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Old 09-29-2003, 04:59 PM
sixto's Avatar
smoke gets in your eyes
Join Date: Jun 2000
Location: SF Bay Area
Posts: 20,319
W126 87 300SDL - ACC unexpected bi-level

When I set the ACC to cool after a hot restart, the side vents give cool air from the main register and hot air from the top register and windshield vents. Eventually the hot air gets cooler to the point of being proper AC. Theories:

- the heater core heat soaks while the engine is off and the monovalve is open

- the monovalve is taking its sweet time closing

- the R134a conversion in unable to quickly overcome residual heat in the heater core

This isn't a problem in the S420 or SE (SD AC doesn't work) or I don't notice it.

I replaced the monovalve cartridge yesterday. The old one looks as good as the new one. No change. I haven't checked the monovalve ground signal but I don't know what to expect.

Any ideas?

95 S420
91 300SE
87 300SDL
83 300SD

Last edited by sixto; 09-29-2003 at 05:54 PM.
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Old 09-29-2003, 05:44 PM
Registered User
Join Date: Mar 2003
Location: franklin,mass
Posts: 211
Sixto, now that my 300sd heater is working, thanks to you, I have noticed the same thing. I thought it was something to do with the instant on wind screen defrost. Mine will fog the screen momentarely, then clear right away. What I have started to do is turn off ac before I shut down engine. This seems to help, I think, I was begining to think it was the nature of the system. steve
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