How-To: Disabling Automatic A/C - Defrost modes

Yes, you can change the fan speed settings.

The fan speed control is a five-way switch.
Off-1-2-3-4

It is wired to three resistors that are connected directly to the blower motor.

4 is straight battery to blower to ground. As you turn the speed DOWN (Towards 1) it adds another resitor to the series.

The three resistors I assume are power resistors.
I don't know what wattage, but they should be located somewhere behind the glovebox.

In theory you could replace the five-way switch with one with more positions, like a 6 or 8 way.
It still needs to handle 100% of the power from the blower motor, so you need fo find out how many watts the blower motor uses.
You would then add more resistors to the series to get more speed options.

Another option as above is to use a PWM controller.
This would allow for very precise control of fan speed from zero to ~100%.
You again need to know how many watts (Or Amps) the blower motor uses, and find a PWM circuit or chip that can handle that much power.
http://www.digikey.com/scripts/DkSearch/dksus.dll?Criteria?Ref=124615&Site=US&Cat=31523794
 
Yeah i looked that up last night because it was stuck on my brain.

i measured 10A on fan speed 3. i couldnt measure more as my multimeter can only do 10A, but im assuming ~15A on fan speed 4 or 180W!

i was going to look into PWM but i dont think it would be good for the electrical system. no data backing that up, just a feeling. lol
 
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PWM won't hurt the electrical system, if the power rails have some caps close to the PWM controller.

My guess is also 15 to 20 amps. I am thinking about trying to locate the motor or resistors to be sure. The fuse it runs off of is 40amps though.

But lets try to reverse engineer it:

Looking at the service manual it lists the resistace of each resistor:
---.18---.36---1.96---

Total resistance at switch:
1: 2.50 ohms
2: 0.54 ohms
3: 0.18 ohms
4: ~0.01-ohms

It also says the voltages at each position:
-This is across the resistors.
Off: +12volts
1: 0.12 volts
2: 0.65 volts
3: 0.60 volts
4: 0.30 volts

Therefore:
1: +12volts--motor---2.50ohms---GND
2: +12volts--motor---0.54ohms---GND
3: +12volts--motor---0.18ohms---GND
4: +12volts--motor---0.01ohms---GND

1: Rv= 0.12v Mv= 11.88v A=0.048 amps
2: Rv= 0.65v Mv= 11.35v A=1.2 amps
3: Rv= 0.60v Mv= 11.40v A=3.33 amps
4: Rv= 0.30v Mv= 11.70v A=30 amps

We can now figure out the resistor watts.
1: 0.06 Watts
2: 0.78 watts
3: 2 watts
4: 9 watts
That means they should be 10 or 20 watt resistors.

Did I do some math wrong somewhere it seems to be very low wattages?
Logic to me seems like the resistors should be absorbing more watts when slowing the motor down.
These numbers are all from the service manual, actualy measuring the thing would be more accurate.

In order to finish completly we need the wattage of the blower motor.

(Do you guys have any idea how much time you guys make me put into these things?)
 
heh nice work zap.

oh i checked out my other question, the one about the difference between the user A/C switch and the internal switch in this mod. I go to the ECU and checked the A/C relay wire (pin 96 iirc) and it behaves the same if i push the A/C button in, or manually press the internal switch.
 
Excellent instructions, as always. Did this in about 40 mins. in the dark (well, I had a flashlight!). Thanks!
 
Question. My control has two microswitched directly conected to the Air Flow Direction Control. Disconection that should also prevent this from hapeneing...
 
No what i meant to say was why not just break off the little metal flap on the microswitch rather than cutting the connections...
 
Brian MP5T said:
No what i meant to say was why not just break off the little metal flap on the microswitch rather than cutting the connections...

Oh, well if you ever wanted to restore this feature, for example selling the car . Or transfering the car to say, Mom or the wife, who doesn't want to mess with the manual settings.

It's much easier to just apply solder to bridge the connection, rather then try to replace the whole switch.
 
The metal lever just comes out and in as you need it. With it not in place, the rotating cam never gets to contact the switch. You how to is stellar, just another way to do it is all I was suggesting...
 
There's another reason for the A/C to come on with defrost, other than automakers thinking we're stupid or dehumidifying the air.

In cold climes, where the A/C may not be run for 6-9 months a year, lack of compressor use is a problem. The linking was designed to give the compressor exercise. This goes way back with domestics. With newer compressors and better seals and oils, perhaps the exercise isn't needed. But the design is a holdover I'm sure, probably in the interest of reducing warranty work.

That said, we on this board are clearly not average owners, and can be trusted to run the A/C once a month to lube the compressor.

Doug
 
I rarely use my A/C or Defrost. Last week when I turned it on after it being off for about 2-3 weeks it kept turning on and off on the highway. I was going about 70 and when I looked down the A/C light would flicker off then come back on a few seconds later. After about 25-30 minutes it stayed on. I hope there is nothing wrong. I know it would be one of those things where the Technicians cannot duplicate it until after my warranty is up.
 
chuyler1 said:
I rarely use my A/C or Defrost. Last week when I turned it on after it being off for about 2-3 weeks it kept turning on and off on the highway. I was going about 70 and when I looked down the A/C light would flicker off then come back on a few seconds later. After about 25-30 minutes it stayed on. I hope there is nothing wrong. I know it would be one of those things where the Technicians cannot duplicate it until after my warranty is up.

Bump; are you still getting this issue?
 
i just completed this mod. sooooo very easy!

times:
10mins to get unit out (could do much quicker now that i understand the black clips)
10mins to get to and cut the circuit board traces
15mins to install red LED's (see link- change HVAC illumination colors link)
10mins to reassemble

45mins for a killer FREE (if you have 4 led's laying around) mod! thanks you so much for this how to!
change HVAC illumination colors
 
Hey guys, i have a problem. When my car is hot, i have hot air coming out from anywhere i select, BUT AS SOON AS I CHOOSE DEFROST FRONT WINDOW, the air is COLD COLD COLD.... if i put it back on like head only, its still cold, i need to put the fan back to zero and put it back to 1-2-3 or 4 then i can expect hot air again.

WTF? ??
 
my only thought is maybe you forgot to plug in one of the two harnesses? sorry i can't help more... well- wait, i just thought up something else-
can you turn the two dials (temp adjust and heater position adjust) all the way in both directions? it might be possible that when you pushed the black part of the cables back into the clips, that they were not in the right positions. i turned mine so that the metal rod comming out of the black sheath was as close in (read- the opposite of extended all the way out) as possible. once it is as close in as it goes, then hook the wire loop, and then push the full cable into the black clips. this will insure everything is as it should be... hope this is clear and that it helps
 
does anyone know where I can get some replacement bulbs for the Hvac in blue? I can't find any at the parts stores.
 
also, my rear defrost button stays lit at all times while the ac button doesn't light at all. What could be the problem?
 
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