PURPOSE: Reestablish positive control of air and water flow by cleaning, replacing seals, and checking of heat exchanger.

It has been about 15 years since doing this project on my 1966 250S. I do not know for sure, but think that all W108 chassis have essentially the same heater box and any variations found would be minor. I regret not making notes and taking pictures at that time. However, when the heater box is removed for replacement or repair of the blower motor this would be a good time to recondition the box, air control doors and the heater core.

DISASSEMBLY:

The box is comprised of two large halves, made of a plastic or Bakelite type material, and held together with spring steel 'U' shaped clips that slip over tabs, into recesses cast into the box. The airflow control doors as well as the attach points and hardware necessary for operation is also a part of this box. Mounted on the upper left is the blower motor and squirrel cage fan.

Remove all the external hardware, and if concerned about reassembly, make sketches, bag and label the hardware as it is removed. I normally reinstall bolts, nuts, and screws loosely in their respective locations so I know exactly what went where.

Now carefully remove all the spring 'U' clips. I believe I used a flat blade screwdriver to pry these up and then Channel Lock pliers over the upper edges to hold and keep from flying away. Don't know if these are available as an over the counter part, so if you lose or break one you may have to make a trip to a salvage yard so be careful.

Separate the box halves. Note, some heater boxes had a rubber O ring like seal in a groove; others had a smooth mating surface. Remove the heat exchanger (heater core) from the back half. It will have old rubber seals or a sealing putty on the inlet / outlet tubes which may have to be scraped or cut away. This old sealing material will need to be completely cleaned off both the box and tubes.

It would be prudent to take the heater core to a local radiator shop for an inspection, flush, cleaning and leak check while you clean and prepare the box. Also, the motor and squirrel cage fan should be cleaned, refurbished (including bearings and electrical brushes), and thoroughly checked out - preferably at a qualified electrical shop.

A cleaning of the water control valve and replacement of soft goods - O ring seals - insure ease and freedom of operation. I used hi-temp plumbers grease to lubricate the O ring seal during assembly and it assists in smooth assembly and long-term operation. The disassembly of the water control valves was discussed in a recent THE STAR article do not know the specific issue.

You will find years of dust, dirt, and grime built up on the heater box inside and out. Strong detergent, a stiff brush and water will do wonders to clean it up, make it look like new, and easy to handle during reassembly and installation.

The airflow control doors had a foam type seal. I say had because after all these years you will be lucky to find much of the foam left and what is still there will fall apart when disturbed. I do not recall exactly how the air control doors were attached to the box, but they were removed for cleaning and installation of new seals.

ASSEMBLY:

Inspect all the soft goods (seals) removed and replace all that are damaged or questionable. By now the work and effort necessary to get this box out is quite evident, so repair or replace anything questionable. Some items may not be available from MB suppliers so you will have to be creative. The idea here is to close all unwanted paths for air to escape the box. Items like foam rubber, RTV, body joint seal (like play dough) etc. can be used, just remember around the inlet water tube the material could see temperatures as high as 220 F. Duck tape is not an acceptable item.

New seal material needs to be installed on the airflow control doors. I used the thin, narrow, self-sticking weather stripping insulating material found at Home Depot or Lowes. Be sure it is not too thick or dense preventing the airflow control to close and seal properly - the foam width is not a concern. Reinstall the airflow control doors on the heater box in their proper location making sure they move freely and seal completely with a very light force. You should see 50% or less compression of the seal, more compression will put undue stress on the plastic heater control levers.

Do a mating or fit check of the heater box halves. Look for any indication of warping or uneven contact between the halves. Make a note or mark any areas found not making good contact for reference at final assembly. Fit check the heater core into the heater box and note the penetrations and the type and amount of sealing material that will be required. Make one final inspection of the heater core fins for any that may have been bent over due to handling. A commercial fin comb or two 1-inch plastic putty knives, on either side of a bent fin, make a good straightening tool.

Final installation and assembly of the heater core into the back half of the heater box using new, salvaged, or home made seals to duplicate the seal configuration or arrangement found on disassembly. Prepare the outer half by inserting new O ring or applying a very thin film of RTV to the mating surface. A slightly heavier film on any areas that indicated warping. Mate the two halves and install the spring steel U clips. Wipe any excess RTV that may have squeezed out after all the clips are installed. Now install the external seals over the inlet and outlet water tubes and verify all unwanted openings have been sealed.

The installation of the blower motor and fan is the reverse of disassembly. Do not forget the large foam rubber seal on the fan motor that interfaces with the cowling.

Note: Before installing the heater box in the vehicle insure that the area of the cowling underside, where the blower fan foam rubber seal mates, is clean and free of debris.

When installing the heater box in the vehicle, locate the box and start the mounting hardware on the fire wall, but do not tighten. With the external air intake grill and filter element removed, a small hand can reach into the cowling opening and verify or reposition the foam rubber seal on the blower motor is properly mated to the underside of the cowling.

Note of Caution: Duck tape is a useful item in the workshop. It should not be used anywhere on the heater box. Duck tape that is found at the local hardware store is of a lesser quality and should not be used where you expect pressure differential, heat, and long term application. Heat causes the adhesive to become soft; heat and age will cause the adhesive and material to become brittle, crack and fall off.

Frank Mokry