Welcome to one of the reasons you love your Mark VII
over a plain-Jane lesser model: the fantastic automatic temperature control
system (ATC). The ATC always keeps you at the right temperature; not too
cold when the clouds roll in, and not overly hot when your drive home faces
a blazing sunset. When you want the heat, it will not blow until the engine
has warmed up enough to give you that precious heat. But when it breaks
you have a problem!
Take it to the dealer? Maybe, but at least look at what I learned
from a 1985 Ford Shop manual.
THEORY: these are the ATC inputs
- An in-car temperature sensor
- An ambient temperature sensor
- Engine temperature sensor
- Customer input, from a head unit
- The head unit has LED lights for the buttons and a Vacuum Fluorescent
Display, both of which are used for diagnostic purposes. The display
also switches between degrees Celsius and Fahrenheit depending on what
button you push on the trip computer
THEORY: these are the ATC outputs
- Blower speed controller (High, Low, Auto, and Medium for
later model cars)
- Blend door actuator (for automatic temperature control?)
- Mode door actuator
- PANEL/DEF door actuator (for front Vent or Defroster control)
- Fresh air recirculate door position (Vent or Recirculate
control)
These 4 doors are each driven by a DC motor with a feedback wire to tell the
computer which position each door is in. Think of these systems like the throttle
position sensor in your EFI system, as they work on the same principle.
Now, with the proper knowledge of this system, try diagnosing it yourself. To
get an accurate diagnosis, TESTS 1 and 2 need to be done right after each other.
TEST 1: The head unit has a self-test feature that you can
use to diagnose just about any problem with the ATC system.
Put the ignition key to the "on" position
and set the head unit to 90 degrees and OFF. Wait 40 seconds.
If the display begins to flash then you have a wiring problem
with the blend actuator, a bad actuator, or a bad head unit. If the LED
light begins to flash then there is one or more of the problems previously
mentioned.
Address the problems or continue with the test. After no flashing occurs, set
the head unit to 60 degrees and DEF. Again wait 40 seconds and look for a flashing
display or LED lights. If there is no flashing, there is no problem with the
actuator or circuitry.
TEST 2: Push the OFF and DEFROST buttons at the same time. Within 2 seconds,
push the AUTO button. If an 88 is displayed, everything supposedly is fine according
to the head unit. If you look below, the shop manual also says the number 9 is
fine too. Take it for what is worth, I guess. Any other numbers show a problem
with the ATC system. Now is the time to write down the number of numbers displayed
on the ATC head unit. When you are done with the codes press the COOLER button
to put the ATC back into regular mode. The temperature control will now be one
degree cooler than what you had set it before running the test. Cool, huh? Now
turn off the ignition and figure out how to fix your problem.
The Codes
1
, Blend
Actuator out of position, display flashes
2
, Mode
Actuator out of position, LED flashes
3
, PANEL/DEF
Actuator is out of position, LED flashes
4
, Fresh
Air (Vent) Actuator is out of position, display flashes
1, 5 Blend Actuator output shorted, display flashes
2, 6 Mode Actuator output shorted, LED flashes
3, 7 PANEL/DEF Actuator output shorted, LED flashes
4, 8 Fresh Air Actuator output shorted, LED flashes
9
, No
failures found
10,11 A/C clutch is either always on or always off (you can figure
this out easily)
12
, System stays
in full heat (In car temp above 60° for this test to be valid)
13
, System stays
in full a/c
14
, Blower stays
at full speed
15
, Blower never
runs
