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A fully loaded Audi A6 came from a garage that told the customer that a new heater control unit was needed to fix the heater blower motor (i.e. not working due to no hot or cold air blowing though heater vents) at a cost of £1250.
So I plugged in my new car diagnostic machine from vaglaptop with ODIS software on it and it found a fault in climate control unit 1. Fault listed Lin bus defective.
So I started to run through guided fault finding test plans as follows as directed by ODIS;
Step 1. ODIS told me to disconnect one multi plug from climate control module and test for a Lin bus signal coming out of control unit. I did this and all was OK.
Step 2. ODIS told me to refit multi plug and recheck signal to find signal failure. I did this and it went to a short to positive i.e. 14volts constantly.
Step 3. ODIS told me to disconnect control module from blower motor and test at multi plug pin 4 for a Lin bus signal. I did this and fault was still the same with constant 14volt supply.
Step 4. ODIS told me to leave blower motor control unit disconnected and disconnect air con pressure switch and test for Lin bus signal at multi plug pin 2. I did this, and on disconnecting air con pressure switch Lin bus signal came back switching OK.
Step 5. ODIS told me to reconnect multi plug to blower motor control unit and test Lin bus signal. I did this and still all OK.
The minute I disconnected the air con pressure switch, the blower motor started to work with blower motor control unit plugged back in.
The test program told me to check wiring for a short to positive on the air con pressure switch. I did and no faults found.
The end result of the test was air con pressure switch was faulty and ODIS told me to replace it.
I replaced air con pressure switch and tested climate control unit and blower motor; all working OK.
Cost of pressure switch was £56 from TPS plus 2 hours labour. There was no fault with the control unit. This fault would be very hard to find without this VAG laptop and software.