Project Description:

Being able to start my ford crown victoria's engine by remote control and have the climate control system run at the setting it was left at when I exited the vehicle is convient during the cold winter months and the hot summer months. But my police interceptor was not equipped EATC (electronic automatic temperature control). So to pre-warm or pre-cool my vehicle, I had to remember to manually set the middle knob on the climate control to either the full warm or full cold position prior to exiting the car. And when I loaned my vehicle to someone else, I usually found it inappropriate to give them a lecture about leaving the climate controls set in a certain position when they returned the vehicle.

But closer examination of the manual climate control system in my police interceptor revealed that the function that the middle knob controls, whether the air that flows from the dashboard vents is hot or cold, could be setup so that when the car was remote started it would always revert to a certain position regardless of the physical position of the knob. But as soon as the brake pedal was pressed to disengage the remote starter, the climate controls would again behave normally. The climate control system would also behave normally if the vehicle was started using the ignition key.


The manual heating & air conditioning control panel in 1995 & later crown vics has three knobs on it.

The left most knob controls the speed of the blower motor. When set on low, the blower motor will spin slowly and minimal air will flow out of the dashboard vents. But when set on high, the climate control system will move relatively high volumes of air.

The right most knob controls which dashboard vents the air will be discharged from. This is accomplished by moving a series of doors inside an enclosed tunnel located under the dashboard.

The middle knob controls the temperature coming from the vents. When the knob is rotated to the full counter-clockwise position, the air discharged from the vents will be cooled. When the knob is rotated to the full clockwise direction, the air will be heated. Turning the temperature knob sends an electronic signal to an electronic actuator under the dashboard which then moves a door inside the heater box called the blend door. When in the full cold position, all air is directed through the A/C evaporator. When in the full hot position, air is first directed through the A/C evaporator & then through the heater core. Since the introduction of the AOD-E transmission in the mid-1992 model year, all crown vics have used electronic servo motors to control this door. Earlier AOD vehicles used mechanical cables that offered less precise control of the blend door and were also prone to binding and slipping.

Of note: on recent crown vics temperature control is accomplished through the blend door without the use of any mechanical valves to coolant coolant flow. Engine heated coolant flows through the heater core continuously when the engine is running even when the climate control is set on "Max AC".

Also of note: A typical drive cycle is to start the vehicle's engine, turn the climate control knob to either full hot or full cold to get the vehicle's interior to the desired temperature. Then adjust the climate control temperature door knob midway between hot and cold to maintain the desired temperature. If you normally take short trips, the modification mentioned here will likely be of little use since you never get to the stage where the temperature knob is adjusted midway between hot and cold.

Below are diagrams of the electrical portion of the climate control system.

The part that we're particulary interested in is the temperature control potentiometer with three wires on it DB, Y/LG and R/LG.

When in the Full Hot Position:

Dark Blue
Yellow/Light Green
3100 Ohms
Yellow/Light Green
0 Ohms

When in the Full Cold position:

Dark Blue
Yellow/Light Green
0 Ohms
Yellow/Light Green
3100 Ohms

The potentiometer in my crown vic was a little "scratchy" and readings were not a smooth linear increase like they should have been. This probably explains why the knob had some "dead spots" on it where the climate system would heat the car when it should have been cooling it. Perhaps a little contact cleaner would fix the problem, or maybe i'll pay the dealer $15 for a new knob if i can't get the old one smooth enough for my liking.

Anyways, when my vehicle is running I usually always have the climate control system active. That means that the climate control head should almost always provide power to the blower motor when remote started. But if this becomes a concern, I could use a relay to jumper the BR/O with DB/LG then the blower motor would spin even if the right most knob was set on "OFF". In addition, I could make sure that the blower speed was always at or above the medium-low setting when remote started by using a relay to jumper the O/BK wire with the LG/W wire.

As long as the blower motor is spinning, i'll get air out of one or more of the dashboard vents. Which dashboard vent the air comes from is controlled by the climate control head directing engine vacuum to the a series of doors inside the heater box. Whatever position the doors are in ought to be acceptable for this project. Although running cool air conditioning cooled air out of the defroster or floorboard area vents may seem a little odd, it will cool the interior of the car. Similary running heat out of the dashboard a/c vents will heat the interior of the car. The exception is that heating the car with the outside recirc door closed can cause the interior temp to climb close to that of the 200 degree engine coolant flowing through the heater core.

The rightmost knob in question also controls whether the a/c compressor engages. If the compressor does not engage, directing air through the evaporator will not lower it's temperature. I could use a relay to bypass the switch during remote starts and engage the a/c compressor regardless of the switches position. But if the blower motor were not engaged, i'd also run the risk of freezing the evaporator core. And besides, I almost always have the a/c on either max or normal during the summer anyways. And during the winter or on humid days it's usually on defrost or mix. So the compressor should be cycling on when the engine is running.


