Subject: [MB] 1987 W126 intermittent climate control
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1. Subject MB 1987 W126 intermittent climate control w SOLUTION LONG Date Wednesday May 8 2002 11 30 PM From Christopher J Pikus lt cpikus san rr com gt Reply To mercedes mercedeslist com To lt mercedes mercedeslist com gt Good evening all I have been meaning to write this for the past week or so but hadn t gotten around to it After I wrote that last climate control message I thought I d sit down and just write it up It s a bit long but I think it will be useful to anyone who has a modern climate control system W124 W201 amp 1986 up W126 Basically my Type III IIA climate control would oscillate from working perfectly to blowing hot air out of the defrost vents The car would stay cool but it would be fighting itself and I found it really annoying and obtrusive This is kind of long posting because I ll be going through the diagnosis in a kind of narrative style You can skip to the bottom if you just want to see the answer and the reason behind it The story starts about three weeks ago when I was getting ready to drive from San Diego to Phoenix in order to attend my brother s wedding Since I was also going to be driving my parents too the 42 SEL was the mandatory choice for travel Having been recently inspired by Marshall Booth s posting saying that he needed to drive each car in his fleet at Least 100 miles week in order to keep them in fighting trim I decided to take the 420SEL out of its coccoon i2. and not sooner Nonetheless since it s common wisdom that these things are troublesome in their old age I decide to unplug it as a test I m sure my Dad thought it was strange that I was going to fix the A C by unplugging something under the hood But it s an easy thing to try so I go ahead SO I UNPLUG THE AUXILLIARY WATER PUMP AND IT WORKS PERFECTLY I take it for a victory drive and the ACC works as it should It reaches the optimum temperature and the fans go to their lowest speed setting just like they re supposed to And it keeps working Why It has been covered before that the electrically driven Aux water pump starts drawing more current as it ages And this is what happened The device is only supposed to draw about 1 0 amperes in normal operation But as it gets older it starts blocking up and takes more current to operate If it truly was stuck then that 5 ohm load would be drawing almost 3 0 amps Now the climate control unit is all electronic and all the output signals are really transistors arranged as open collector drivers Each of these NPN power transistors have their emitters connected to a common ground rail which in turn passes through a current measuring shunt resistor prior to being connected to the real ground When the current measurement gets too large the climate control unit cuts out all the outputs Csince it can t know which output is shorted for 30 seconds and retries What was happening
3. ared for this I get the car back and it does indeed work So I m happy I depart for phoenix on Thursday at noon I m on the road twenty minutes and it goes into heat mode again I m about the abort the trip and take my cramped coupe because of flakey climate control This means buying plane tickets for the parents because there s no way to fold them into the back seat of the coupe Back to the shop We discuss it a bit and it occurs to me that wild temperature fluctuations are often caused by little airflow across the temperature sensor in the cieling I blow on the sensor and the system decides to blow cold again Now that I have a temporary fix I decide that I can risk a drive across the desert in the car So I m on my way and whenever I notice that the car is no longer blowing cold air I blow some hot air of my own into the temperature sensor and it gets the message It still bugs me but I can live with it So on friday in between some pre wedding festivities I pull the glove compartment out of the car and try to see why the air is not flowing over the sensor The aspirator fan is always powered whenever the ignition is on so it s not electrical And the hose seems solid if it had a leak the temperature control would never be stable not be intermittent So I rule out that cause and put the glove compartment back together Is it air currents that accidentally blow a snootful of cold air into the sensor and cause it to go i
4. circuit has been eliminated Mercedes Benz ACC Service Manual Model 126 CLUE CLUE So the next day I m driving around and I ask my dad to look at his watch and time the inteval between when the recirc Light goes out and when it comes back on Whadaya know It s approximately 3 seconds CLUE CLUE CLUE So it s back to the schematics to see how many output sources there are and which ones could be causing this problem There are several 1 Monovalve Seven vacuum actuator solenoids Aux water pump Blower motor control Compressor control CKlima relay Recirculation switch LED aAmKRWN So I pull the control unit and start probing the resistances of the output connections 1 Monovalve 12 ohms in spec 2 vacuum actuators 58 ohms in spec 3 aux water pump 5 ohms no spec 4 blower motor control goes to a solid state module with high impedence and draws negligible current Besides it s not part of the short circuit protection logic 5 KLIMA relay Another high impedence solid state input with negligible current draw and also above suspicion 6 recirc LED draws so little current it too is above suspicion So everything looks normal But there s an overcurrent condition And everything meets spec those that have specs But an electric motor that is 5 ohms would draw about 3 0 amps neglecting back EMF but the book says 1 0 amps But why would that be a problem when the car reaches optimum temperature
