Heater fan
Team Owner
Joined: Mar 2001
Posts: 54,124
Likes: 30
From: Cincinnati, Oh USA
On my 87, the blower motor controller is located in the top of the evaporator housing which is on the right side of the firewall in the engine compartment. This module receives a d.c. voltage from the control head, 0.5 volts (low speed) to 5 volts (high speed) and the motor controller uses a power transistor and other circuits to supply a pulse width modulated voltage to the blower motor to control its speed. Usually the power transistor fails with a short from emitter to collector (if a bipolar transistor) or from source to drain (if a MOSFET transistor) and this causes the motor to run at full speed in spite of the control voltage from the control head inside the car. On my 87, the control voltage wire is brown and if you measure the voltage on this wire while someone changes blower speed, you can isolate the problem to the motor controller if this voltage varies but the fan stays on top speed. If the control voltage stays at 5 volts, then the problem is in the heating a/c control head.
Le Mans Master
Joined: Dec 2000
Posts: 7,839
Likes: 22
From: San Diego, Ca
Specs on the signal or Brown wire are 2.5 volts "1" to 6 volts at "10". Output, purple (or whatever is hot at the blower motor) are 4 volts "1" and 12 volts "10". '90 signal is from the Programmer, not the Controller. The Controller sends the signal over a serial data line to the Programmer. Actual blower voltage is returned to the Programmer through a 5 amp fused circuit. I believe the fuse is in one of the boxes on the '90; prior to that it was on the firewall, left of the Evaporator Housing. If the fuse opens, system defaults to high blower, so check that fuse first. Then check your input and output voltages at the Module. Otherwise, a Tech 2 or bidirectional scanner will emulate the Controller and communicate with the Programmer via the CCM. If it works, you need a new Control Panel. If it doesn't, check input and output voltages again. If they're not within specs, the Programmer is bad. If you don't have access to a scanner, you could try jumpering a 1.5 volt flashlight battery to the tan wire (you'll need to isolate it). If it produces low blower speed, the Module is ok - it's probably the Programmer.
