The thermostat bulb in the AAV on my 73 450SLC (D-Jet) has never worked in my ownership of the car. I'd balked at the price of a new AAV but when AAV Parts (aavpart.com) came out with a cost effective bulb replacement I bit the bullet and bought one. Note that the AAV on this earlier 450SLC has the air inlet and outlet at different heights (see images below). Later AAVs had the inlet and outlet at the same level and this procedure will need to be modified to suite them.
1. Drain some coolant out of the system
Because the thermostat bulb is dipping into the coolant you'll need to drain a couple of litres so that the level falls below the AAV.
2. Remove the inlet and outlet air hoses
Remove the air filter to gain better access.
The inlet hose is a "y" hose shared with the idle air mixture so this will also need to be removed.
3. Remove the AAV
Remove the two bolts holding down the AAV and break it loose from the gasket.
4. Confirm the problem
There is a piston inside the AAV that is controlled by the thermostat bulb. When the coolant is cold the piston will be retracted allowing air to flow from the inlet to the outlet. As the coolant warms the thermostat bulb pushes on the piston which cuts off airflow from the inlet to outlet. I put the AAV bulb into near boiling water and the piston did not move (Note: I have had the AAV apart before and confirmed that the piston moves freely).
Here is a view of the piston through the inlet port. It didn't move regardless of the bulb temperature.
Performing the same hot water test on the replacement bulb saw an extension of around 16mm from cold (retracted) to hot (extended). It should also be noted that the extension rod from the bulb will hold significant force. The piston spring in the AAV is quite strong yet the extension rod will easily overcome it. There would need to be significant friction within the piston to stop it from working.
5. Pry off the top of the AAV
Now for these older AAVs they have a perfect lip to allow a pry bar to extract the top of the AAV (I just used a large screw driver with the AAV in a vice). Later AAVs do not have this luxury but I don't have one to try - so you'll just have to get creative.
Here is what you'll get when you open up the AAV - also shown is the pry lip that I've described.
Part 2 to follow...