Below are a couple diagrams on how to use relays and a resistor to force the blend door to the desired position.

The idea here is that when the vehicle is started with the ignition key, the relays will pass along the blend door potentiometer control signal to the blend door actuator transparently through pins 87a & 30. But when remote started, the relays will electrically remove the potentiometer knob from the circuit and send a false signal to the blend door actuator that the knob has been rotated to either the full cold or full hot position.

Note: an orange-black-red-gold resistor = 3000ohms

Since physical space was a concern and current flow through the blend door potentiometer is negligible, I elected to use a couple DEI 451M door lock micro-relay assemblies instead of the conventional 30amp vf4 form factor relays that are commmonly used in remote starter installations. Each of the 451m's contains two spdt relays in a small package about the size of one vf4 relay.

The 451m's are marketed for controlling the door lock actuators when installing an aftermarket keyless entry system, but can be used for other purposes too.  Each of the relay engagement coils is controlled through a 3 wire ribbon cable. The red wire on the ribbon cable is connected to one side of each of the relay engagement coils. The green wire is connected to the other side of one of the lock relay coils, and the blue wire is connected to the other relay's engagement coil. For this project, cut off the prioprietary connector on the ribbon cable and strip some insulation off the wire to access the conductors. Then twist the green & blue wires together on the coil and connect them to a constant ground source. Connect the red wire to a source that provides +12V only when remote started.

The violet/black wire connects to an inline fuse, which then connects to two wires which run to each of the normally open relay contacts. For this project, cut the wire after the fuse where it splits into two seperate violet & violet black wires.


To construct a relay pack that will default the blend door to the full hot position, connect the following wires on the 451m to the dash harness:

violet/black (lock normally open) -> Y/LG in series with 3k Ohm resistor
green/black (lock output) -> DB (dash side)
white/black (lock normally closed) -> DB (control head side)

violet (unlock normally open) -> Y/LG (0 Ohms)
blue/black (unlock output) -> R/LG (dash side)
brown/black (unlock normally closed) -> R/LG (control head side)


To construct a relay pack that will default the blend door to the full hot position, connect the following wires on the 451m to the dash harness:

violet/black (lock normally open) -> Y/LG (0 Ohms)
green/black (lock output) -> DB (dash side)
white/black (lock normally closed) -> DB (control head side)

violet (unlock normally open) -> Y/LG in series with 3k Ohm resistor
blue/black (unlock output) -> R/LG (dash side)
brown/black (unlock normally closed) -> R/LG (control head side)

Dual Hot & Cold with toggle switch

To construct a setup where the blend door can be defaulted to either the full hold or full cold position with the flip of a switch located under the dash, a pair of 451M's can be used.

Connect together:

Brown/Black (Relay Pack#1) -> Blue/Black(Relay Pack#2)
White/Black (Relay Pack#1) -> Green/Black (Relay Pack#2)

Violet (Relay Pack#1)  & Violet/Black (Relay Pack#2) & one end of a 3k Resistor
        Connect other end of 3k resistor to blend door common (Yellow/Light Green)

Violet (Relay Pack#2) & Violet/Black (Relay Pack#1)  & straight peice of jumper wire
        Connect other end of jumper wire to blend door common (Yellow/Light Green)

Ribbon Cable
Blue (Relay Pack#2) & Green (Relay Pack#2) -> winter/summer spdt toggle switch
Blue (Relay Pack#1) & Green (Relay Pack#1) -> winter/summer spdt toggle switch
Ground                                             -> common terminal of spdt winter/summer toggle switch
Red (Relay Pack#1) & Red (Relay Pack#2)  -> +12V during remote start

Green/Black (Relay Pack#1) -> DB Dash Side
White/Black (Relay Pack#2) -> DB Potentiometer Side

Blue/Black (Relay Pack#1) -> R/LG Dash Side
Brown/Black (Relay Pack#2) -> R/LG Potentiometer Side

Note that I am no longer using this project in my crownvic because I have removed the manual climate control system in my crown victoria and installed it's electronic automatic temperature control (eatc) counterpart in it's place. Since, I usually leave the ford eatc system in automatic mode, this project is no longer needed in this vehicle because the blend door will be set at full cold when it's warm outside and full hot when it's cold outside by the eatc system. But the project did not go to waste, since I installed the 451m relay pack and control switch in a remote start equipped ford taurus. Recent tauruses use the same variable resistor to control the blend door that crown victorias do. This hack will work in most other ford vehicles that use electrically operated blend doors too. But some cars such as the ford focus and ford mustang still use a conventional mechanical cable to control the blend door and the project discussed above will not work in such vehicles for obvious reasons.

Questions or Comments?

Email Me: [email protected]