Our Cars

Richard Simonds

Heater Core Crisis

As we drove home over the Pacific Coast Range on a cool August evening in our 1969 280S sedan, I turned on the heater in defrost mode to remove the dew from the interior of the windshield. Suddenly the windshield fogged up inside. I stopped by the side of the two-lane road, opened the vent-wing windows, turned off the heater, and wiped the windows outside and inside. The outside came clean; inside left a greasy film. Not good.

The evidence

The next morning I noticed white spots on the car’s new carpeting and it was soggy and sticky to the touch; I undid the snaps and removed the carpeting on the left side in front and rear. The foam pads on the backside were black from the floor insulation and I could squeeze liquid from the carpet and the foam padding. When I checked the radiator, it was down two quarts below the tang that shows its level when full. Turning on the heat had emptied two quarts of coolant (ethylene glycol) in my nearly new carpeting.

The diagnosis

A heater-core leak had soaked the floorboards and carpets with coolant, which is a corrosive liquid: Thus the fogged interior windshield, the spots where the color was bleached from the carpets, and sticky residue in the carpets and foam padding.

I called the upholstery shop that installed the carpeting, the company that had provided the carpeting kit, and my independent Mercedes-Benz mechanic. All agreed that it definitely was a coolant leak: The white spots could not be dyed and almost everything under and around the dash would have to be removed to access the heater core to remove it for a rebuild – and reinstallation. The carpeting over the tunnel, on the cross beams under the seats, and on the right side was affected by the seeping coolant; that would require a complete new carpet kit.

The repairs

First, the shop mechanic started the engine, let it run a few minutes, then turned on the heat-control levers to allow hot water into the heater core (the left and right sides are controlled separately). Sure enough, the left side was leaking noticeably and the right side was seeping slightly. After removing all dash panels, the instrument cluster, a/c panel and components, glove box, dropping the steering and shift columns, and removing heater controls and cables, mechanics finally had access to the heater core, which was removed after draining the radiator and disconnecting hoses from the engine side of the firewall.

The next step was to send the heater core to a radiator shop for recoring and resealing of the seams where it had split. New O-rings were installed in the control valves, which allow hot water into the heater core. Meanwhile, the shop checked the heater-fan motor and cleaned all the flaps that control airflow and then reglued the rubber strips that seal the flaps in the housing. When the rebuilt heater core returned, shop mechanics glued heavy-duty foam on all sides that contact the housing. Note: It was the deterioration of a rubber strip that had allowed the heater core to rub on the housing that weakened the seams and caused it to rupture. That was the root cause of the failure – after 47 years of use.

Finally, with everything rebuilt, cleaned and checked for functionality, mechanics reinstalled the heater core, rechecked all operations, reassembled all components in, under and around the dash, and again checked for functionality.

Three weeks and $3,700 later, everything except replacing the carpets was done. Another carpet kit was ordered for $915 plus installation; my 1969 280S was ready to hit the road again.

Why spend that kind of money on an older sedan? Given the build quality and engineering of the W108/109 sedans and the excellent condition of this particular 1969 280S, my wife and I made the decision that, as caretakers of our beautiful classic, it is our responsibility to preserve the car for future generations. And thus, it was done.

For other owners, we strongly recommend that if you’re working behind the dash on your classic, consider removing the heater core and replacing the valve O-rings, flap seals and mounting strips to avoid our messy and expensive crisis.

1. What a mess! Accessing the failed heating core on our 1969 280S required removing much of the car’s front interior, including the carpeting, dash, instrument cluster and a/c.

2. The old core clearly shows deteriorated rubber stripping that allowed the core to rub on the housing, leading to rupture.

3. Refurbished heater controls.

4. Rebuilt, padded core.