Climate control problems? Read This!

[gadget73]

I figured I'd toss together something that explains basically how the climate controls work, and how to troubleshoot and repair common problems. This applies mostly to box models since I don't know jack about the climate controls on the aero.

Firstly, there are 2 different systems in use, standard climate control, and automatic climate control (often referred to as ATC). Identifying them is quite simple. Automatic climate control head units say "Automatic Climate Control" on them and the temperature slider has numbers. Non-ATC head units simply have hot and cold markings.

The inner workings of both systems are pretty much the same. Temperature control is done via a blend door. One side of the door has the heater core box, the other side has the AC evaporator. The door can be full hot, full cold, or somewhere in between. There is always engine coolant flowing through the heater core.

Air is pushed by the blower through the box with the blend door, and past that it goes into the air plenum. In here there are a couple of doors that direct heat to the floor, dash, or defrost or whatever mix is selected. These doors are controlled by vacuum motors attatched to the side of the plenum. This whole mess is located directly behind the center of the dash, if you look behind the radio area, the vacuum motors are probably visible. In the event of a loss of engine vacuum, the doors are positioned in such a way that all air goes out the defrost vents.

The big difference between ATC and non-ATC is what controls the blend door. In a manual system, its done through a cable connected to the temperature lever. On the ATC system, it is done with a vacuum motor. Vacuum to this is controlled by the interior temperature sensor. This device is located above the glove box. It samples cabin air, and adjusts the vacuum on the blend door to get the desired temperature. Inside the temperature sensor there is a rubber diaphragm, a bi-metallic plate, a spring, and a small cam. The bi-metallic plate warps with temperature, allowing more or less vacuum through to the blend door motor. The spring is controlled through the temperature lever, more tension = more warping. In the event of vacuum loss, the blend door motor goes to full hot.

stand by for the next installment which will involve fan motor operation and troubleshooting.

[gadget73]

The fan motor on both systems is controlled the same way. The motor gets full +12v all the time. Speed control is done by inserting more or less resistance in the connection to ground. For less speed, more resistor, for full speed the motor is connected directly to ground. The blower resistor is located in the air plenum, and is accessible from the engine compartment. Its basically behind the passenger side valve cover.

ATC systems also provide a second way of controlling the fan. In addition to the 3 manual speeds, there is an automatic setting. On top of the blend door box, connected to the blend door there is a second fan speed control switch. When the blend door is full hot or full cold, the blower motor runs full blast. As it moves closer to the center, the fan speed slows. The auto setting is only for fan speed, it will not disable the automatic temperature control.


Troubleshooting:

**note** all troubleshooting info assumes that the heater core is present, connected, and not clogged. It also assumes no modifications to the air plenum or any of the systems in the car, and that the engine has the proper coolant level. Please check to make sure nothing has been removed or bypassed before moving on to the other stuff. Removed or broken AC will mostly not affect any of this, though obviously it won't get cold.

Air moves to defrost when accelerating:

This is caused by a loss of vacuum. The system defaults to defrost as a safety measure. Common leak areas are the "soup can" under the hood, a cracked line for the soup can, the one-way check valve on the vacuum tree, or the thermal blower lockout on ATC systems. To track down the leak, get a vacuum pump and apply vacuum to the assorted parts. Replace the defective one.

Air gets hot under load:
This is an ATC problem, also vacuum related. Usually this is also accompanied with the air moving to the defrost vents. See above for areas to check.

Fan motor does not work in floor position
Another ATC bug. The thermal blower lockout switch has probably failed. This is the sensor looking thing in one of the heater hoses under the hood. It will have 2 vacuum lines and 2 wires hooked to it. The purpose of this device is to keep the fan from running when the engine coolant is under 120 degrees. To diagnose this, simply unplug the electrical connector and jumper the two wires. To resolve it, either replace the TBL, or simply bypass it. If removing it, the vacuum lines must be sealed off. Do not connect them together.

Poor heat, no heat, or temperature wanders:
ATC problem. The interior temperature sensor has probably failed or it is not sampling interior air. Essentially this sensor is just a calibrated vacuum leak, but the rubber diaphragm gets hard and doesn't seal properly, causing the blend door to stay towards cold. To test if this is the cause for no heat, simply pull the vacuum line off the servo under the dash. This will be located directly behind the glovebox. Its a gold can with a clear vacuum hose. If the temperature wanders, ensure the plastic air sample tube is connected and not full of leaves. Ditto on the sensor itself. The tube connects to a nipple on the firewall just above the passenger's feet and goes up inside the dash to the ATC sensor. If this is pinched, disconnected, clogged, or ripped, the sensor doesn't have good data to work on. Simple test for this is to see if air is moving out of the side of the sensor. Its also possible that small bits of leaves and junk have gotten packed into the sensor from being sucked into the fan over the years. See attached pictures for the blend door servo, the sample tube, and the ATC sensor. Pics courtesy of 86GmLsCoupe

No fan speed other than high:
Likely a bad connection at the blower resistor, or the blower resistor has failed.


L to R, pics are:
sample tube connection at the firewall. SOmetimes this is a white tube.

ATC sensor location in the dash

ATC sensor part number

behind the dash view of the sensor. The control cable is disconnected in this pic. Normally it will be hooked to the arm on the end of the sensor.

The blend door servo with the vacuum line plugged for diagnosis.

