AC relay failure modes and fix, more than you ever wanted to know

[Bernie Benz]

The AC relay drives (supplies battery voltage to) the AC compressor clutch.
This clutch, being a power inductor, stores considerable inductive energy
which must be dissipated when releasing the clutch to stop the compressor.
Audi supresses this reverse voltage spike, which is generated by the clutch
coil when current to it is interupted by the opening of the AC relay
contacts, by placing a supression diode (of marginal quality) across the
relay contacts.  Thus the stored coil energy is dissipated by continued
current flow thru the PS (power supply, battery) after the contacts have
opened.  This diode has two failure modes, BTDT both modes.

1.  If the diode fails by shorting (conducts current in either direction)
the compressor will run all the time when the AC is on and will cause the
evaporator to ice up thus blocking air flow.

2.  If the diode fails open (will not conduct in either direction) the
stored energy causes a large negative voltage spike to be generated across
the opening AC relay contacts which is superimposed on the PS. This pulse
can cause damage to other electronic components and is the cause of the loud
pop that is heard on the radio.

The relay is shown schmaticly and physically located in Bentley:

For the 200Q-20V on pg 590, the offending diode on CT 487, shown within the
relay and across the contacts.  The relay is located at position #1, ref pg
333 (the ref location diagram for the 20V on pg 531 is all screwed up) in
the aux relay panel, driver's foot well.  PN 443-919-578F, marked "295", or
-587H, marked "306".  Maybe other versions?, but basicly all the same.

For the 90Q/CQ, schematic pg 87-20-10, aux relay panel #4, pg 87-20-2, relay
ID as above.

The Fix is to replace the bad diode with a 50V 3A or better diode, RS has
them, about $0.20.  It is one of the two unmarked black axial lead diodes on
the base circuit board.  Trace out the correct one to the relay contacts.  A
good, close quarters soldering project.

Bernie