1986 cadillac deville 4.1 had a/c replaced all new! had to replaced blower motor module! replaced a/c control! heater works fine. will not kick compresser on? fuses all good.

Electronic climate control power module . Don't know what relay you replaced because this vehicle has no blower motor relay . 1990 Cadillac DeVille Power Module

Blower Motor Inoperative at Any Speed (Front) ?
The heater & A/C programmer is probably the problem . The 4 to9 volts on white - that's not how you test that circuit . That is a PWM - pulse width modulation circuit . The programer switches that signal on an off real fast . You need a automotive lab scope to see the square wave that would be produced .
HVAC Blower Motor Circuit

could be switch.some models have resistor in heater box by heater core.

Follow the wires from the blower motor back to the blower motor resistor, normally only a few inches away and held in by a 7 mm screw. Replace it

Your blower motor operates off a heater blower module. If you can take a wire and jump it to your heater blower motor and see if that operates its it your module is bad instead. The module is located on the top of your a/c evaporator case near the wiper area

cooling fan module or blower fan module. there is two different ones. radiator and ac heater you know

this has a "power module" near the blower motor, this is what control the blower motor to come on. very common for this power module to fail.

replace blower control module

Cadillac 1999 DeVille
If the front blower motor is not running with battery level power to it, you may have bad blower resistors which controls the speed of the motor.. Unfortunately the resistors are built-in the motor. You may have to replace the motor. If you remove the motor, you can recheck it by direct wiring power to it. OR you may have loose wires.
Also check the Heater and A/C Programmer module.

Here's some info I found from another posting.

Red is positive 12 volts coming from fuse of circuit breaker. The black wire is either the ground or the main hot wire going to the blower motor (3 pin connection only), and the gray wire (if is is small in size) is a variable voltage coming from the body control module requesting speed for the blower motor.The blower control module varies voltage on the main wire going to the blower motor, supposedly in accordance with the requested voltage from the body control module. Usually if all fuses are okay and present on the apprpriate wire of the blower control module, the a blower motor that will not run or stays running after the key is turned off is the fault of the blower control module. If the blower control module has failed the the cause must be determined (over amping blower motor) or else new blower control module may fail when powered up.Usually blower control modules have more than three wires. Usual connections are: B+ (battery voltage) GND (chassis or engine ground), two large wires going directly to blower motor (can be any color, but are usually 10 guage in size), and an optional wire that reports voltage level back to the body control module.

It has a blower control module instead of a resistor mounted in the box next to the blower motor. The blower control module is bad