My E has 296K miles but today decided to give me too much heat. The heat is constant and does not respond to changes in the dial thermostat. The heat comes out of the side vents but not in the center or down low. I am using the Economy button. Sometimes the fan keeps blowing after I hit the off button. Sometimes the fan speed buttons work, sometimes not. I can drive with a window open or leave the heat off. I checked the diaphram between the coolant near the battery and it did not seem to be torn. What is the best way to evaluate this?

lizem,
I would suggest removing the pushbutton control and resolidering all the connections on the bottom. the job only takes about 2 hours start to finish and it may save your wallet from the rediculous price of a new climate control computer.
Good luck!
Adam

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Yes, indeed resoldering the CC unit MIGHT fix the problem. But you need to check the current draw of the auxilliary coolant pump that is located on the passenger side fender well. It should draw no more than 1.3 Amps. If it is drawing more than 1.3Amps, it probably knocked out an active component in the CC Unit. In this case, you should replace the pump and CC unit. If the cooling system is dirty, you might get lucky and be able to remove and clean and flush the pump and get the current draw below 1.3Amps saving that portion of the expense.

Good luck,

Also check the mono valve, when it fails, it is spring loaded to the open position and there is nothing to keep the hot coolant out.

Harry
86 300 SDL

i've read it somewhere that re-soldering the CC unit
must be done with care and knowledge of how the
unit works, since you may ground out other components
of the circuitry. hence, you need a new CC unit $$$$.

just a word of caution, good luck!!

Here is what I did and for now it works:

Finding a broken wire to the aspirator blower raised hope, but failed to fix
it.
Installing a mono-valve repair kit failed to fix it.
Re-soldering every connection (about 1100) on the two push-button circuit
boards did the trick.
I only saw one questionable connection, everything looked new, but there
must have been some.

My method was to cover the boards with strips of 3/4 inch scotch tape, then
remove one strip and work only that area. I inspected first and noted any
bridges that were suppose to be there, heated each connection for about two
seconds with a 25 watt iron. Inspected, cleaned and moved to the next
strip.

I also sprayed all switches and connections with Caig Lab's R5 contact
cleaner.

Before reinstalling, I replaced the aux heater pump with a 3/8" street el
and a 2' nipple. The pump bench tested OK, so it had not caused my problem.

The most difficult part was removing the engine heat shield, to be able to
get to the aux heater pump.

Thanks for all the tips posted on this list, I would not have even started
without the knowledge gained here.

Harry

86 300 SDL

thanks for the advice everyone.
I assume the cc unit comes out easily after removing the two screws holding the wood-just below cc and above the radio.
I will look for the aux coolant pump in AM.
will go to Radio Shack and get them to check the currant draw. Is the mono valve the one near the battery and held together by 4 screws?

Keep in mind that our MBs are different.

Yes, that is the mono valve and if you buy the repair kit, ($26) just mark for orientation purposes, remove those four screws and lift straight up. Note the order of everything, add the reusable parts to the new part and reassemble.

My aux pump was below the mono valve, it was hard to see and remove.

The CC unit removes as you said. I had to put a steady downward pressure to move it out from under the upper overlap.

I suggest you repair the mono valve and test the system, that may solve all your problems.

If you do the re-solder job, don't put your system to work until the aux pump is verified OK, otherwise, if it has failed, you may fry the CC, as Larry Bible indicated.

It was recommended somewhere, that an inline 2 amp fuse be installed in the aux pump wire to protect the CC. If I had left mine in service, I would have done so.

Good luck

Harry
86 300 SDL

The mono valve looks good-no tears. It is a diaphragm with a needle sticking up into another diaphragm. Should I replace it anyway?

I suggest you clean everything inside and out as best you can, then reinstall. Before you plug it in, test it with jumpers from the battery to make sure it clicks.

Button it up and try the system, if it works, hoorah, if not, only your time is lost.

Please keep us informed.

Harry
86 300 SDL

I have experienced the monovalve syndrome. Close inspection reveals no problems with the valves. But replacing them solves the problem. You cant really tell if they are defective by looking at them.

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lizem,
You mentioned your pushbuttons will not function correctly right?
when you turn it off it stays on? I had the same problem and solved it by resolidering. although the aux pump seizing will fry your controller I would attempt this procedure you may be amazed. I would first try resolidering just the pushbutton switch connections not all 1100 points.
Good Luck,
Adam

Yes. I second Adam's suggestion that you resolder only the pushbutton switch terminals. They are pretty easy to find on the underside of the circuit board. Each switch has two rows of three terminals.

My primary problem symptoms were the fan running in the OFF position, and the AC compressor coming on in the OFF and ECONOMY positions.

The pushbuttons get a lot of stress normally, and a whole lot of stress from people who think violently pushing them in (while angry ) will somehow make the system act right. Once you get your pushbutton switch terminals resoldered - treat them very gently.

It is also worthwhile to resolder the three terminals that connect the round temperature setting dial if you can find them. They are in a row, adjacent to the wheel, on the circuit board.

By resoldering only switch terminals and the dial terminals, you avoid a lot of contact with more sensitive components that are probably still soldered in OK anyway. Its the control hardware terminals that get stressed and cold crack.

You may have other system issues, but this should fix the fan running in OFF.

Ken300D

Thank you all for your advice so far.
I checked the aux coolant pump. There were no obvious external terminals so I partially unplugged the male-female connector and checked the amps between the two prongs. It read 2.3 amps which is higher than 1.3 amps. Thus the aux coolant pump may be bad. I will double check the ammeter reading.
If the amp draw is really 2.3 and I checked it correctly by touching the leads to the plugs, then I assume I have to clean the pump(if lucky) or replace it.
Can I still try to solder the cc unit if the aux coolant pump failed or may fixing the aux pump fix the whole problem? Or does it mean the cc unit definitely need replacement and not just soldering?
Can I ignore the monovalve since I think we have found the problem?

Re advice on fan buttons. Sometimes the unit responds to speed requests sometimes not. Moving the thermostat dial makes no difference. I just get some heat and then shut the system off.

From your test description, I don't think you are familiar with checking current draw. To test current draw you must put the ammeter IN SERIES with the pump. If you did it this way and you indeed have 2.3 amps, the pump is definitely drawing too much current and probably caused CCU failure.

Good luck,