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Firstly, thank you both for your responses and especially ‘neromoto’ for the attachments to his replies.

Secondly – and to my considerable embarrassment – I have to admit that I mis-described the problem in my original posting, due to writing from memory. I had thought it was just the temperature control slider that was the issue, but, having had the car out for a run to check what Theo suggested, it now appears that the problem goes beyond that.

So, while the slider is definitely stuck solid towards the ‘cold’ end of its range of movement (see attached photo), I can now report that the air distribution controls are not working either. While it is possible to depress any of the ‘A/E’ ‘RISC’ and ‘SBR’ buttons this makes no difference to the actual air distribution – air is coming out of the screen demister vents only. On the upside, depressing ‘Stop’ has the proper effect of shutting off all airflow, albeit that the slider control stays stuck when it should move to the extreme left of its range.

The German language information covering the Heating and Ventilation System provided by ‘neromoto’ sent me to the English language version of the Concise Workshop Manual (CWM) at pp. 59-63 and so I now have a much better grasp of how the whole system works. In particular, I understand that the temperature control slider does not act to control the amount of hot engine coolant passing through the heater core, but rather to control the mixing of ambient temperature air and warm air, heated by passing over/through the heater core. So it is more accurately described as an air blender control.

I actually spent some considerable time looking at this section of the CWM, working out how the different system modes set out in the table on p. 60 mapped onto the diagram on p. 59. It was also very helpful to have ‘neromoto’s coloured versions of the individual system statuses. I’ll come back to those later.

The fact that I can only get air via the front windscreen demister vents regardless of which of the three air distribution option buttons is depressed indicates that Shutter C is stuck in position 2, where it blocks air from exiting via the cabin vents, forcing it up onto the windscreen instead. According to the CWM, that is the correct position for Shutter C when button ‘SBR’ is depressed. Shutter A should then be open in position 1, admitting outside air, while Shutter B should be in position 1, forcing all of that air to pass over/through the heater core before being direct onto the front windscreen by Shutter C in position 2. All of these shutter positions should be controlled by the vacuum system. However, the air I am getting out of the windscreen vents appears not to be heated, but rather seems to be at ambient temperature. That is currently relatively warm, due to it being summer. This implies that Shutter B is actually in position 2 (or a position very near that), allowing the air admitted via Shutter A to bypass the heater core on its way to the demister vents.

Selecting ‘RISC’ should produce a different set-up. According to the CWM, Shutter A moves to position 1 so as to admit outside air, while shutter Shutter C also moves to its position 1, blocking the path to the demister vents and forcing the airflow towards the cabin vents. Both these actions are controlled by the vacuum system, but vacuum control of Shutter B ceases in favour of manual control via the temperature control slider, which is is connected to Shutter B by a sheathed cable. This allows Shutter B to be set in a variable position between its (vacuum controlled) positions 1 and 2. This is the air blender function in action, since this manual control allows the operator to control how much air passes over/through the heater core and how much is allowed to bypass it. The resulting blended air then passes into the cabin as directed by Shutter C.

However, ‘neromoto’ has added some handwritten annotations in his native German language to his coloured diagrams of the heating and ventilation system and I think these are saying that the system as shown in the diagrams is only valid for early cars, up to chassis no. 2785 (‘bis 2785 Chassis’). More specifically, on the diagram for the system in ‘SBR’ mode, I think he is saying that on subsequent cars there was a modification allowing manual control of the temperature of the air sent to the windscreen demisting vents. Here I refer particularly to his annotation ‘Altes System warm, neues System regulierbar’, which Google Translate renders into English as ‘Old system warm, new system adjustable’. This makes the modified system’s ‘SBR’ mode identical to ‘RSC’ mode in operation, save for the final direction of the air (to the windscreen rather than into the cabin). My Gamma berlina was built in 1978 with the chassis no. 5798, so, if I have interpreted the handwritten German annotations correctly, it would benefit from this modification.

In turn, this would explain why the air exiting the windscreen demister vents of my car (with the air distribution vacuum system seemingly stuck in ‘SBR’ mode) is not warmed air, as the CWM indicates it should be, but rather seems to be at summer-warm ambient temperature. With the temperature control slider ‘in play’ per the modification referenced above but seized in place towards the ‘cold’ end of its range of movement, the fresh air admitted via Shutter A is largely, (if not quite entirely), bypassing the heater core and therefore emerging from the windscreen vents still at ambient temperature, (or something very close to it).

Of course, all this speculation on my part doesn’t really get me any closer to a diagnosis of what has actually gone wrong. On the face of it, a seized temperature control slider cable shouldn’t affect the operation of the vacuum system other than the operation of that system on Shutter B, so why doesn’t depressing button ‘RISC’ sent air to the cabin vents (by causing Shutter C to move to its position 1)? To my mind this points towards issues with both the cable and the vacuum system.

On reflection, I think I shall be handing this problem over to my lucky mechanic! The car is already booked into the workshop for 17 July, for other matters needing attention. Meanwhile, I should be grateful;

1)   if ‘neromoto’ would please let me know whether or not I have understood his annotations correctly – and does the chassis number referenced refer to Gamma berlina models, not coupés?
2)   for any further suggestions as to what has gone wrong, now that I have given the correct full description of the fault; and
3)   for any information as to the physical location of the likely faulty parts and how to access them Presumably the dashboard cowl will need to come off?

I’ll feed this information to my mechanic, who I am sure will be very appreciative of it.

Thank you, PB.