ACtion Magazine - July/August 2016 - (Page 34)

Freightliner M2 HVAC Control Testing Attention! The evaporator sensor design was changed after January 8, 2007. This new design changed the resistance values, which are different on vehicles built before January 8, 2007. Evaporator Probe Circuit Test Evaporator temperature probe check: With key off, engine off, disconnect and remove the probe from the evaporator box. Fill an insulated cup with ice and add water. (Figure 11) Insert a thermometer and allow the mixture to reach 32 degrees F. Place the tip of the probe into the mixture for five minutes. Measure the resistance across the pins of the probe while the probe tip is still in the ice and water mixture. For trucks built before January 8, 2007, the resistance at 32 Degrees F, should be: 16,000 to 16,730 Ohms. For trucks built After January 8, 2007, the resistance reading at 32 degrees F, should be 8910 to 9090 Ohms. If the resistance is not within the correct range, replace the temperature probe. Evaporator temperature probe circuit test: With key on, engine off, and the sensor probe connector disconnected, measure the voltage across the temperature probe connector terminals. The reading should be 5 volts. If the reading is not correct, check for an open in circuits 338K and 338GP. If wiring is ok, replace the control head. M2 Blower Motor The M2 uses a brushless magnetic motor rather than a conventional design that utilizes carbon brushes contacting a commutator to make the motor turn. The advantages of brushless motor are long lifespan, low maintenance, high efficiency, no electronic noise and precise speed control through use of an electronic controller. The blower motor power and ground are supplied directly to the blower motor. The blower motor speed is controlled by the fan switch on the control head. The control head sends a pulse width modulated (PWM) signal to the blower motor. The frequency of this signal is approximately 2000 Hertz (Hz). The PWM varies with the fan switch position. The blower motor incorporates three protection modes: reverse voltage protection (the motor will not operate if the power circuit (98F) and the ground are reversed); current protection (if the motor exceeds the maximum limit of 23.5 Amps, the speed will be reduced until the current is within limits); and temperature protection: If the motor's internal temperature sensor detects too high a temperature, the motor's speed will be reduced to 1000 RPM. If the temperature remains too high, the speed is reduced to 500 RPM, and if the temperature is still too high, the blower motor will shut down until it cools off. (Figure 12) Blower Motor Circuit Tests Main power to the blower motor: With key off, blower motor connector removed, measure voltage with a DMM between pin 4 of the blower motor connector and the negative battery post. The reading should be 12 volts. If the voltage is not present or is below 12 volts, check fuse F2 in the power distribution module 34 ACTION * July/August 2016 (PDM) mounted on the driver's side inner fender well. If the fuse is blown, check for shorted wiring or a damaged blower motor. Check for an open in circuit 98F. Blower motor ground circuit test: With key off, blower motor connector disconnected, measure voltage between Pin 3 of the blower motor connector and the positive battery post. The reading should be 12 volts. If the reading is not correct, check the motor ground circuit for damage or opens. PWM signal from the control head to the blower motor: With key on, engine off, blower motor connection connected, back probe pin 4 and 5 at the blower motor connection, with a DMM set to the Hertz range. Change the blower speeds and observe the frequency change. Blower off and in the high speed setting, the correct reading is 0 Hz. At all other speeds, the Hz reading will be about 1995 Hz. (Figure 13) If the values are incorrect, check circuit 338H. If circuit is good check the control head. Blower motor voltage drop tests: With key on, engine off, blower motor connector connected, back probe pin 4 at the blower motor connection with a DMM, connect the other lead to the positive battery post, and measure the voltage with the blower motor on high speed. The reading should be less than 0.5 volts. If the value is not correct, look for high resistance in circuit 98F. Check the ground side with a DMM. Back probe pin 3 at the blower motor connection, with the blower motor on high speed. The voltage reading should be less than 0.5 volts. If the reading is not correct, check the ground circuit for excessive resistance. ❆

Table of Contents for the Digital Edition of ACtion Magazine - July/August 2016

T/CCI's Wind Tunnel: Let's Test It
CAT Training @ T/CCI
Outlook
Service Port
Leonard's Law
Virtual View
Last Watch
By the numbers
Industry News
Association News
New Products

ACtion Magazine - July/August 2016