JoelsBuicks Posted May 20, 2015 Share Posted May 20, 2015 My 74 Lesabre A/C is charged and cooling nicely but the fan motor is unresponsive to the switch. The fan seems to be running on high speed whenever the Heater and A/C control is switched on. The fan speed switch cannot turn down the fan. The only diagnostic that I have done is to check the fuses under the dash and they all look ok. I do not have a wiring diagram for this car. I was hoping someone could direct me to the highest probable cause or route me through a reasonable diagnostic sequence so that tearing into the dash comes at a last resort. This stuff is much more modern that what I'm used to. Thank you, Link to comment Share on other sites More sharing options...
avgwarhawk Posted May 20, 2015 Share Posted May 20, 2015 (edited) The fan is then running off the 12 volt wire only. Under the dash is fan speed controller. Usually on the passenger side of the dash. Bolted into the ducting for the blower motor. Sometimes located on the firewall cowl where the blower motor is mounted. Easy to access in most cases. . Needs to be replace. Looks like this: Edited May 20, 2015 by avgwarhawk (see edit history) Link to comment Share on other sites More sharing options...
avgwarhawk Posted May 20, 2015 Share Posted May 20, 2015 Another example. Link to comment Share on other sites More sharing options...
avgwarhawk Posted May 20, 2015 Share Posted May 20, 2015 This should be yours: http://www.oldsobsolete.com/products-page/1971-buick-parts-for-sale/1971-74-gm-blower-motor-resistor-nos-3998401/ Link to comment Share on other sites More sharing options...
NTX5467 Posted May 21, 2015 Share Posted May 21, 2015 Otherwise known as "blower motor resistor". On many GM vehicles, it's the "plate" with the wiring harness plug-in on it, under the hood, on the a/c evaporator case. All the instrument panel switch does is to change "the circuit" used in the resistor set-up to vary the fan speed. On many GM vehicles of that era, when the a/c control was placed on "Max" (or inside air), then blower motor would automatically go to "high", via a switch on the back of the a/c control head, which ran the "High Blower Relay". Should have an extra wire on the back of the instrument panel control, as I recall, which ran that situation. Unplugging that wire would give your FULL fan control on "MAX" or "outside air" for a/c use. NTX5467 Link to comment Share on other sites More sharing options...
Brad Conley Posted May 21, 2015 Share Posted May 21, 2015 The blower motor resistor is under the hood, on the passenger side, bolted to the AC box. It will have a red/maroon color as noted above and be easy to see. Replace that part (it's quite available even at you local auto parts store) and you will regain fan speed control once again. If you're not too concerned with exact authenticity, it can be obtained for much less than the NOS part listed. O'Riley has it listed for $8.99 in even the correct color if you can believe their website. http://www.oreillyauto.com/site/c/search/Blower+Motor+Resistor/02627/C0331.oap?year=1974&make=Buick&model=LeSabre&vi=5134081 Link to comment Share on other sites More sharing options...
JamesBulldogMiller55Buick Posted May 21, 2015 Share Posted May 21, 2015 When the fan is running at high speed , it is getting full voltage.The voltage resistance is what allows the motor to run at slower speeds.The resistor degrades from heat, after a time ...It stops resisting Link to comment Share on other sites More sharing options...
JoelsBuicks Posted May 21, 2015 Author Share Posted May 21, 2015 Wow, I had to depart from the site urgently to attend my Dad's surgery and when I returned, I found the answers I needed. Dad is doing great and I don't have to tear my car apart! Thank you all very much!Joel Link to comment Share on other sites More sharing options...
avgwarhawk Posted May 21, 2015 Share Posted May 21, 2015 Wow, I had to depart from the site urgently to attend my Dad's surgery and when I returned, I found the answers I needed. Dad is doing great and I don't have to tear my car apart! Thank you all very much!Joel The information is free. However, please report back the repair was successful. Link to comment Share on other sites More sharing options...
