The fan works but is slower than normal across all speeds, when you brake the fan speeds up to its proper speed. It doesn't seem to be linked to the braking related circuits as braking while stationary does nothing. It seems to be affected by the physical deceleration of the car, I assumed a loose connection somewhere but a brief check shows all fuses in tight, the fan resistor is connected tightly and the connections I can see around the blower seem to be fine, can't get behind the fan sped controller yet to check that. Any ideas?

Have you checked the battery voltage ?

I think the blower motor will be worn out so only running at full power when perfectly aligned by deceleration. 

On the Mk1, isn't there enough access to remove the blower without removing the dash?  Just 3 Torx screws?  (Might be confusing it with another car so do double check)

13 hours ago, unofix said:

Have you checked the battery voltage ?

Not yet as all the other systems seemed fine but I'll have a look.

You beat me to it, Tom. I was about to say the same. A worn commutator and/or brushes and enough endfloat on the shaft to restore good contact when it moves under braking.

7 minutes ago, TomsFocus said:

I think the blower motor will be worn out so only running at full power when perfectly aligned by deceleration. 

On the Mk1, isn't there enough access to remove the blower without removing the dash?  Just 3 Torx screws?  (Might be confusing it with another car so do double check)

Haven't looked yet, I assumed the worst but if it can come out easily I'll pull it and check, could be the brushes have gone maybe and deceleration is making better contact.

Just checked, the return spring holding the brush against the commutator was broken, a rare easy fix....