Purchase the Maintenance manual on Cd rom. I think you can get it at this web site. The diagnostics for the climate control are pretty good if you have the time to troubleshoot it. However fixing most stuff requires that you remove most of the dash panel. So I hope you have a spare car or like to get in way deep into a repair project. The cd rom gives you step by step analysis of the system.

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1990 190E 2.6
1996 Grand Voyager 3.3
1985 Mustang GT 5.0 5 SPD
1982 Suzuki GS 750T