[87chevy.com]

From a few wiring diagrams I have, 84, 87 and 91 they seem to be the same, 4 wires:

Tan, Orange, LT Blue and Brown/White

Orange wire - High Speed Signal
Tan - Low Speed signal
LT Blue - Medium Speed Signal
Brown/White - Power in from Selector Switch

Orange, Tan and Light Blue wires go to the Blower Resistor on the Evaporator box case.

You can check voltage, ground to each of the wires and verify you are getting approx the same voltage from each wire. It is s switch, not a resistor so the voltage should be about the same.

If switch reads fine:
DK Blue wire at the Blower Motor Relay should be the POWER out from the resistor. This wire will have the different voltages that goes through the relay and to the Purple wire that connects to the Blower Motor... Low speed should have lower voltage, medium speed should have higher voltage and High should be even higher voltage, but not battery voltage

Max Speed signal goes from Switch to Blower Motor relay. It activates the relay and gets power from the 10 Red gauge wire (that should be Hot at all times) and goes through relay and out the Purple wire for Blower Motor. This should be same as battery voltage. If the reading is more than 1 volt difference, I would check the wiring and if ok, replace the relay as the contacts are worn.


clint