9.1.1 - Overview ( Martin Briscoe,  )

See warning at beginning of section 9.1 page *.

These devices contain an electronic circuit which gives speed control for all settings but "high" This comprises a Darlington transistor 2N6284, a 68 Ohm 2.5Watt resistor, a 1N5401 diode and a small glass diode, probably 1N4148 or 1N914. All these parts are easily obtained cheaply from electronic component outlets (example; in the UK Farnell Electronic Components, +113 2636311, but ask for a catalogue to get the right order numbers).

Blowers that won't run at low speeds almost always have a failed transistor, but replacing that alone won't always fix the fault. The resistor is there to protect the transistor from voltage spikes produced by the motor, so check the value and replace if it isn't right (most hobby multimeters have an Ohms measurement feature). The small diode feeds the blower motor voltage back to the AC computer, and has steel leads which corrode away, breaking the connection and preventing the speed control from working. They can be replaced with 1N4004 types, which are much more rugged, more easily handled, and cost just a few pennies more. These diodes are fitted with their cathodes (marked by a bar on the body) towards the transistor collector (the steel case). The 1N5401 diodes are very rugged, and unlikely to need replacing.

When fitting new diodes or resistors, don't try to fit them inside the blower interior, like the originals, where they are prone to corrosion. Fit them on the solder side of the PCB (having snipped out the old parts) and then cover them with the original plastic flap using some tape.

You can also fit new brushes to the blower motors, using power drill spares filed down to the right size (Kirby's tip!). Inside the blower assembly is the high speed relay, which is also prone to dirt and corrosion. Standard car accessory shop units can be used here, but check the pinout - they may not be standard.

Considering the cost of a new unit, changing the parts mentioned above gives "as new" performance at a tiny fraction of the cost.