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

" . . . so i guess new resistor pack is necessary". Not mentioning to beat you up but to reinforce how it important RULE #1 is in electrical trouble shooting. We've all done this. It was trained out of me and then has been proven by over 30 years of troubleshooting both electrical and mechanical systems.

Sounds to me first thing to do is to pull the resistor pack. Verify -- don't assume.

Resistors usually fail open. It is very rare for a resistor to fail as a short.

If you have 0.8 ohms on all three, it sounds to me like the bimetallic switches are continiously closed. Could be similar to Brent's situation. In his case, he had one of them closed all the time and tweaked it with a pair of pliers to be normally open as it should be. Testing with DMM can verify this.



However, if this is the case, you really should have 3 high speeds already.

You can bench test the fan with 12v battery and some wire jumpers. Did you try that by chance?

Lack of airflow could have overheated the resistors and caused the bi-metallic switches to become deformed and to have become normally closed instead of normally open per the schematic. That is a theory and an assumption. A bench test of the reistor pack and of the fan itself will verify or disprove that hypothesis.

[DRPHIL914]

@DRPHIL914

" . . . so i guess new resistor pack is necessary". Not mentioning to beat you up but to reinforce how it important RULE #1 is in electrical trouble shooting. We've all done this. It was trained out of me and then has been proven by over 30 years of troubleshooting both electrical and mechanical systems.

Sounds to me first thing to do is to pull the resistor pack. Verify -- don't assume.

Resistors usually fail open. It is very rare for a resistor to fail as a short.

If you have 0.8 ohms on all three, it sounds to me like the bimetallic switches are continiously closed. Could be similar to Brent's situation. In his case, he had one of them closed all the time and tweaked it with a pair of pliers to be normally open as it should be. Testing with DMM can verify this.

However, if this is the case, you really should have 3 high speeds already.

You can bench test the fan with 12v battery and some wire jumpers. Did you try that by chance?

Lack of airflow could have overheated the resistors and caused the bi-metallic switches to become deformed and to have become normally closed. That is a theory and an assumption. A bench test will verify that hypothesis.

completely agree, but just didnt have time last night,

plan for tonight is to bench test wit the batter charger, put 12 volts in there and see what we get, then pull it out so i can inspect the resistor pack up close, see if that bi-metallic strip is stuck together. so will verify . the resistance values seemed strange to me to, was trying to figure out how it would fail that way. fan does not spin freely so so doubt i have to get this one pulled apart and cleaned up and see if lub will help. Ive heard that the motors really dont fail or burn out often, dont know if that is true,
But talked to Bruce Stone over the weekend and he suggested doing the same bench test, inspection, etc. and i have all new seals and hardware coming from Mark too.

will report back tonight.

[Superhawk996]

. . . plan for tonight is to bench test wit the battery charger . . .

Would be better to use a 12v battery as the source if you can. Battery chargers vary widely and some are really only 1/2 wave rectified and don't have a full 12 volt steady output. It can cause the fan motor to run slower than it would with a battery.

Likewise, it may be current limited. I tested stall current with a battery and it pulled 15A or so. If the battery charger can only deliver 10A, you may not get full speed and/or have sufficient amperage to turn motor if it is binding a little.


Here is what I mean by 1/2 wave output. Can go to site if you want more info.