348 Heater Valve Motor Testing

I can only give you the basics, some of the system is straightforward, some of it is too complex for me to understand.

The servo motor for the heater valve has five wires going to it. Four of these wires run to the ECU for the climate control system on the center console. The fifth wire runs to a common chassis ground. The illustration in the wiring diagram shows that the servo motor itself is operated by two wires: a positive (red/white) and a negative (yellow/black). The other three wires are apparently operating a circuit that can read the open/closed position of the valve through a potentiometer of some sort.

Taking my spare heater valve in hand, I can see that the travel of the arms is limited by stops, both on the electrical servo and on the plastic valve itself. The first thing to do is to make sure that these arms are "clocked" properly, so that the range of motion between the stops on the servo is in alignment with the range of motion between the stops on the valve. If not, then you are not able to obtain full travel in one direction or the other. It's easily adjustable .

Next, I hooked the servo up to a 12 volt power source: positive to the pin that corresponds to the red/white wire, and negative to the pin that corresponds to the yellow/black wire. This moves the lever from the full cold position (no coolant flow through the valve) to the full hot position (valve all the way open). By reversing the polarity, the motor moves the valve through the full travel in the opposite direction.

Therefore, the climate control ECU must read the temperature of the ambient air, and compare it with the temperature set by the driver on the control panel. If more heat is needed, it tells the servo to open the valve further and allow more heated water to flow through the valve, and into the heater core. If less heat is needed, it reverses the polarity to the motor and reduces the flow of hot water through the valve.

The E5 error code tells you that something in the servo is giving a reading outside the normal range. It could be that the motor is sticking. It could be that the valve is sticking and the motor is not able to open and close it. It could be that the ground wire isn't making proper contact. It could be that the potentiometer is fried. It could be that the ECU is sending conflicting information to the servo. About all you can do is to compare the operation of a known good unit with a suspected faulty unit, and see if something is obviously wrong.

It may be possible to open the servo and examine its inner workings, as it is screwed together rather than bonded or riveted. I haven't tried that.

Good luck!
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