Automatic Climate Control "Auto" fan speed

[grandmarquisdriversince98]

Hey guys:

My Automatic Climate Control fan speed doesn't change speed in the Auto setting. Well, it does in one aspect. When you move the lever through the temperatures, it will lower speed in the middle and then speed up again at the extremes. That is how the shop manual tells me to test the ATC servo.

On the other hand, the feature where it is supposed to start on high and then slow down eventually automatically as the temperature reaches a set desired level is not there. The Auto setting is more like medium speed forever when the temperature lever is not moved. Meaning it starts and stays on medium even when the car starts a journey as hot as Hades inside from the baking sun and the Auto is selected. My understanding is that the fan speed should be hands free, always start on high, and work itself down as the car gets to the desired temerature. The high setting is clearly always faster than Auto when I start the AC and I end up choosing high instead of Auto.

I've already replaced the temperature sensor some time ago to see if that helped. Could the culprit be the High-Medium-Low-Auto switch or the ATC? servo?

[gadget73]

What year is the car? Different climate control systems for boxes vs later models.

[grandmarquisdriversince98]

1988 Grand Marquis

[Martin Blank]

Does the blower respond to the manual side low-medium-hi?

I'm looking at the wiring diagram for the circuit. With the blower circuit in the auto position, the circuit goes to a second switch called the "ATC Servo Programmer". It's outputs are wired the same as the manual switch. High goes directly to ground and low-medium go to the blower resistor assembly but a seperate leg than the manual.

From the book, "The ATC Servo Programmer is a mechanically operated switch which controls the blower speed when the Blower Switch is in the auto position.

The Servo Programmer is mechanically linked to the Temperature Sensor Servo Motor. This motor controls both the Temerature Blend Door and the programmer."

I'm going to try and attach a scan of the schematic and parts placement.

[gadget73]

If you've got the lever to either extreme, it won't ever slow down. Sounds like the electrical side of things is working if you can hear the fan speed up and slow down as you cross "center". Sounds like either the interior temperature sensor isn't responding as it should, or the car is just never reaching the proper temperature. I'd guess more likely the issue is the temperature sensor since thats what governs the vacuum servo. They're kind of known to get wonky and cause the heat to not work right, and random hot/cold surges when its failing.

And "high" is a little faster than auto at full speed. Mine is like that too, not sure why exactly. My guess would be a little bit of added resistance with the extra wiring in auto mode vs manual fan control.

[grandmarquisdriversince98]

Martin, I noticed the wiring diagram also so we are on the same page on that. The manual settings work. The wiring diagram kind of looks like the Auto setting does not use low. It looks like it uses Hi (H1), Medium High (M#), and Medium Low(M#). I was under the wishful thinking from the description of how it works in the manual that Auto uses more magical settings than there are manual positions so that Auto had free reign of High, Medium High, Medium Low, and Low when you can only manually select High, Medium, and Low but I guess it only has free reign of High, Medium High, and Medium Low perhaps.

Gadget, that would figure that Auto High is not as good as manual High due to resistance. Perhaps the manual books should go like this, "The speed will slow down as it gets to your comfortable temperature. By the way, Auto high is not as good as manual high so you are going to want to use non-auto when you get in the car on a hot day plus it will never go down to low so don't think it will work its way down to the bottom so get used to Mediumish all the time."

I do remember reading in the operator manual where something says that when you have it on the extremes, some setting doesn't make any difference. Must have been the auto speed. Maybe I'm just on the extremes too much and not satisfied with the not-high-as-high high auto speed. Why put it on Auto and not on the extremes when you get down the road for a half an hour and feel like you could have been chilling better and quicker with manual High and 65 all that time you had it on Auto and not on 65?

Well, I have a new blower resister that I've had for a couple of years and I'm going to be putting in anyway when I put on new valve cover gaskets. That could eliminate some resistance if the old resistor is rusty looking compared to the new? I still want to see the fan speed change more on its own because it is cool to see it in action when not sliding the lever from side to side on Auto but it is kind of disappointing if it doesn't start on real high and the wiring diagram shows it doesn't use low.

[gadget73]

I think the resistance is in the wire and the various connections, not the resistor itself. The blower resistor isn't used at all in high fan speed anyway. Theres a lot of current draw on that blower motor. Its fused at 30 amps, so figure probably 20-25 amps on full speed. Copper wire doesn't have a huge amount of loss per foot, but by the time you add in all the connections and the contacts within the switch itself, its probably enough to account for a little loss in fan speed. Headlamps lose a good volt from the battery to the headlight socket, and they pull even less current so its entirely feasable.

The fan fully on low moves so little air that they probably figured it would not be practical to try and make use of it. The newer cars with fully electronic climate controls actually have fully variable fan speeds instead of 3 positions, so it changes speed in a much more gradual manner. I don't think they ever quite get to minimum either, just because it requires a certain amount of air movement to maintain temperature.

[grandmarquisdriversince98]

I forgot about how it bypasses the resistor in High. At least I have a better understanding of Automatic Climate Control now.

I figure I would notice when the fan speed changes automatically while going down the road. I started making sure to use 70 more instead of the extremes but even after driving an hour to the city, I don't recall ever noticing changes to Medium High and then later Medium Low. Well, it seems kind of Mickey Mouse with only a few components so if I eventually replace everything then I will know if it's just me.

[gadget73]

if the temperature sensor isn't actually working and adjusting the position of the air door, it won't change speed either. Mine works well, but much of that I credit to having fixed all the vacuum leaks and replacing that air temperature sensor.