JoelsBuicks Posted May 21, 2015 Author Share Posted May 21, 2015 I removed the resistor cluster and was expecting to see something obvious like a burn or separation that would cause open circuits. It actually looked good and I'll attempt to post a picture here with my iPhone. So I'm guessing the switch is next?Thanks again for your help,Joel Link to comment Share on other sites More sharing options...
avgwarhawk Posted May 21, 2015 Share Posted May 21, 2015 Looking will not determine if it is functioning. Check continuity on each terminal. The diode could be bad as well. As stated, once this item goes bad the fan will function on straight 12 volts only. Link to comment Share on other sites More sharing options...
JohnD1956 Posted May 21, 2015 Share Posted May 21, 2015 My experience with the resistor is that the fan will run on high speed only when the switch is set to high. Otherwise if the resistor has a short, the fan did not work when the switch was set to the level with the faulty circuit. I assume your car has climate control AC, as opposed to automatic climate control AC?If so then slide the temp lever towards heat and see if the fan works. If not I'd b looking for a manual to read up before tearing into any other parts of the system. 1 Link to comment Share on other sites More sharing options...
JohnD1956 Posted May 21, 2015 Share Posted May 21, 2015 Btw, may we also assume Dad is on the mend? Link to comment Share on other sites More sharing options...
JoelsBuicks Posted May 22, 2015 Author Share Posted May 22, 2015 John, I will try that and meant to do it after reading Willis' post. The car doesn't have automatic climate control. I'll also do a post-to-post continuity check on the resistors. The switch feels ok in that it has subtle stops at the different positions.Dad had eye surgery, nothing bad but Mom was unable to drive him. At 81, I feel lucky to have him around.Thanks! Link to comment Share on other sites More sharing options...
JoelsBuicks Posted May 22, 2015 Author Share Posted May 22, 2015 (edited) ? Edited May 22, 2015 by JoelsBuicks (see edit history) Link to comment Share on other sites More sharing options...
JoelsBuicks Posted May 22, 2015 Author Share Posted May 22, 2015 (edited) Not sure what's happening with my duplicate posts but boy am I feeling stupid right now. I took the temp control off of "Recirc" and guess what? Yep, the fan speed control worked! I don't like putting my ignorance on display but it's not the first and won't be the last. Thanks to you all, I learned how this switch speed control works and I learned mine works just fine.Thanks again,Joel Edited May 22, 2015 by JoelsBuicks (see edit history) Link to comment Share on other sites More sharing options...
JohnD1956 Posted May 22, 2015 Share Posted May 22, 2015 Sounds good on dad's eye work. That's good news. Link to comment Share on other sites More sharing options...
NTX5467 Posted May 22, 2015 Share Posted May 22, 2015 Thanks for that update. That deal of GM cars' a/c blower motors being on "high only" in the "Recir" mode is something that I know there's a reason for, didn't really like on my '77 Camaro, and many have forgotten about over the years. NTX5467 Link to comment Share on other sites More sharing options...
Ben Bruce aka First Born Posted May 22, 2015 Share Posted May 22, 2015 Not sure what's happening with my duplicate posts but boy am I feeling stupid right now. I took the temp control off of "Recirc" and guess what? Yep, the fan speed control worked!I don't like putting my ignorance on display but it's not the first and won't be the last. Thanks to you all, I learned how this switch speed control works and I learned mine works just fine.Thanks again,Joel Welcome to the club, Joel. There are only two kinds of people in the world. Those who have made those kinds of mistakes , and those who will. Glad everything worked out and that Dad is improving. Ben Link to comment Share on other sites More sharing options...
avgwarhawk Posted May 22, 2015 Share Posted May 22, 2015 Not sure what's happening with my duplicate posts but boy am I feeling stupid right now. I took the temp control off of "Recirc" and guess what? Yep, the fan speed control worked!I don't like putting my ignorance on display but it's not the first and won't be the last. Thanks to you all, I learned how this switch speed control works and I learned mine works just fine.Thanks again,JoelAll is well that ends well! Link to comment Share on other sites More sharing options...
JohnD1956 Posted May 22, 2015 Share Posted May 22, 2015 At least you know your resistor switch is okay. And so is your Pop, and your car! Sounds like a win-win-win to me. Link to comment Share on other sites More sharing options...
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