Replaced the heater control valve in a 88, 190e 2.3. Heater blows hot, then cold, then back to hot. The heater doesn't seem to operate at a comfortable warm temperature. I've heard there is another valve behind the glove box but before I throw money at this issue, perhaps someone has some advice! Thanks for the help!

Behind it you will probably find a deteriorated foam tube connecting the dash-mounted interior temperature sensor with a plastic tube going to the blower. That tube allows temperature regulation by sampling inside air.

The tube may be replaced with OE foam, foam pipe insulation, or a simple piece of plastic tubing. I did the latter on my 1988 560SL and it has been regulating fine ever since.

Yoiu need a new auxhiliary water pump .The main coolant pump is not enough to pump water through the heater on its own .Look under the expansion tank it is the location of it and as three water pipes going on to it .and an electrical conection on the rear of the pump .Put 12 volts to it and see if it works before you get another one .

On a 190, the aux pump will be on the firewall. But its unlikely to be the problem. When the pump fails, it shorts the system and trips the circuit breaker. Its remotely possible that its failing intermittently, the way to check is to just unplug it. Its not critical to the operation of the heater.

The 190 system is enormously complex. Temperature is set by the head unit, but there are half a dozen hard to reach components involved. First there os a sensor near the mirror. There is a tiny fan connected to this via a hose as described. But again, the fan and hose could be discarded and ot would have noimpact on your particular problem.

Behind the dash, just above the passengers knee, there is a five port switchover valve. This reacts to various signals from sensors and the head unit to apply or vent vacuum to any if five vacuum pods. Mercedes made a test box for dealers to verify its operation. Without that, testing is a complicated process of applying various electrical signals and verifying vacuum operation with a Mityvac.

There are five vacuun pods in the heater circuit. The two most relevant are the water valve located behind the battery, and the air blend flap located at the very top of the heater box (requires pulling the entire dashboard). There is also a blend flap rheostat next to the vacuum control that needs to be tested.

Theres a fairly good writeup on this system online. Google "190 Cacavas"

One last point. This system is vacuum controlled. A leak anywhere in your vacuum system can cause the problem. Start at thenintake manifold and check sll the rubber connectors in the engine compartment

Wow, thanks for your help. You have given me a lot to check out. I will report my findings. Hopefully, sooner rather than later!