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Author Topic: 1974 Deville Blower Motor - Switching to manual  (Read 351 times)

Offline pmantos

  • Posts: 1
  • Name: Peter Mantos
1974 Deville Blower Motor - Switching to manual
« on: November 09, 2021, 04:58:14 PM »
I'm throwing in the towel on my ATC climate controller in my 1974 Cadillac Coupe DeVille w/AC.  I've installed an electronic heater valve bypass that works well.  Now I want to control the speed of my fan manually.  I was thinking that I would just run a positive lead (from the battery?) through a fuse and to a potentiometer mounted under the dash, and to the blower motor.  However, I'm no electrician and I don't know the range of acceptable current that I can send  to the blower.  I've looked online for new motors and have found them, but I cannot find specs for them that include voltage and amps. 

I understand that I never want the fan completely off when running the car but I need those specs to determine my wire gauge, fuse size, and potentiometer range.

Does anyone know what acceptable voltages and/or amps I can send through that motor? 

Offline bcroe

  • Posts: 3895
1974 Deville Blower Motor - Switching to manual
« Reply #1 on: November 09, 2021, 09:59:55 PM »
Your fan motor uses quite high current, esp as you approach
high speed.  The usual range of electronic potentiometers is 
entirely unable to deal with this.  Note a manual control (from
most cars) works by switching in several high power resistors,
which must be cooled by the air flow to survive.  Perhaps one
of these could be hijacked. 

Another approach (probably adapted on modern cars) is to use
a DC-DC switching converter, with output voltage controlled by
your potentiometer.  This eliminates the heat problem, but needs
to be in the 20A range and able to survive the transients and
spikes that happen on car wiring.  good luck, Bruce Roe

Offline TJ Hopland

  • Posts: 10188
Re: 1974 Deville Blower Motor - Switching to manual
« Reply #2 on: November 10, 2021, 12:55:30 PM »
Do you know what was wrong with your system?   The amplifier and transducer failed on my 73.   While I was troubleshooting to determine that those were my issues I came up with a work around that is working great.  I ended up hooking my hand vacuum gauge/pump to where the transducer hooks into the system.   

I'm not sure if 74 is the same as 73 but I assume they have to be similar.  Basically all the controls and sensors generate variable electrical signal.  The transducer changes that electrical signal to a variable vacuum signal that operates a servo.   The position of the servo selects various vacuum ports to operate the various doors and valves as well as contacts for the blower. 

I unhooked the vacuum line between the transducer and servo and hooked the servo to my hand vacuum pump.   I can set the control to auto then adjust the vacuum and get the system to do what ever I want by adding or taking away vacuum.   I get variable temp.  I get variable blower speeds.   Half a pump which is maybe an inch of vacuum will change the speed or temp so its not actually that hard to control.

Sure setting the dial and having it just work would be better but I had the hand vacuum pump and didn't have the $500 to buy the replacement parts. 
StPaul/Mpls, MN USA

73 Eldo convert w/FiTech EFI
80 Eldo Diesel
90 CDV
And other assorted stuff I keep buying for some reason

Offline J. Doogla

  • Posts: 45
  • Name: J. Doogla
Re: 1974 Deville Blower Motor - Switching to manual
« Reply #3 on: November 11, 2021, 06:52:56 AM »
Taking bits from a different gm carline would probably be a fairly simple way to work around the non working atc. Might be able to make the cable operate manual controls work on the cadillac. Then you would have the fan control switch with 3 speeds...i think "high" speed uses a resistor mounted on the hvac plenum under the hood. Then a cable controlled heater valve inline with the heater hose. Still woild have vacuum controlled blending, but with an HVAC vacuum and electrical diagram for the cadillac, and whatever gm donor car you found, it would be easier to figure everything out. And the control panel would probably be very similar in size to the cadillac, so you could have it mounted in the dash wjere the cadillac controls went. It would be some effort, but doable.

Re: 1974 Deville Blower Motor - Switching to manual
« Reply #4 on: November 11, 2021, 09:52:18 AM »
I've heard that a shortcut is the fastest way to get somewhere you really don't want to go.  You apparently got that '74 because you wanted what Cadillac could offer, or perhaps not. In the case of the former, fixing the ATC to provide year round comfort conditions in the cabin is simply a matter of diagnosing what is wrong and correcting it.
Using the factory service manual (FSM) to verify that all the components, their wiring and vacuum lines are present and correctly installed would be the first step. 
This "workaround" to get heat might be a temporary fix, but just try and explain it to a potential purchaser, if in fact you ever plan on selling this vehicle.
Greg Surfas
Cadillac Kid-Greg Surfas
Director Modified Chapter CLC
CLC #15364
66 Coupe deVille (now gone to the UK)
72 Eldo Cpe  (now cruising the sands in Quatar)
73 Coupe deVille
75 Coupe deElegance
76 Coupe deVille
79 Coupe de ville with "Paris" (pick up) option and 472 motor
514 inch motor now in '73-


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