R107 cabin heat flap servo - how do you actually get the servo apart without breaking the housing

Started by Chris_K, 09-05-2015 01:14 PM. 9 replies. 986 views.

10-05-2015 09:44 AM

Chris_K ★ Gold - M.A.L.A.K.A.S.

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Join Date: Nov 2009
Location: Brighton-Le-Sands NSW
Posts: 1,516
Club: M.A.L.A.K.A.S.
Car: 1962 Mercedes-Benz 220SE Auto

one for the brains trust.

R107 heater box vacuum element and flap servo reference

R107 450SL 1982. Cabin heat flap is stuck in the cold position. Diagnosed as a failed servo motor (vacuum hold-and-release type, not electric). Servo is mounted behind the dash inside the heater box.

The WIS procedure says "remove servo housing, replace servo, refit". Housing has 4 plastic tabs holding it together that are now 40 years old and brittle. Heard of people breaking the tabs and having to replace the whole housing.

Whats the technique to get the housing apart without breaking the tabs. Heat gun? Specific tool? Hoping a member has done this and remembers

10-05-2015 09:44 AM

Vasilios_Billy

Lifetime Benzer

Join Date: Mar 2008
Location: Campbelltown NSW
Posts: 2,031
Club: M.A.L.A.K.A.S.
Car: 2020 AMG GT C

Chris.

Yes! The tabs are the problem. Do not use a heat gun (the surrounding plastic deforms before the tabs flex). Use a cold approach:

Remove the servo housing as an assembly from the heater box. WIS section 83-200 covers this.

Place the assembly in your freezer for 4 hours. Yes, the freezer. The polypropylene tabs become more flexible at low temperature, not less, because the material has high coefficient of thermal contraction.

Within 5 minutes of removing from the freezer, use a thin flat-blade screwdriver to release each tab while pulling the housing apart with steady pressure. The tabs will release without snapping.

Service the servo. Refit. Allow the assembly to return to ambient before installing in the car so the gasket seals correctly.

ive done this about 14 times across R107, C107, W116, and early W126. Zero tab failures with the freezer method. Approximately 60% tab failure rate without it (the old approach was a heat gun).

10-05-2015 09:44 AM

Hugo_Darlinghurst Senior Member

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Join Date: Feb 2010
Location: Darlinghurst NSW
Posts: 524
Club: Mercedes Classic Drivers Australia (MCDA) - chapter committee
Car: 1968 W108 280SE 5.0 Coupe

Freezer method is not documented in WIS.. it is correct. I have used it on three of my own cars without tab failure. The Werkstatthandbuch was written before the long-term ageing characteristics of the polypropylene were observed..

10-05-2015 09:44 AM

Chris_K ★ Gold - M.A.L.A.K.A.S.

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Join Date: Nov 2009
Location: Brighton-Le-Sands NSW
Posts: 1,221
Club: M.A.L.A.K.A.S.
Car: 1962 Mercedes-Benz 220SE Auto

Done. 4 hours in the freezer, housing apart in under 90 seconds, zero tab damage. Servo swapped, refitted, working.

Mrs Chris saw the heater box sitting in the freezer between the lamb shanks and the spanakopita. She did not say a word. 30 years married to a Mercedes guy has trained her well.

For future searchers: the freezer method works.

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