86-91 HVAC head unit vacuum routing

[BA_051]

So now it’s winter and getting into the 20’s here in Albuquerque and I desperately need heat... I have completed my heater box and now need to hook up the head unit. I am running the ford SD electric vacuum pump.

The head unit has 4 vacuum lines:
Solid black, rigid: this is the vacuum supply line
Black with red tracer: this is the firewall door flap. Has vacuum in all but off position
Black with green tracer: has vacuum when in heat/def mode and def mode
Black with blue tracer: has vacuum only in def mode.

To sum it up:
Heat mode: vacuum to black/red only
Heat/def mode:vacuum to black/red + black/green
Def mode: vacuum to black/red + black/green + black/blue

Based on the length of the hoses, black/red goes to the firewall door diaphragm
And the black/green and black/blue are the same length.

Does anyone know where the black/green and/or the black/blue lines go? I know one goes to the heat/defrost diaphragm but where does the other go?

[BA_051]

Well thanks to 78 Widetrac, he is parting a few late model grand wags, we were able to figure it all out.

The 85 and earlier FSJ’s use a different heater duct than the 86-91 FSJs... the early use 2 single action diaphragms. One to open the firewall door and one to move a door between heat and defroster. With no vacuum, it defaults to defroster. Both diaphragms have a single vacuum nipple.

The 86-91 fsjs have a different setup for defroster/heat. It’s a single acting diaphragm for the firewall just like the early models, but the heater and defroster diaphragm is a double action, having 2 vacuum nipples. There is also a different setup internally. There is no spring and the lever arm is longer to allow the double diaphragm to move it to the middle and all the way up for heat/defrost and just heat.

I am going to have to swap the early style with the late style to retain full function of the heater control unit (I am OCD and it has to function fully for me).

I will get pictures comparing both the pre 86 and the 86-91 u it’s when I have a replacement to swap in.

[78 WIDETRAC]

Do you have the dash heater control for the 86+ wag?

[BA_051]

That I do! I swapped an entire dash from an 87 or 88 into my 74. I thought the heater duct was the same for all years but it does change from pre-85 to 86-91

But yea, I have swapped everything in to make a new dash cluster panel that holds all 10 gauges. I like the more modern look of the dash in the old school cab/body.

I removed the tailgate and headlamp controls and moved the headlamp switch to the bezel under the column. It

[woodrow]

Wow, you put a 88 dash in a 74 vehicle?
I wouldn't think that would be possible.
Love to see pics

[BA_051]

Yes I did. It was pretty much bolt in except the parking brake assembly. One bolt isnt used but the other lined up fine.

Ill have to get pics this weekend.