Cut and pasted from BANNED, as its a good discussion for this list: So, my 450SLC 5.0 has the all-heat-all-the-time thing going on. Everyone once in a while the climate control works exactly as it should, but about 95% of the time its just heat. I've been doing some diag, following the processes in the manual...
Heres what I can gather about the system so far: As far as vacuum issues go, if it doesn't work at all (except defrost) its because you never get vacuum to the main vacuum switch to turn the system on. If you have enough vacuum to switch that switch (you don't need much), then you consider if all the flappers work as you'd expect to see if they're leaking by old-fashioned trouble shooting with a tester. They're controlled by a combination of what button you press and what position the servo is in (based on temps). When you really study the diagrams for a while, its really not very complicated. You've got normal auto, a few overrides for bilevel, and the servo comes in to determine if the center vent should open or not (heat on/off). There are a few more things that happen for AC, but this is really just overriding again to close fresh air vent, etc. Now, electrically, except for the vacuum switches, the system doesn't care what is going on vacuum-wise. From what I can see, control of the temperature (heater valve) is only dependent on the sensor chain and amplifier telling the servo what to do. I don't see any connection between a vacuum issue (leak) causing the servo to not respond to temperature correctly, as long as there is enough vacuum to keep the main switch closed. Heres my situation: Vacuum: -I have what appears to be low vacuum when running... 7-10 inches on the test connection. The line from the vacuum tank is supplying 20+ inches to the push buttons, so thats no issue. -Vacuum falls off shortly after shutting off engine. So there is a leak somewhere, but its not obvious after checking a few lines. They all seem to be leaking?! -I think the leak might in the connections inside the servo... these rubber lines are very loose fitting. I'm going to try replacing them. Electrical: -If I connect power directly to the servo motor, it moves. But, it seems the amplifier is never sending power to the servo motor when needed. It never parks, or moves at all. -I replaced the amp with a new (really new) one, and it didn't make any difference. -I checked the interior temp, exterior temp, and temp dial and all are working correctly. So, if my understanding is correct, the vacuum leaks aren't necessarily good, but should effect the fact that the amp doesn't move the servo. So I have some issue with the electrical part of the servo (like the feedback potentiometer), or I've got a bunch of bad amps. Does this sound logical?! Jaime _______________________________________ http://www.okiebenz.com For new and used parts go to www.okiebenz.com To search list archives http://www.okiebenz.com/archive/ To Unsubscribe or change delivery options go to: http://mail.okiebenz.com/mailman/listinfo/mercedes_okiebenz.com
