Defrost fan rehab and rebuild thread (fresh air fan), or How the heck do if fix this thing? Options

[DRPHIL914]

I have been dealing with this for 12 years, a non-running defrost fan. i even pulled this out 2 years ago and rebuilt it with new flapper seals and main body seal then made sure fan turned but didnt bench test the motor or resisitors, so as a result it still does not work properly. i will be documenting this because 914rubber and Mark will soon be putting together a kit for this that includes all the hardware seals and even a motor i believe, BUT i still have problems and questions!!! so i am starting this thread to get information on diagnosing the common causes of it not working and how to fix it, and then document the rebuild and replacement of the motor, fan, resistor and maybe the control unit in dash. Others have documented the reinstallation of the fan and the cables so i will not duplicate that. and we may want to link other threads hear that have done that as well.
Mark is sending me a kit soon, so while i wait for it, i will have to get some more information about the wiring and resistors function and how to test them .

I had this out last week and tested 3 different control units due to thinking that my issue was a control issue, because it runs on one speed, #2, and anything else does not work and it will then throw the fuse. most assume this would be caused by bad slider unit but i tested 3 of them and 2 are like new with no wear on the sliders , still same result. so i am suspecting the resistors /plug aparatus .
If anyone has done this and cares to share how to examine and test that for proper function, lets start there.


Resistor function, which lead is which and examination of the control units
I will take pictures of mine tonight and post those soon. once this figured out and fixed i will do a full step bystep on reassembly too.

Looking for lots of help and input on this, thanks!!!

Dr. Phil

[Superhawk996]

@DRPHIL914

Agree with @bbrock 's comments, need to know exactly where you measured from both initially and on this latest round of measurements.

Before we go there, let's acknowlege something changed that doesn't make sense. If you were measuring 0.8 ohms on all three settings before and now you're measuring higher resistances with the resistor pack disconnected from the motor, something is wrong.

The motor + a resistance value should always be higher than just measuring the resistors alone. Since resistances went up since 1st round, something isn't correct.

Another key item that is basic to electronics work is that in order to measure resistance, the component (or system) under measurement must be disconnected from the rest of the system. For example, we cannot measure the resistance of the fresh air motor + resistor with the motor assembly plugged into the rest of the vehicle wiring and HVAC slider controls. I assumed the fresh air box was completely disconnected from the vehicle when the 0.8 ohms was reported but maybe I made a bad assumption despite my own Rule #1. (IMG:style_emoticons/default/laugh.gif) See, we all do it despite our best attempt.

This is probably a good point to talk about Rule #4, half splitting. The goal is to divide things in 1/2 to isolate which side the problem is on. By disconnecting the fan motor & resistor assembly, we dive the system into two. The vehicle side (vehicle wiring, HVAC control, connectors) and the fresh air box assembly (motor, resistor, and the short bit of wire inside).

I had assumed we were working on just the 1/2 system of the fresh air box completely disconnected from the vehicle and wiring. Is that valid?

To continue the 1/2 split conversation, we can now split the fresh air box system in 1/2 again. We can divide system to the motor only disconnected from the resistor pack, and the resistor+stub of wire that connects to the motor.

So on the resitor pack; disconnected from the motor, (1/4 of the sytem as we currently have it divided) I'd like you to measure the resistors only (measure one resistor post to the other resistor post) with nothing else connected to it. That will give us the value of the two resistors alone.

By inspection we can see the bi-metallic switches are open. We might measure that later just to be sure but for now let's just get the resistor values and see if they come close to Brent's or BeatNavy's measurements and/or markings on the housing.

The other thing we can do is to measure the other 1/4 of the system (the motor only). With the resistor pack disconnected from the motor, what does the motor resistance measure? Does it change drasticallywhen you rotate the position of the fan impeller from one position to another?

We can also physically test just the fan -- it can be hooked to a 12 volt battery to see if it spins.

Let's try this and see where we end up as we try to resolve why the system changed. Worst case, we will put the motor + resistor 1/2 of the system back together and can see if we can duplicate your original 0.8 ohm measurements. Not likely to have to do this once we go through the steps above.