Pictured below is a schematic of the
modifications necessary to implement manual
control over the stock air ride system on 93-98
Lincoln Mark VIIIs. It is a simple network of
relays, diodes and switches that energizes the
various solenoids to control ride height. When
the system is activated, the stock air ride
system is disabled until the car is either
restarted, or the manual reset switch is
depressed. It is importatnt to note that without
a diode at the ignition switch, the starter will
be energized whenever the manual reset switch is
depressed. All parts are readily available at
your local Radio Shack, except for the latching
relay. The one I used was an ECG Electronics
Dual Coil Latching Relay P/N RLY7942D. You can
find them at your local electronics store, or
order one online.
For raising the car with stock lines and valves,
I recommend using non-momentary rocker switches.
This will allow the compressor to run without
having to hold the switch.
NOTES ON PINS 21 and 12
PIN 12 on the Suspension ECU is a 12v INPUT.
To wire the relay as depicted above, cut the
wire and connect the 12v side to pin 9 on the
Latching Relay. Connect the other end of the
wire to pin 3. This is the end that is still
connected to the Suspension ECU. You are
essentially placing the relay inline with the
12v supply and the Suspension ECU.
PIN 21 is the Message Center OUTPUT. Cut this
wire in two, then connect the wire that is still
connected to the Suspension ECU to pin 1 on the
Latching Relay. Now take the wire that is
connected to the Message Center and connect it
to pin 7.
Additionally, some people have said that they
needed to cut PIN 1 as well. Since 21 and 1 both
provide 12v supply voltage, these two can be cut
and connected to the same terminal on the
Latching Relay.My car is no longer
hooked up this way so I can not test the need to
cut pin #1. Be prepared to do so if you are
unable to kill power to the module.
IGNITION AND MANUAL RESET SWITCH
Wiring the ignition reset switch is optional. It
allows you to automatically reset the system
when the car is turned off and restarted. You
can use a manual reset instead or in addition to
the the ignition reset switch. The reset switch
is necessary to reset the contacts of the
Latching Relay.
SYSTEM OPERATION
RAISING THE CAR
In order to raise the car you need two things:
1) The compressor has to be running.
2) A bag solenoid has to be open.
The suspension ECU controls the compressor by
grounding PIN 18. PINS 14,15,16 and 17 are
energized to open the bag solenoids. With the
compressor running and a bag solenoid open, air
flows into the bag therefore raising the
vehicle.
The logic behind raising the car is simple: When
12v is applied to the bag solenoid, the
compressor control relay starts the compressor,
the check air ride message is disconnected and
power to the ECU is cut to allow you to revert
back to the stock system.
To control the system, there are four three-way
momentary switches that raise or lower each
corner. When one of the switches is pressed up,
current flows from the 12v terminal of the
switch to the inflate circuit. This applies 12V
to the bag solenoid, terminal 86 on the
compressor control relay, as well as terminal B
on the latching relay. Voltage at the 86
terminal switches the relay contacts and grounds
pin 18 on the suspension ECU, which in turn
grounds the stock compressor relay causing the
compressor to run. Note that there is a relay
controlling a relay. The actual compressor relay
is a solid state relay with cooling fins mounted
on it to disperse heat. It is located on the
compressor bracket assembly. A diode prevents
the vent solenoid from opening and diodes at
each switch allow them to operate individually
(they would share a common contact without the
diodes, essentially connecting the four
switches). With the solenoids open and the
compressor running, air goes into the bags and
the car raises.
Voltage at the B terminal causes the relay
contacts to switch and remained switched until
pin 5 is energized. This will be done
automatically when the car is restarted or can
be done manually via a manual reset switch. This
temporarily cuts power to the ECU and the
message center lead. Power to the module is cut
because the module will error if left on. Pin 12
is cut in order to prevent the "Air Ride Off"
message from being displayed. In order to bring
the stock system back online, it is essential
that the system operate without errors. It is
also important to kill power to the module to
prevent the system from correcting any changes
you made to the vehicle trim height.
LOWERING
In order to lower the car you need two things:
1) A bag solenoid has to be open.
2) The vent solenoid has to be open.
Air will always go from an area of high pressure
to an area of low pressure. Opening the bag
solenoid allows air to escape the bag and
"equalize" with the lines and dryer. Opening the
vent solenoid allows air to escape entirely. The
control module does this by energizing pins
14,15,16,17 and pin 30 for vent control.
Pressing one of the switches down is very
similar to pressing it up. With voltage at
terminal B on the latching relay, power is cut
to the control module and the message center
lead is disabled. Depending on which switch was
depressed, a bag solenoid opens and air escapes
the bag. Additionally, the vent solenoid is
opened allowing air to escape out of the system.
This will cause the car to drop. Just as in the
raising circuit, diodes prevent the compressor
relay from receiving this voltage, as well as
the other switches.
RELAY INFO:
Manufacturer Part No. RLY7942D
NTE Part No. R50-11D10-12C
