I have great heat in city Traffic, but as soon as I get to freeway speeds I loose all my heat. Could it be the thermostat or something else?

1984 500SEC euro


Cardude

A torn diaphragm on the mono-heat valve will do exactly what you describe. If its a European car with dual temperature wheels it has a heater valve with two inserts.The heater valve should be behind the false firewall to the right of the battery(looking from under the hood). It has 4 slot-head screws in the top that hold the insert in the housing. The insert costs around $30 and is easy to replace.
Regards,
Randy D.

It only has one temp wheel. Is it the same part and same location?

Cardude