Troubleshooting the Heating System in a Pontiac Montanaby Don Bowman
In order to troubleshoot the heating system in a Pontiac Montana, it's important to understand how the heating system works in this vehicle. The hot antifreeze is picked up at the water pump and travels through a hose to the heater core. The hot antifreeze travels through the heater core and out the other side and sent back to the thermostat by another hose. The heater blower and the heater core, along with the air conditioning evaporator, are all encased in a housing under the dash.
The housing is rectangular in design and is segmented with a segment for the blower motor, the heater core and the air conditioning evaporator. All of these segments have vertical doors that either open or close, changing the direction of the air flow. The doors are actuated by the control panel on the dash, and an individual electric servo does the work of moving the doors by pushing or pulling on a rod for each door. The control panel has a control box that receives the temperature signals, and when the door is open or closed controls the fan speed to keep the temperature constant. Whether the fan speed is in automatic or manual, the speeds are achieved by the use of a blower motor resister in close proximity to the blower motor.
The most common problem is that the blower motor will not work or will work only on one speed. To diagnoses this problem, a common circuit tester is necessary. Check the fuse and the relay first. They are located in the fuse block in the driver's side compartment. Check the fuse for power and make sure it is good. If the fuse is good, check the relay by turning the ignition key and pulling the relay out of its socket. Check to make sure there is power to two of the terminals in the relay receptacle. If not, and there is only one with power, turn the key to "Off" and check again. If there is still only one with power, the fan control switch is bad. If there was no power with the switch off, there is a problem with the power wire from the fuse to the relay.
If the fuse and the relay are good, check the blower motor resister. Remove the bottom of the dash on the passenger side to gain access to the blower motor. The blower motor resister is right next to the blower motor, just to the left side and toward the rear. Pull the electrical connector off and turn on the ignition key. Turn on the fan for the blower motor. Check the connector for power on one of the terminals. If there is no power, the control head on the dash is bad. If there is power, plug it back in and pull the 2-wire connector off the blower motor and test the connector for power. If there is no power at the connector, the blower motor resister is bad. If there is power, the blower motor is bad.
If the fan works OK but some or all positions of the level control do not work and the air cannot be controlled, one or more doors are not functioning properly. If one of the doors does not open, there will be no heat. Remove the glove box to gain access to the heater and AC housing. Start the engine and turn on the heat and set the fan on high. Rotate the knob to regulate the position of the air exchange while watching the motors on the housing. The motors can be seen moving the arms on the doors if they are operating properly. Replace the one that does not move. If none of the motors move, pull the connector off the closest one. Check for power at the connector while repositioning the knob on the control panel. If there is no power at the connector, replace the control head.
Don Bowman has been writing for various websites and several online magazines since 2008. He has owned an auto service facility since 1982 and has over 45 years of technical experience as a master ASE tech. Bowman has a business degree from Pennsylvania State University and was an officer in the U.S. Army (aircraft maintenance officer, pilot, six Air Medal awards, two tours Vietnam).