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1973 450SLC
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88 Posts
Discussion Starter · #1 ·
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.

IMGP4886.JPG IMGP4883.JPG

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.
IMGP4839.JPG

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.
IMGP4838.JPG

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.
IMGP4841.JPG

Cheers,
Pete

Part 2 to follow...
 

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1973 450SLC
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88 Posts
Discussion Starter · #2 ·
Auxiliary Air Valve Overhaul - AAV (Part 2)

6. Remove and clean the piston and spring
At this stage it is worth giving the piston and cylinder faces a good clean and make sure that the piston moves freely.
IMGP4843.JPG

7. Press out the bulb carrier
The thermostat bulb is held in a carrier that is pressed into the base of the AAV housing. To press this out I used a deep socket.
IMGP4846.JPG IMGP4847.JPG
IMGP4848.JPG IMGP4850.JPG
IMGP4851.JPG

Part 3 to follow...
 

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1973 450SLC
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88 Posts
Discussion Starter · #3 ·
Auxiliary Air Valve Overhaul - AAV (part 3)

8. Remove the bulb from the bulb carrier
The bulb is held in the carrier by a captive washer. This is a pain to remove and I couldn't achieve it without destroying the bulb. No big deal, it doesn't work right?
First remove the extension rod from the bulb. It is a simple pin (the replacement bulb has the same).
IMGP4852.JPG

Next, press on the end of the bulb to apply some pressure to the washer. This will deform the bulb so use some discretion on how hard to press - it won't be enough to push out the washer.
IMGP4853.JPG

Now you'll have to get in there with a pry of some sort the get the washer out. This is where I had to break the top of the old bulb to get enough purchase under the washer to remove it.
IMGP4854.JPG

And, there you have it. The carrier, the washer and the bulb. That's the hard part done.
IMGP4855.JPG IMGP4857.JPG

Part 4 to follow...
 

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1973 450SLC
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88 Posts
Discussion Starter · #5 ·
Auxiliary Air Valve Overhaul - AAV (part 4)

9. Clean bulb and carrier
There is bound to be a certain amount of crud on the carrier. The instructions that come with the bulb recommend using steel wool to clean all the pieces and that worked well.
IMGP4859.JPG

10. Install new bulb into bulb carrier
The top washer of the carrier fits neatly over the new bulb. The bulb needs to be sealed into the carrier so that no coolant leaks into the AAV. For this I've used Waterpump and Thermostat Housing sealant. This seems to be an appropriate use but any high temperature silicone should do the trick. The sealant was applied liberally around the bulb before placing into the carrier.
IMGP4861.JPG IMGP4864.JPG

Then use a socket (as close to the outer diameter of the washer as possible) to press home the top washer. Any sealant that is squeezed out can be wiped away.
IMGP4866.JPG IMGP4867.JPG

And there we have it - a nice newly populated bulb carrier. The sealant will need to cure for at least 24 hours before sealing and testing the AAV.
IMGP4869.JPG IMGP4870.JPG
 

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1973 450SLC
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Discussion Starter · #6 ·
Auxiliary Air Valve Overhaul - AAV (part 5)

11. Press the bulb carrier assembly into the AAV housing
A socket was used to press the bulb carrier back into the AAV housing. The bulb instructions recommended using loctite 638 bearing mount. This is a high gap filler - I only had regular loctite 641 bearing retainer and I think that is a reasonable compromise. It is a good interference fit.

IMGP4871.JPG IMGP4872.JPG

Press in the carrier until it is exactly flush with the bottom of the AAV housing.
IMGP4873.JPG

12. Reinstall the piston, spring and AAV top
The outlet port of the bottom AAV housing has machining to channel air to the piston. Mark the extents of this machining so that it can be lined up with the cutout in the AVV top.
IMGP4874.JPG IMGP4876.JPG

With the spring and piston installed into the AAV top, press it by hand into the AAV housing, ensuring that the port machining and cutout match up.
IMGP4877.JPG

Press the AAV top into the AAV housing until it bottoms out. Note that no sealant or retaining fluid is required on this part.
IMGP4878.JPG IMGP4879.JPG
 

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1973 450SLC
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Discussion Starter · #7 ·
Auxiliary Air Valve Overhaul - AAV (part 6)

13. Test operation
Once the sealant is cured it is time to run a test on the new assembly. In the following pictures you can see the piston through the inlet port. On the left is the cold condition where air will flow through the port. In the right picture a heated bulb has pushed the piston up to cover the port, restricting airflow through the AAV.
Result!

IMGP4880.JPG IMGP4882.JPG

14. Reinstall the AAV
The mating surfaces for the AAV and the inlet manifold must be cleaned up and either a gasket or sealer applied. I've opted for more of the Thermostat Housing Sealant.
IMGP4885.JPG

Once its bolted up, all that is required is to wait for the sealant to cure, re-attach the air hoses, install the air filter and refill the coolant.

Sure enough this provided a big difference in air supply between cold and hot. I'm pretty sure that the idle mixture has been adjusted to counteract the inoperative AAV because I've got very high revs at cold. But that will be the next adventure...

Hope this helps someone!
Cheers
Pete
 

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Premium Member
1987 560SL
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2,391 Posts
Wow excellent pictorial and detailed step by step. Wish I had this when I squeezed my AAV bulb in a vice. The link is also good news for anyone with this issue AAVpart selling for $69 as of this posting.
I think this should be nominated for a sticky in the EGv107. Note the AAV transition temperature is 55 degrees C.

 

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Premium Member
2007 CL600, 2003 SL55AMG, 1987 560SEL ECE, 1985 280SL
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1,522 Posts
Great work! A candidate for the EGV107!
 

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'72 350SL, '85 300D, '98 E320, '19 Subaru Outback (sold '14 GLK250)
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11,947 Posts
Great work! A candidate for the EGV107!
Agreed! There are at least two others there already, but they don't cover bulb replacement.
 
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