HVAC Question

My HVAC makes a wierd popping/clicking noise; likely the flap. I just keep the temp set to full hot and it doesn't do it. If it's anywhere other than hot, it will pop/click. Haven't had time to look into it though...

 

Hard to troubleshoot until someone can identify what causes the flap to stop in the open or closed position (and not somewhere in the middle) under normal circumstances (when operating correctly).

 

I had the same problem with my 355. When I first purchased the car a few years ago, the problem was intermittent and then last spring it developed into the problem you are describing. I took the actuator motor (that controls that flap, located under the fuzzy trim piece that surrounds the vent) off of the heater unit. Being a curious person, I cut the actuator motor apart to see how it works. It is very similar to a servo motor for an RC airplane. It has a small electric motor, multiple gears, and a circuit board. In mine, one of the gears had a tooth missing, when the flap reached the end of it's travel to close the vent, the missing tooth would cause the flap to "drop" or re-open and the HVAC ECU would attempt to close the flap again (repeat this every 4-5 seconds). I wasn't able to repair the motor so I bought a new one from Ricambi (PN 63306700).

As you have found out, this is an endless cycle. The owners manual and workshop manual both describe how the HVAC system, no matter what mode you are in (heating, cooling, off) will open and close the flap once every so often to let in "fresh" air. (I guess the designers thought the cars were so air tight that we would suffocate!) . Bottom line, replace the actuator motor.

 

Hard to troubleshoot until someone can identify what causes the flap to stop in the open or closed position (and not somewhere in the middle) under normal circumstances (when operating correctly)

Eric,

When I was having the problem with my "fresh air" flap and also the evaporator vent flap (AC was blowing out of the defrost), I found that both motors work similar to a servo for an RC airplane. The HVAC ECU appears to "learn" where the flap stops are (or should be) and stops the motor at that point. I tried stalling the motor using a pair of pliers and it doesn't stop. At least not without breaking something internally. That was particularly the case with the evaporator vent motor. It (the ECU) has to know where to stop to be able to blend the air at the appropriate vent. As an aside, each time I replaced the motors, on initial start-up, the ECU would run through the flap cycle.

With my evaporator vent actuator motor, I also took it apart to see if I could determine why it wasn't blending the air properly, and I was actually able to "re-time" the motor to make it work properly for a few times, but it went back to it's old habits. I just went ahead and replaced that motor also.

Randy

 

Hi Randy,

I'm not familiar with the evap. vent flap but I would have to say the fresh air flap motor doesn't work like an RC servo (okay, they both have gears inside). RC servos, as you probably know, have built in pots. so their position is "known." The flap motor does not (and thus the reason there are only two wires coming out of it).

Based on your experience then, that just seems to leave a current sensing circuit in the climate control ECU that reads the stall current when the flap is at either end of travel as a way to turn off voltage. However that would not explain why your flap motor would not stop when you grabbed the shaft with pliers UNLESS you simply hooked up the flap motor to a 12 volt source instead of to the "proper" connector on the car.

 

Eric,

Yea, it was probably an over simplification to say the fresh air actuator works "just like an RC servo" since it doesn't have a third wire or pot. To be honest, I don't remember if I left the actuator plugged into the wiring harness or if I used a separate voltage source. However, after reflecting on your comment about the stall voltage being measured by the ECU, I probably just hooked to 12V. That would certainly account for the actuator not stopping when held with a pair of pliers!

Now the evaporator vent actuator....that unit has 3 or 4 wires associated with it and I do remember the internal components being different than the fresh air actuator. I'm pretty sure I saved both actuators (not sure why), I'll look at them again and see if the evap. actuator has a pot.

 

This happened to me as well.

Turned out the actual pin that holds the "fresh air flap" in place was brittle and broken.
I replaced it with the brass pin from Ricambi as well as the actuator motor that went with it. I think all in it was about $150 and an hour of my time.

Now the HVAC all works as it should. Switches vent direction, fresh air to re-circ, etc.

Call Daniel or Mike and tell them what the problem is, they know which parts you need.

RICAMBI FTW!

 

I SEEM to remember plugging my new motor module into the normal connector and then holding the little motor shaft with my fingers trying to stop it turning but I could not. That kind of made me wonder if there really is a current sensing circuit to turn off the darn thing. Maybe there is and mine is bad. Have to do some more checks. Right now mine remains unplugged. Hate the idea of maybe stripping the gears on the new one if there is some other problem :-(