Blower motor trouble shooting

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With the weather finally warming up in NW, PA I’m starting to work on my 72 imperial project again. And the next big hurdle in my way is HVAC, I want to keep the rv2/auto-temp ii system if possible. But long story short I figured the best place to start is getting the blower motor working. I believe it’s an electrical problem. I’ve tested the motor on its own and it works fine. I’ve also checked the fuses. But I’m still not getting any power to the blower motor. Could it be the switch? The rear blower works and it’s on a separate switch. Or could it be a autotemp servo issue? I verified I’m getting good vacuum to the servo and the dash switch as well. Not sure if the auto temp servo has anything to do with the blower. As far as I know my servo is good. I pulled it from my parts car it looks almost new I suspect it’s been replaced once already.

Thanks!
-Josh
 
Look in the FSM for what is in the place of the normal "blower motor resistor" that varies the fan speeds on the AT II system. Whatever there might be that does that will be between the inst panel blower switch and the motor itself. Not sure if that is a part of the AT II module or not. I suspect it should be in the wiring schematic.

Just some thoughts,
CBODY67
 
Usually when there is no blower motor activity taking place when turning on the Autotemp II system, it is due to a failed vacuum check valve in one of the various vacuum lines in the system. Those valves are usually in the lines hanging down on the floor hump or otherwise in the vacuum lines under the dash. They are plastic check valves that are white circular plastic about an inch in diameter with with vacuum nipples on each side where the very small vacuum hoses connect. You might want to check with Performance Analysis Company that is one of the two remanufacturers of ATC II modules to see where to buy these check valves. When I last needed one of those valves, I believe I used a very similar check valve that was made by GM. You can contact Performance Analysis at 865-482-9175 Eastern time, hours 9:00 - 5:00.

I am pretty sure this will fix the blower motor issue since each time I have encountered this issue, that has been the cause. After years of use they just fail with no warning.
 
Last edited:
Usually when there is no blower motor activity taking place when turning on the Autotemp II system, it is due to a failed vacuum check valve in one of the various vacuum lines in the system. Those valves are usually in the lines hanging down on the floor hump or otherwise in the vacuum lines under the dash. They are plastic check valves that are white circular plastic about an inch in diameter with with vacuum nipples on each side where the very small vacuum hoses connect. You might want to check with Performance Analysis Company that is one of the two remanufacturers of ATC II modules to see where to buy these check valves. When I last needed one of those valves, I believe I used a very similar check valve that was made by GM. You can contact Performance Analysis at 865-482-9175 Eastern time, hours 9:00 - 5:00.

I am pretty sure this will fix the blower motor issue since each time I have encountered this issue, that has been the cause. After years of use they just fail with no warning.
Thank you! That’s what I’ll check first.
 
Easy way to check it is to put a jumper wire on the blower motor master switch. If it starts up with the jumper wire you have proven continuity of the whole circuit and it will be a vacuum concern.

You will have to check for vacuum signal at the manifold on the engine for a baseline then at the servo and the selector switch on the dash and then the blower motor switch. If you find where you lose vacuum you have found your problem.
 
Usually when there is no blower motor activity taking place when turning on the Autotemp II system, it is due to a failed vacuum check valve in one of the various vacuum lines in the system. Those valves are usually in the lines hanging down on the floor hump or otherwise in the vacuum lines under the dash. They are plastic check valves that are white circular plastic about an inch in diameter with with vacuum nipples on each side where the very small vacuum hoses connect. You might want to check with Performance Analysis Company that is one of the two remanufacturers of ATC II modules to see where to buy these check valves. When I last needed one of those valves, I believe I used a very similar check valve that was made by GM. You can contact Performance Analysis at 865-482-9175 Eastern time, hours 9:00 - 5:00.

I am pretty sure this will fix the blower motor issue since each time I have encountered this issue, that has been the cause. After years of use they just fail with no warning.
My check valve was bad so I replaced it with a OE one that tested ok. That still did not solve the problem i’m getting power to the dash switch but not to the blower I have two switches and they both test the same high auto/high DEF work but low auto/DEF does not (assuming that’s because I didn’t have a resistor wired in when testing?) I also checked the resistor and it looks brand new. What would you recommend I check next?

Thanks!
 
My check valve was bad so I replaced it with a OE one that tested ok. That still did not solve the problem i’m getting power to the dash switch but not to the blower I have two switches and they both test the same high auto/high DEF work but low auto/DEF does not (assuming that’s because I didn’t have a resistor wired in when testing?) I also checked the resistor and it looks brand new. What would you recommend I check next?

Thanks!
Is there more than one check valve in the system - I thought there was but I am not sure either.........................................?
 
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