As a slight aside but relevant part of the ‘fan not working’ story I will update this thread with the continuing result. With of word of caution.
Recap:- fan not working, as the dashboard was out for loom change, original MOSFET controller repaired as the fan was not working.
On reassembly initially working and tested fan stopped working again very soon after.
Cable detached from MOSFET controller and repaired unit left in foot well.
This unit also failed not long afterwards.
For me clearly the MOSFET replacements were not the total solution, the first failure was no random incident and the two replacements were not due to faulty repair or faulty components or for lack of air flow to cool it in the open air.
I had tested the motor historically and had it running at full speed direct from a battery, I was initially concerned about the wires feeding the test heating up but could not measure the current as it was greater than my meter could measure at over 10 Amps, I also measured the armature resistance of the motor but found no immediate cause for concern. The motor had been completely dissembled and cleaned, lubricated and refitted not obvious signs or distress.
MOSFET replaced and blew again under observed conditions in the foot well.
With the repeated failure of the MOSFET, I repeated all the testing and evaluated the results.
I was left with three possible fault conditions, the heater controls were feeding erroneous signals to the MOSFET controller and was causing the failure, the motor was defective and causing high current to destroy the MOSFET or three the MOSFET’s were not fit for for the job and failing.
I was looking at higher rated MOSFET’s and reviewing the replacement of all three components as a solution.
The testing I under took replaced the motor with a load resistor of 10 ohms and measured the voltage output of the heater controls to the MOSFET and this appeared as expected, it varied as you increased the fan speed and the auto was full on as you would expect.
This proved the thrice repaired MOSFET controller and wiring was sound, what then was causing the issue.
The motor was inspected more closely and found that one of the armature segments was short circuit causing short circuit, high current, to flow once every revolution. Missed on the initial inspections as the very low resistance (2-3 ohms) of a heathy segment and the normal irregular readings of a brush armature motor masked the shorted segment. Add to that that the motor actual worked when connected to an unlimited current supply.
Moral of the story for me is take more notice of your instincts and double check when is appears right but smells wrong.
A replacement motor and the system appears to be working fine with the MOSFET controller in the foot well not over heating but getting very warm.
Next job… test some of these theories that the MOSFET controller can be changed without removing the gas from the A/C and the dash from the car.
This was an unusual fault that consumed a lot of time fortunately not a lot of cash as I have the capability to repair the MOSFET myself.