Let me try and explain the use of the Darlington pair transistors as used on the blowers in our motors.

The Darlington pair is actually two transistors encapsulated in one housing, hence the name, the first transistor of the pair cascades its output onto the input of the second, this is done to increase the high current that can be switched (This relatively high current is why the transistor cans themselves are mounted on a hefty heat sinking base).

A transistor is in fact a controllable, variable relay, but instead of just having two states, on or off, it has various stages of open from zero to max with all the values in-between.

In the blower circuits the first transistor of the pair is triggered by the “Climate Control Microprocessor” the subsequent output of the second is fed directly to the blower.

We have one Darlington pair for the right blower and another for the left blower. The microprocessor receives the settings from the climate control panel switch of low / Med / High speed and depending on the ambient temperature regulates the fan speed via the Darlington pair, however, in the case of the high speed setting a relay is triggered which effectively bypasses the Darlington pair and drives the fan directly.

So the microprocessor controls the fans at low or medium speed via the Darlington array and then directly when on high speed, which is why the fans may work on high speed but not on low or medium if the Darlington has failed.

When changing out the Darlington pairs I cannot stress enough the care that must be taken to ensure that they are electrically isolated from the heat sink plate, but at the same time thermally “connected” via the mica washer and a liberal layer of white thermal conducting paste. They can get quite hot when in service.