HVAC Blower Motor only runs on high

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22 Mar 2011 14:28 #4115 by srshaw3
HVAC Blower Motor only runs on high was created by srshaw3
In my '90 GT the HVAC Blower Motor only runs on high. I am wondering if anyone knows it this a control issue (is there a resister pack, etc.) or a blower motor issue?

It is going in for service next week;
1. Fix the A/C not working issue (died after a complete replacement 2 years ago)
2. Find electrical drain(s) Battery gets run down after just a few days of sitting.
3. Find out why it goes into some kind of limp mode, even with IGR bypass relay in place (new ignition control units, coils, dist. caps, dist. rotors & plugs. Wires are just a couple of year old Magnecor. The manifold refresh with most parts replaced was done 2 years ago).
4. Review HVAC blower only runs on high issue.
5. Review ball joint, as they noticed a boot tear last time it was in.

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22 Mar 2011 15:05 #4116 by 87 Shark
Replied by 87 Shark on topic HVAC Blower Motor only runs on high
My 87 has started doing the same thing, however it is intermitent. Some days all speeds work and others just the high . I'm hopping there is a module to replace as was done on my GMC pickup. The pickup was an easy fix . Hope thecar is to. My AC is not working but it is not in the cards this year (needs new compressor) .This should not affect the blower settings.

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22 Mar 2011 15:41 #4118 by srshaw3
Replied by srshaw3 on topic HVAC Blower Motor only runs on high

My 87 has started doing the same thing, however it is intermitent. Some days all speeds work and others just the high . I'm hopping there is a module to replace as was done on my GMC pickup. The pickup was an easy fix . Hope thecar is to. My AC is not working but it is not in the cards this year (needs new compressor) .This should not affect the blower settings.


Thanks, yes on my M3 it is supposed to be a Final Stage Unit, about $60, so hoping the 928 has something similar.

Yes, I know the A/C problem is unrelated, rumor is that the clutch in the Griffiths kit dies quickly :(

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22 Mar 2011 16:28 #4120 by 928mac
Replied by 928mac on topic HVAC Blower Motor only runs on high
Stan, The power to the blower goes through the resistor pack that controls the speeds.
If the motor starts to drag or draw to much it burns them out one at a time. The high speed has no resistor and is driect power that is why that speed still works.
The resistor can just be old and burn out as well.

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22 Mar 2011 16:53 #4121 by srshaw3
Replied by srshaw3 on topic HVAC Blower Motor only runs on high

Stan, The power to the blower goes through the resistor pack that controls the speeds.
If the motor starts to drag or draw to much it burns them out one at a time. The high speed has no resistor and is driect power that is why that speed still works.
The resistor can just be old and burn out as well.


Looks like a fun project: members.rennlist.com/pirtle/svc.html

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22 Mar 2011 18:13 #4122 by pcar928fan
Replied by pcar928fan on topic HVAC Blower Motor only runs on high
Isn't this (or something like it) known as Magic Blower Syndrome?

James
78 Silver / Black-white #295
84 Ruby Red / Black AO84
88 Dark Blue / Linen-Black
92 Polar Silver / Dark Blue 92EURO
93 Arrow Blue / Black

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22 Mar 2011 18:34 #4127 by SeanR
Replied by SeanR on topic HVAC Blower Motor only runs on high
The resistor pack is pretty easy to do if you have hands the size of mine (small). Takes about 20 minutes. That is what is causing the 'magic blower syndrom'

Pull back the boot between the blower and the core, take some long pliers so you can put them in there to grab the resistor pack after you've unscrewed it. (two screws and it falls inside)

Putting it back in might be the harder part for most nomal sized people, but once you get one screw started, the rest is a breeze.

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22 Mar 2011 19:12 #4128 by srshaw3
Replied by srshaw3 on topic HVAC Blower Motor only runs on high
Thanks Sean. I couldn't find a part number at a couple of sites I search. Do you know the official part name, or preferably the part number?

P.S. I did email Roger as well :)

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22 Mar 2011 19:39 #4130 by srshaw3
Replied by srshaw3 on topic HVAC Blower Motor only runs on high
Think I found the part number 928.616.101.00 $126 at one vendor's site.
Part name known as A/C Thermo Switch (1978-93)

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22 Mar 2011 19:52 #4132 by 928mac
Replied by 928mac on topic HVAC Blower Motor only runs on high
BTW Stan that is a great RL site.

We should have that link posted.

I bookmarked it.

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22 Mar 2011 21:41 #4136 by srshaw3
Replied by srshaw3 on topic HVAC Blower Motor only runs on high
Getting the contents of that site into the technical library, along with the nichols site, would be a great project for someone...... ;)

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22 Mar 2011 22:09 #4138 by 928mac
Replied by 928mac on topic HVAC Blower Motor only runs on high
ha ha ha you bad. How the hang would I do that. I don't know where to start

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22 Mar 2011 23:03 #4139 by srshaw3
Replied by srshaw3 on topic HVAC Blower Motor only runs on high
I am only too happy to train volunteers :D

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04 Dec 2011 22:28 #7276 by kohi30
Replied by kohi30 on topic HVAC Blower Motor only runs on high
For those addressing the "Magic Blower Syndrome" here is a little detail that was not immediately obvious to me when I tackled this on my white 86.5 5-speed: When you are looking at the resistor pack from the outside of the tunnel, you are actually looking at the back of a 7-pin plug that connects to it. I know the instructions allude to this, and tell you to disconnect the wiring. In fact the linked instructions tell you to unplug it BEFORE undoing the two screws that hold the resistor pack to the tunnel. However, my 7-pin plug was on there pretty good, and I had to be sure of what I was dealing with, before I started prying on it hard to take it off. Below is a picture after I got it off. Note that I had already taken off the bottom right screw. That screw is a bear to get to anyway, but it is much easier AFTER unplugging the 7-pin plug.

Also, when plugging the 7-pin connector back, I heard a distinct "click", so there may be some sort of tab-slot fit that holds it securely, which is what may have caused resistance to my pulling it off. I did not look closely at it before putting it back on however (and was DONE with it by the time I got it plugged back in, so I didn't unplug it again).

Finally, I recommend NOT messing with the Blower Motor assembly, unless you have very clear access to it (i.e., the hood is off). I started going this route with my hood on, and dropped an 8mm socket in there, which took me a while to fish out! That is why I recommend against it! After that, I abandoned that route, and simply got a long needle-nosed plier and used it to grab the resistor pack after the last screw was off the front, and "hand it" to myself through the rubber boot gap (having pushed aside the rubber boot). Small hands (like mine) are helpful here, however. When putting it back, do the reverse, and push the resistor pack carefully through the rubber boot gap, until you can grab it from outside the tunnel through the small rectangular opening it sits in. Another pair of hands would be very helpful here - I did it myself, but I was able to stick my hand into the tunnel to hold the resistor pack from inside while I pushed the plug on halfway, allowing me to leave it there while I got the screws and screw driver and was able to start the top left screw, then unplugged the plug to do the bottom right screw (again, quite difficult to get to), then plugged the 7-pin connector back on. Whew!

Anyway, I hope the picture, below, helps others!

Attached files [img]media/attachments/7444=811-IMAG0249 w notes.JPG[/img]

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