It seems like I am always having to fiddle with the temperature setting in my '93 500E. It's always getting too hot, or going too cold. Sometimes it seems to work properly, and other times I'll have to make a big adjustment to get it so I'm comfortable. When the A/C is on it blows really cold, and when the heat is full on it works, too. What's up with this thing? Anyone had this?



E

I haven't had to work on that problem on a 500E but I have seen it on other 124 chassis cars.

Aside from electronic problems there is one other common failure that gets over looked. That is the auxilary water pump. On other chassis the pump is just used when it gets real cold (like below 59 deg - we hardly ever see them work {bg}). On the 124 chassis the aux water pump is necessary to keep a uniform temp in the heater core.

The Climate control system uses mixing of heat and cold to get the desired temp. If the heater core is changing temperatures because of different engine speeds the controls can't keep pace.

This problem doesn't occur if you are on the road at a constant speed. It occurs in traffic and after leaving a light. While idling the heater core drops in temp as the flow is reduced (without the aux pump). In order to compensate the heater valve is opened more of the time. As one starts from the light and the engine speed increases the valve is open too much and the temp gets hot fast; and of course the controls try to stop it and often go to far the other way.

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Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician

Eric,

As always, Steve prefectly described the way the system works.

There is one thing I would like to add, since the auxilliary water pump came up as a possible cause of your problem. If the auxilliary water pump gets a little flaky, probably from a slight amount of cooling system contamination, it can draw too much current which will blow your climate control push button unit in the dash, a part that is even more expensive.

It might be a good idea at this point to check the pump for current draw. If it draws more than 1.3 amps, it would probably be a good idea to replace it. As Steve said it could be why you're having varying temperature, and if it draws too much current, it can and probably will blow the pushbutton control unit.

Good luck,

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Larry Bible
'84 Euro 240D, 516K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in
Mercedes automobiles

Thanks for your intelligent and well informed replies.

Steve, the one thing you mentioned about a problem being associated with the aux. water pump would not occur while driving at a steady speed makes me believe my problem is something else. I do have to continue to adjust the setting on trips as the cabin temps changes up and down.

Any other thoughts?

I would check the amperage of the auxillary pump while driving. If it is contiuous and not over 1.3 amps you are fine. I have seen the aspirator pump fail on these causing the in car temp senor to be strange.

The service CD has this system in detail. we have them in partsshop.

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Benzmac:
Donnie Drummonds
1992 500E (very soon I hope
1981 280GE SWB
ASE CERTIFIED MASTER AUTO TECHNICIAN
SERVICE MANAGER FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM

Benzmac,
Realizing that this was covered under warrantee, I took my car in to the local dealer. They think it is the aspirator as you suggested. It was too late for them to finish testing, but I hope this is it.

Thanks Guys!


[This message has been edited by EricH (edited 05-08-2000).]

I have a 93 190E 2.6 with similar problem. Replaced leaking evaporator but still got hot air thru central vents. I froze one of the "inlet doors" and saw temperature dropping and worked fine for a while. When I got hot air coming in again, I placed a valve in the water hose going to the heater pump. Valve can be opened manually at will. As I live in Texas I do not mind having only cold air coming in as climate here is hot almost 10 months. However, such "solution" makes me wonder if it will "damage" something else in the A/C-Haeater system. Any comments are much appreciated.

Had the same problem once. Sampler motor was seized. Replaced it and that fixed the problem.

Sampler motor is to the right and below the glove box, check this first. The pull the trim off the climate control pushbutton unit (two screws underneath on the earlier cars, not sure about later ones) and squirt some contact cleaner in the rheostat for the temp wheel.

Last, and this will be a pain if they are bad, check the vac servos for leaks. You need to remove the glove box (push rivets -- pull the center "plug" out and the rivet should lift out and two screws for the latch loop. Pull the top down to release the hook and pull out, disconnect the lamp wires). Attach a Mitivac each of the vac lines on the vertical switchover valve and see if they hold. If not, you have air direction problems, too, and the dash has to come out to replace them.

I expect either the aspirator or the temp wheel to be the problem. My TE, prior to the recent evaporator/vac servo replacements would do just what yours is -- get cold, then blow hot, then get cold, with the fan randomly running at full speed, then slowing way down. Works perfectly now.

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!