I am working the bugs out of a new to me '78 Bronco that had air conditioning, for some reason I had no power to the heater fan switch, but I got it wired to work correctly, now I am trying to get the heater control working (switching from A/C-Vent-defroster), I have power going to the switch but as I slide the leaver across nothing changes, what all controls the the different settings. I know there are all the vacuum hoses along with this but I am stumped where to look next?


Also is the any tricks to the lights on the instrument cluster? If I wiggle the bulb to the left or right of the speedo cluster my lights will come on and off as I spin the bulb, I guess the circuit board is getting old !

 

The doors that control the heater and a/c vents are vacuum operated. The main hot/cold door is operated by a cable and runs to an actuator under the hood on the a/c box.

As for the instrument lights, you can try cleaning the bulb contacts and the circuit board where they get power with rubbing alcohol. One big problem with these old instrument clusters is that they used a plastic that rots over time. The back of the cluster can just turn to dust. You can get a replacement from Dennis Carpenter Ford Parts or from late '80s to early '90s Econoline vans. Just make sure if you have oil and amp gauges or lights to order the correct cluster back. They are not the same.

 

1973/79 F100/350 & 1978/79 Bronco with factory installed integral A/C

Located at the backside of the AC/Heater control panel is this:

D5TZ-19B888-A .. A/C Vacuum Control Selector Valve Assy (Motorcraft YH-285).

All the vacuum hoses from the various damper door motors connect to it.

It also controls the vacuum controlled heater water valve (D4AZ-18495-A) that is spliced into the inlet to heater core heater hose.

The valve also includes the 'kicker switch' for the compressor.

When you slide the lever on the control panel, this valve opens one damper door, closes another.

Look at the ends of the vacuum hoses, they age crack...causing vacuum leaks.

It's possible that one or more of the vacuum motors has failed, ditto this valve.

Left (upper) A/C/Heater control panel and 19B888 valve / Right: The maze of parts behind the dash, there are so many vacuum motors (18A318) that Ford numbered them.

Left (lower-upper pic): 18495 heater water valve

 

Ok thank you for the help. ill try looking into that valve, hopefully ill get a chance to tonight.
ill also try the alcohol on the back of tge cluster, the plastic housing is still holding up ok on it.

 

The damper doors that switch from defrost/heat and dash vents are vacuum controlled. If moving the lever does nothing, the first thing to check is that you have vacuum going to the controls under the dash. There should be a small vacuum reservoir on the drivers side splash pan, on the '78 Bronco is should be a small plastic "ball". It will have two lines going to it. One goes to the vacuum tree on the intake manifold, the other goes through the firewall to the vacuum switches on the A/C control.
Where the vacuum line goes through the firewall you'll see a second line, this is a vacuum line from the A/C control, it goes to the hot water shut off valve located in one of the heater hoses.
The vacuum lines at the heater control are (or at least were originally) color coded. There are two switches, one on top of the control and the other located on the bottom of the control. It's not uncommon, especially on the bottom switch, for the switch to fall apart.
The first two pictures are of the upper and lower vacuum switches, the next is a picture of a lower switch that has fallen apart.
The next pictures are of the vacuum line harness. The lines are all color coded and the plugs only go on one way.