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Old 12-11-2007, 05:55 PM
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Join Date: Oct 2007
Location: Greater Metropolitan Beaverdam VA
Posts: 2,873
1988 560SL Climate Control Repair Report

Symptoms: Heat from footwell, defroster and two outer dash vents is inconsistent, sometimes barely warm, sometimes none. AC operation inconclusive since it is cold now but in any case does not seem to be colder than ambient air temperature. No flow from center dash vents.

Fault isolation: Engine coolant temperature is o.k. and well controlled by the thermostat. Since air is sometimes heated even though barely warm, basic water flow path appears o.k. Problem lies in cabin temperature control system. Pushbutton controller apparently operates normally. Tube from the cabin temperature sensor had been previously replaced and is in good shape.
Research on this forum and ALLDATADIY points to Monovalve as probable suspect.

Repair: Purchase and install a monovalve repair kit (about $45). Without dismounting the Monovalve simply unplug the electrical connector, remove the four cover screws and remove exixting innards. CAREFUL TO OBSERVE THE ORDER IN WHICH THEY ARE INSTALLED. Replace with the new parts in the kit and reinstall cover then plug the connector back in.

Result: Blast furnace level heat in footwell, outer dash vents and defroster nicely controlled by the auto climate control system. Air conditioning works now too. Still no center dash vents. A project for another time.

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Old 12-11-2007, 06:53 PM
Join Date: Apr 2003
Location: NB Canada
Posts: 1,173
Thank you for a nice clear and concise presentation. We all appreciate the time you took to post this.

1986 560SL
2002 Toyota Camry
1993 Lexus
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