Some MBZ engineer, who apparently was having a bad day, buried the pump that circulates coolant through the heat exchanger in the car that is part of the climate control in a very inaccessable place on the engine side of the firewall.
I think the pump is not operating properly as I have very limited heat available. I get heat with the blower off, but with the blower on, heat only lasts for a few seconds. It seems as though there is a very limited flow through the heat exchanger and high blower quickly exhausts the heat in the coolant quickly.
Any experience out there with this problem?
On my 88 TE, the pump has been relocated to a sensible position, so they realized the error of their ways.

Chances are you have a bad water control valve. The repair is simple. The guts can be replaced by removing the four small screws on the top (with the engine cold preferably).

When you remove it if the diaphram is broken it is the problem for sure. Unlike the 124 body the 126 body doesn't use the pump much (at least not in Florida). There is enough water flow without the pump when the car isn't idling.

Thanks, Steve. Your advice sounds good. As I've only owned this car for about a month and it is very different from the 240's I'm used to, I'd appreciate a heads up on where the "water control valve" is located. On the firewall by the aux. pump?, by the thermostat? Many thanks for your patience with us shade tree mechanic types.

It is in the comparment in front of the windshield. It is a few inches to the center from the battery.

Is it also called the "Monovalve"? I know that it is in that general location.

Yes it is a monovalve unless it is a duovalve. I think all automatic systems use a monovalve.