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If you have 4 Zone Automatic Climate Control there is a neat residual heater function. After you have shut off the engine to lets say fuel up, wait for the kids at school or go into the convenience store with passengers left in the car to face the bone chilling cold, you can push the "REST" button once on the climate control panel. This energizes the heater blower on LOW with an auxiliary electric water pump in the engine compartment to circulate residual warm water thru the heater core in the passenger compartment even with the engine off and the key removed. It will eventually time out so it doesn't drain the battery too much. It also works somewhat in the summer with the AC albeit just circulating residual cool air and not really activating the AC compressor.
Hmmm, wonder if you could hot-wire the "electric auxiliary water pump" to help you limp home if the main water pump failed? Probably not a high enough of a flow rate.
On my W211 I installed 4-zone climatronic to have additional menu for reading HVAC sensors. even it had REST function on old one. . It was plug & play.
I wonder if the same can be done on W212?
I wonder why MB makes climatronic without REST function, when looks like all the cars are ready for it, so it is just a matter of active button.
Anyway, I did drive car with broken belt few times and electric pump is big help. The trick still is to do extreme ECO driving.
When the climatronic basic functions have been very clear on W210 - MB on W211 created chaos. Some don't have REST button, but one member reported that with no REST button, he had REST display showing up on his cluster in certain condition. .
Still even when you have REST button on 2z - those climatronics still don't allow sensor readouts.
So if you want to have hidden functions display - you need to install 4-zones climatronic.
Again plug&play and I paid like $40 on ebay.
That is good part of the story. Bad part is that even on the car with factory 4-z I had sensor #0. I could conclude it is interior sensor, either front of rear, but all my research could not find any info coming from MB about it.
There is a temp sensor in the overhead control unit (dome light assembly) and one near the ignition key behind the dashboard. Both just look like a few tiny slots in the plastic. The sensors actually have a mini electric fan to cycle cabin air over a thermistor device. These thermistors have very thin wires that are prone to oxidation. I had to solder these wires in the overhead console to get it to work again. The one near my ignition key probably has a similar issue because it shows as failed on the MB2 reader. I just haven't looked into it yet as the system seems to perform just fine in automatic without it.
The REST button also has another cool function. If you have the windows down (in any position) and the sunroof open (in any position) and come upon a bunch of dust, smoke or zombies simply push and hold the REST button and all the windows and sunroof will close at the same time. After you have passed the dust, smoke or zombies, push the REST button again and all the windows and sunroof will open again to the same position they were at prior.
I recently switched out my climate control unit for one with a REST button. As some of you know partway through the 2005 model year MB removed the REST button from all U.S. bound W211's (while the rest of the world continued to have the feature until the end of the series run). It took some research to find the correct 2-zone unit for my car, as there are multiple different units (and part numbers) depending on the year, equipment line and climate control system.
With the part number verified, I found an inexpensive used climate control unit on eBay and had it shipped from overseas. The picture tutorial that Drag0na5h posted was extremely helpful and I was able to get my old unit removed and the new one installed without too much trouble. Then with my SDS, I erased the stored codes and reprogrammed it. A necessary step as these units are not plug and play, especially if there are codes stored and/or you own a diesel (a unit from a gas car will not have the electric heater booster setting enabled). So far it's working great!
kajtek1, REST being display on the later (07+) digital climate control clusters in certain conditions is (I think) due to the "Air dehumidification via evaporator" feature to help remove moisture from the A/C system. See the below SDS screenshot.
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