hi to all and thanks for a very informative bord!
i have a problem that i hope some one will be able to help me with,
a few days ago i got into my 83 380SL, after starting the motor i attempted to get the heat on, pushed the bottons and nothing happened... meaning, the fan will not go on at all, now this was the final blow i guess.. ( the climate control would only give full blast heat since i have bought the car abut a year ago,)any wey, knowing that the control unit(the box with the bottons) has acted up before, i hurried to my local Mercedes salvage yard and bought a used control box(pn 107.830.26.85)and as expected,the fan came back on,(still only full hot) so, being who i am, i was looking for answers, i opened the bottom cover on the old control box and noticed that the center of the printed circuit is showing signs of getting hot, i qwikly touched the newly hooked up box and it was very hot!!! i promptly disconnected it, next thing to do was to research this great web site, came up with two good ideas, first, to check and see if the auxiliary water pomp is not seized or dragging ..(simple enough, i disconcerted it) second thing, to check the monovalve (took it apart, looks fine, also left it un hooked!!) i reconnected the control box ,ran the engine, and to my dismay, the box is still getting warm ,and fast!! i need help!!! thanks, mike

I'm not sure that the heat is a problem, but the other outputs to check are the switchover valves that control the vacuum to the control elements.

The logical reason for all the heat from the system is that the monovalve is not being closed (allowing hot water into the heater core). The pushbutton does this by grounding the valve. Test the voltage at the two valve connections. One should be battery voltage and the other should be arouind 1v (the PBC doesn't pull the thing all the way to ground). Power on both sides means that the PBC isn't controlling. Power on one side and zero on the other side can mean an open circuit in the valve coil (also keeping it from functioning).

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

thanks Steve
what i did today is to disconnect every thing that was i sight that might have any thing to do with the climate control system that includes the auxiliary pump,monovaleve ,and all four switch over valves , i then started the car and with in 3 minutes i could sense that the solenoids that are inside the pbc are getting hot( remember that in the first post i told you that i took the bottom cover of the bpc so that i can easily feel if it is getting hot) well.... with no other choice i resorted to an ancient method... i went to my 83 300D , got the pcb out, took the bottom cover off and started the engine and you guessed it.. this one gets very warm also..and its been working for years.. so.. what can i say..