motor. The water valve has an internal feedback potentiometer (2a) (approximately 5000 ohms), which relays the actual position of the rotary water valve to the electronic control.
When the operator adjusts the temperature selector (F), the electronic control sends +9 to 10 Volt or -9 to 10 Volt power to the water valve motor (2b). The motor causes the valve to move in the desired direction until the position of the feedback potentiometer (2a) matches the position of the temperature selector (F).
Component Description
The potentiometer assembly (temperature selector) (F) has a resistance of approximately 5000 ohms from one end to the other. The board control assembly (module assembly) (1) applies approximately 8 to 9 Volts DC across the potentiometer, and measures the voltage of the center terminal to determine the position of the knob. Troubleshoot the potentiometer (F) by disconnecting it from the circuit and measuring the resistance with a 9U-7330 Multimeter (or equivalent). The resistance of the center terminal should vary smoothly as the knob is turned and the total resistance should be about 5000 ohms.
The board control assembly (module assembly) (1) provides +9 to 10 Volt or -9 to 10 Volt power to the water valve motor (2b) when the position of the temperature selector (F) is different than that of the water valve potentiometer (2a).
The water valve assembly (2) consists of a motor (2a) and a feedback potentiometer (2b). The motor is connected to the pink and blue wires. It can be checked by connecting those two wires to a 12 Volt DC source. The motor should run in either direction depending on the polarity.
Do not apply 12 Volts to the water valve motor leads for more than a fraction of a second after the valve reaches the end of its travel, or motor damage will result. Do not apply more than 12 Volts to the water valve motor.
The water valve feedback potentiometer can be checked with a 9U-7330 Multimeter