Interior and Upholstery Unable to switch A/C controls

May 8, 2005
So, after the previous fight with getting the car running, she's operating absolutely wonderful. HOWEVER....

Now I cannot change the settings for the climate control. I know normally if there's bad vacuum, it defaults to the defrost. That's just the thing. I replaced all the vacuum lines because I thought that was the problem, but it is also blowing out of the floor vents too. The only thing I can seem to control is the blower speed. Right now I don't appear to have heat either, but have yet to take a look at the heater core to identify that problem. It's still in the upper 80s here so AC is still a concern. There does not seem to be any positive detent on the mode selector switch, so I'm not entirely sure it's good (and finding a replacement seems impossible). Any one have any advice on some other things I should check?
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SN Certified Technician
Mar 10, 2000
Dublin GA
Vacuum, Vacuum and more Vacuum. There are several vacuum servos that open and close the various doors that control airflow. Check the vacuum feed from the engine first and then check for vacuum at the vacuum servo when they are selected on the HVAC control panel.

The following are diagrams courtesy of Tmoss & Stang&2birds

Vacuum diagram 89-93 Mustangs

HVAC vacuum diagram

See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host) for help on 88-95 wiring; Everyone should bookmark this site.

TFI module wiring for 94-95 Mustang GT

Complete computer, actuator & sensor wiring diagram for 94-95 Mustangs

Complete computer, actuator & sensor wiring diagram for 91-93 Mass Air Mustangs

Complete computer, actuator & sensor wiring diagram for 88-90 Mass Air Mustangs

5.0 wiring diagram for Fuel Injectors, Sensors, and Actuators

Ignition switch wiring

O2 sensor wiring harness

Vacuum diagram 89-93 Mustangs

HVAC vacuum diagram

TFI module differences & pin out

Fuse box layout

Mustang 5.0 Lights and Radio schematic, by TMoss:

87-92 power window wiring

93 power window wiring

T5 Cutaway showing T5 internal parts

Visual comparison of the Ford Fuel Injectors, picture by TMoss:

Convertible top motor wiring

Engine mounted fuel injector harness

Location of the TPS, IAB, and the 10-pin connectors on a 5.0, picture by TMoss:

Starter circuit

Alternator diagram for 94-95 Mustangs.