Sounds like a good shift there. IS the one you've taken out serviceable then or is it borked? Only curious after your breakdown and assessment, not sure what could go wrong with them?
Thanks for the question Sam, and yes I think they are both now serviceable. From my experience of similar things electronic; servos, mixing desks etc., I've seen a number of 'modes of failure' that seem to put these flap motors at risk. Too much torque for the plastic gears being one, but doesn't seem to be happening here ( but it takes a lot of force to move that flap it strikes me ). Worn brushes and commutator in the motor again does not seem to be an issue and there looks to be a potential to repair. Lubrication failure again - no, the grease still had the right effect even if drying out.
But the thick film servo potentiometer track and wiper looks like a weak spot. The track itself can wear and create a 'soup' of carbon particles or debris, dust, grease etc., that can prevent a good, even contact with the wiper. In this case there are two tracks and the wiper has many fingers that bridge them at a specific point, so 'double trouble' you could say. Of course, all this manifests itself for the servo, just like a very noisy volume control on an old radio. It could even result in the servo jittering and hunting around constantly which would accelerate the wear.
So what I found was lots of drying up grease, especially under the potentiometer wiper. I removed all the grease including from gears etc., and replaced all but what was on the pot with silicon grease that does not attack plastic ( and you should be using on your battery terminals since it doesn't catch fire either ! ). On the wiper of the pot I put some RadioSpares ( showing my age again, probably RS Components to you ) 'Contact Treatment Oil'. Not sure of the exact formulation of that, but probably silicon oil amonst other things. Anyway, it really is magic stuff for all sorts of contacts and again doesn't catch fire.
Having shoved them back together, I needed to test them. Another potential problem with the flap motor is incorrect temperature feedback from the temperature sensor on the HVAC console. A tiny fan motor behind vertical slits with a thermistor just underneath the facia. So I had blown this out previously with an air duster, before deciding to work on the flap motors. Please note not to 'play' and let the thing scream away like a little siren. To send it to phenomenal RPM like that can break the oil film in the bearing and ruin it. I often put a finger ( but not when moving ) or plastic spout between blades to stop them spinning when doing this.
To test the flap motors, I first tried them by plugging in the cable and dangling them down so I could see the output shaft. A bit of bent wire, serving like a watch hand would enhance visibility here. I then put the ignition on and made sure the fan was on 'auto' and 'mode' to vents and floor, but not all the way to the right - demist, as that seems to behave differently. Then I slowly changed the temperature settings down to LO and then up to high and watched the shaft move. Since it got no corresponding 'feedback' from temperature sensor, it did not behave quite normally, but showed it was moving smoothly thru a considerable angle to try to decrease / increase heat by moving the shaft that would normally move the flap.
Sorry for the long winded explanation. My job used to be to try and fix oilfield systems around the world, by looking at spare equipment in the workshop from a base in the UK. They were often places you did NOT want to go to. So I found that clarity and completeness saved by butt many times. Cheers - Noel