[FatNasty]

Good info! I'l need to put this in a tech article, and hopefully get some pics to go along with it.

[Tiggie]

Keep up all the good HVAC info and I mightbe tempted to get my ATC working again.

No fan speed other than high:
Likely a bad connection at the blower resistor, or the blower resistor has failed.

I have low and manual high, no mid-range and always low on "Auto". Does this sound like the blower resistor or something else?

[Chris84]

and don´t forget the swich for the vacuum-lines


i´ve tested the soup can, the check vavle, and this swich.... it´s the swich, unfortunately... ;(

if i disconnect the main vac-line and put my finger onto it, it holds the vacuum (i sat there for about 8 minutes) if i connect all the lines, i hear a hissing sound coming from the swich... and, after 30 seconds or so, i hear this typically "ping" from the soup can, when the air comes into it...

[gadget73]

Keep up all the good HVAC info and I mightbe tempted to get my ATC working again.


I have low and manual high, no mid-range and always low on "Auto". Does this sound like the blower resistor or something else?

I'd bet its a bad connection at the resistor. The medium speed taps in the middle of the blower resistor, so if low works the resistor itself isn't fried. Probably the plug is crudded up.

[gadget73]

and don´t forget the swich for the vacuum-lines


i´ve tested the soup can, the check vavle, and this swich.... it´s the swich, unfortunately... ;(

if i disconnect the main vac-line and put my finger onto it, it holds the vacuum (i sat there for about 8 minutes) if i connect all the lines, i hear a hissing sound coming from the swich... and, after 30 seconds or so, i hear this typically "ping" from the soup can, when the air comes into it...

yeah, unfortunately sometimes they go bad too, but its not as common as the soup can. Actually I think you're the first person I've known to have one fail. Lucky you

[1987cp]

Would info on the specifics of the climate control circuitry be found in one of the following manuals?

http://cgi.ebay.com/ebaymotors/1987-Ford-Car-Manual-Electrical-Climate-Control-Auto-g_W0QQcmdZViewItemQQcategoryZ6759QQihZ004QQitemZ140085261583QQrdZ1QQsspagenameZWD1V

http://cgi.ebay.com/ebaymotors/FORD-BODY-CHASSIS-ELECTRICAL-SERVICE-MANUAL-1987_W0QQcmdZViewItemQQcategoryZ6759QQihZ001QQitemZ110170470123QQrdZ1QQsspagenameZWD1V

I'm wondering since I saw this table of contents for a Town Car EVTM manual, and it doesn't look like it covers the heater function except for ATC (unless of course ATC was standard on the Town Car): http://pix.faxonautolit.com/1987TownCarEVTMTOC.jpg

[Grand Marquis GT]

Brought to you by Gadget, the resident HVAC master!

[gadget73]

Would info on the specifics of the climate control circuitry be found in one of the following manuals?

http://cgi.ebay.com/ebaymotors/1987-Ford-Car-Manual-Electrical-Climate-Control-Auto-g_W0QQcmdZViewItemQQcategoryZ6759QQihZ004QQitemZ140085261583QQrdZ1QQsspagenameZWD1V

http://cgi.ebay.com/ebaymotors/FORD-BODY-CHASSIS-ELECTRICAL-SERVICE-MANUAL-1987_W0QQcmdZViewItemQQcategoryZ6759QQihZ001QQitemZ110170470123QQrdZ1QQsspagenameZWD1V

I'm wondering since I saw this table of contents for a Town Car EVTM manual, and it doesn't look like it covers the heater function except for ATC (unless of course ATC was standard on the Town Car): http://pix.faxonautolit.com/1987TownCarEVTMTOC.jpg

Probably the first one, though I have never seen one of those particular manuals. The body/chassis/electrical one has a lot of info on how to work on the climate controls. The EVTM is more of a troubleshooting guide (well, it is called the electrical vacuum troubleshooting manual after all). The BCE book is the ultimate word on fixing stuff on these cars, I'd highly suggest getting one if at all possible.


And yes, the Towncar EVTM only has info for ATC. Manual controls weren't available on them that I'm aware of. Possibly on earlier models.

[Chris84]

yeah, unfortunately sometimes they go bad too, but its not as common as the soup can. Actually I think you're the first person I've known to have one fail. Lucky you

perhaps the reason is the odometer
it says 380.634 kms...

[gabegt90]

thanks thain this is good info to have since I have atc

[Nathan in MI]

Thain, was the interior temperature sensor the piece that you recently mentioned having replaced and then it failed again very soon after? What are the options when that happens? Seems like you'd have to have it working in order to be able to control the heat, since there's no physical connection between the temperature slider and the blend door, correct?

[gadget73]

Thain, was the interior temperature sensor the piece that you recently mentioned having replaced and then it failed again very soon after? What are the options when that happens? Seems like you'd have to have it working in order to be able to control the heat, since there's no physical connection between the temperature slider and the blend door, correct?

That was the new China made thermal blower lockout that exploded on me. The ATC sensor is pretty much required for temperature regulation, though I guess if its really problematic, it would be possible to switch out the cable on the climate control head unit for one out of a non-ATC car, and just directly cable operate the blend door. Once the vacuum lines for the sensor were sealed up and the vacuum motor removed, it probably wouldn't be all that hard to make it manual.

[Mercmarquis]

cool stuff, need to look at some of this for my car.

Oh... wait, I ripped it all out