I guess my previous post kicked everybody's @ss, so I'll try a different approach. Does anyone know where to obtain air flow diagrams (inlet air to blower, through AC evaporator and heater core and on into passenger compartment)? I bought the service manual for my 89 E30, but no flow diagrams were there. I had my AC evap replaced, and since then I've had no heat flow into the passenger space. Sure does s#ck driving around in cold weather with no heat. Your inputs are appreciated.
Air flow diagrams for 325?
- Thread starter themoonandthestars
- Start date
You don't need air flow diagrams to diagnose this problem. I have never seen a schematic showing the air flow in the HVAC unit, but having worked on the A/C & heater unit in my car, I can describe the airflow path for you.
With the recirculation button "Off," air is drawn from the air plenum behind the hood, through the blower fans, and enters the HVAC box. The first thing that the air encounters upon entering the HVAC box is the A/C evaporator. All air passes through the evaporator, whether or not the A/C is on and whether or not you are using heat. After passing through the A/C evaporator, the air encounters the mixing valve, which is essentially just a flapper/damper connected via a cable to the air temp knob on the control panel. The setting of the air temp knob varies the position of this damper which controls the percentage of the air that goes through the heater core before entering the ductwork system and the percentage of air that bypasses the heater core before entering the ductwork system. After that, the air enters the ducting (the route of which is controlled by a few dampers/valves connected via cables to the three airflow selectors on the control panel) and is discharged into the passenger compartment. In addition, there is an electrically controlled valve in the inlet coolant line carrying hot coolant to the heater core. This valve defaults to an open position so that coolant flows to the heater core. When the temperature knob is turned to the coldest setting, a switch in the knob is triggered and sends current to the coolant valve, closing it and stopping the flow of hot coolant to the heater core.
So, one of four things is wrong with your HVAC unit to prevent the heat from working. 1) The cable connecting the temperature control knob to the air mixing damper is either disconnected or adjusted incorrectly, 2) The heater control valve (that electrically controlled coolant valve) is stuck in the closed position, 3) The switch in the temperature control knob mechanism is stuck, continually energizing the solenoid in the heater control valve, resulting in no coolant flow to the heater core, or 4) The heater core is clogged.
I've got my money on 1) above, since this cable would have to be disconnected in order to remove the HVAC unit to replace the evaporator. You'll have to pull the center console out of the car to see the cable and the plastic arm it connects to that operates the air mixing damper.
With the recirculation button "Off," air is drawn from the air plenum behind the hood, through the blower fans, and enters the HVAC box. The first thing that the air encounters upon entering the HVAC box is the A/C evaporator. All air passes through the evaporator, whether or not the A/C is on and whether or not you are using heat. After passing through the A/C evaporator, the air encounters the mixing valve, which is essentially just a flapper/damper connected via a cable to the air temp knob on the control panel. The setting of the air temp knob varies the position of this damper which controls the percentage of the air that goes through the heater core before entering the ductwork system and the percentage of air that bypasses the heater core before entering the ductwork system. After that, the air enters the ducting (the route of which is controlled by a few dampers/valves connected via cables to the three airflow selectors on the control panel) and is discharged into the passenger compartment. In addition, there is an electrically controlled valve in the inlet coolant line carrying hot coolant to the heater core. This valve defaults to an open position so that coolant flows to the heater core. When the temperature knob is turned to the coldest setting, a switch in the knob is triggered and sends current to the coolant valve, closing it and stopping the flow of hot coolant to the heater core.
So, one of four things is wrong with your HVAC unit to prevent the heat from working. 1) The cable connecting the temperature control knob to the air mixing damper is either disconnected or adjusted incorrectly, 2) The heater control valve (that electrically controlled coolant valve) is stuck in the closed position, 3) The switch in the temperature control knob mechanism is stuck, continually energizing the solenoid in the heater control valve, resulting in no coolant flow to the heater core, or 4) The heater core is clogged.
I've got my money on 1) above, since this cable would have to be disconnected in order to remove the HVAC unit to replace the evaporator. You'll have to pull the center console out of the car to see the cable and the plastic arm it connects to that operates the air mixing damper.