5. n order to shake it down prior to the big trip Normally I drive my other Benzes and the W126 only gets driven occasionally about 20 30 miles per week So here I am taking the cats for a drive and everything is great Then all of a sudden the vents are no longer putting out cold air I play with the switches and nothing changes Even the fan speed switch has no effect Then I feel around the vents and note that hot air is coming out of the defrost vents And it s 90 degrees out 32C for you metric folk A little later the AC kicks on full blast and the cabin cools down again Five minutes later it goes into heat mode Then cooldown Again the buttons have no effect even the fan speed But the fan speed isn t the full blast speed that the defroster goes into The astute reader will realize that that is a CLUE but said CLUE was lost on me at the time I had visions of myself trying to drive across the Arizona desert with the ACC in defrost mode And my parents would be withering in the back seat This is clearly unacceptable So I make an appointment at the local independent shop to look at it Tuesday morning That s about t minus 48 hours for departure to PHX Tuesday comes and the car is in the shop I stress the need for speed as I need to start my drive to PHX on thursday morning Given the symptoms they diagnose it to be the climate control unit and replace it CZ had discussed this with them before hand and was prep
6. nto heat mode Blocking direct airflow into it doesn t affect it That theory shot down Fast forward about a week Through the wedding the drive back and shuttling the ancestors around San Diego that week One evening before bed I read through the MB shop manual for the climate control and memorize the schematic diagrams for the 126 xx system And I ponder some more It s got to be an intermittent sensor but which one So I m driving down the expressway because how else do you get around in California and I have the recirculate switch on just to keep the cold air in the car And I notice whenever the system goes on the blink the indicator light in the recirc switch goes out CLUE Aha says I The system is losing its ground all the sensors take on unusual values and the system goes haywire Well if it s an intermittent ground then I have something to look for even though I don t relish digging around behind the dash looking for a Loose wire But when the system comes back the recirculation Light comes back on If we truly were losing ground then the control unit would be reset and the recirc light should have stayed off CLUE So I go back an read through the MB climate control manual again And I come across the following passage on page A15 ALL output signals are switched off except the blower control in case of a short circuit Outputs are switched on again within 30 seconds after the short
7. rn the printed circuit trace in the electromechanical climate control unit If you have a pre 1985 ACC you d be better off just umplugging the water pump That s my story chris pikus 1995 E320 coupe 1987 42QSEL 1977 240D OK KK K OK OK K K K K OB K K 2K K K KK K 2K OR KK OK K K 2K K K FK 2K KK KK KK K 2K KK KK KK KK KK KK KK KKK KKK KKK Christopher J Pikus San Diego California Email cpikus san rr com Mobile 1 858 254 4159 FKK 8 OK OK K K K K K K K K 2K K K KK K 2K 2K K FK 2K K KK K 2K K K KB KK K FK KK KK KKK KKK KKK KK KK KKK KK KKK KKK This message is from the Mercedes Benz Discussion List This list is sponsored by Performance Products Parts and Accessories for Mercedes r Catalogs 1 800 243 1220 http www performanceproducts com default asp referer 1026 amp source TXX2T35 Home http www mercedeslist com mailman listinfo mercedes FAQs http hsb baylor edu html easley autofaqs main htm Archives http www mercedeslist com mailman private mercedes Admin mailto mercedes admin mercedeslist com
8. was that the aux water pump was drawing 3 0 amps Keeping the monovave in the cold position takes another amp Each of the vacuum actuator solenoids take about 1 5 amp each So the water pump plus the monovalve plus a few flap closures ran the current right up near the limit As the cabin cooled down and neared the correct temperature the control unit tried to close one more flap which added even more current to the mix That last 200 mA was the straw that broke the camel s back so to speak The climate control unit shuts down all the outputs Thirty seconds later it retries since the car has warmed up significantly in the heat the system goes into max cool mode which requieres less flaps closed Cand hence less current everything seems to work perfectly Again as the car cools down it tries to close one more flap and cuts out again The moral of the story keep an eye on the aux water pump especially on a 10 year old car Years ago Richard Easley suggested wiring a 1 0 amp fuse inline with the pump I had the kit in the glove compartment but hand t gotten around to it yet If you live in the warmer climes you may want to disconnect the aux water pump entirely it s only used to circulate extra hot water through the heater core when you re idling at a stoplight This is expecially important to those with the non electronic Type II climate controls W123 amp pre 86 W126 In those cars an overcurrent situation could bu
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