1983LU5TA

Trying to get the heater up and running on my 91 Formula. The heater control valve isn't opening due to no vacuum being present at the vacuum line. All of the mode doors operate properly for each setting of the control head. There don't appear to be any breaks in the line under the hood, so which vacuum line on the back of the control head operated the heater control valve? I've been searching all over the internet for a diagram, but to no avail. Anybody have any ideas?

OrangeBird

iTrader: (1)

Were this mine to diagnose , I'd remove the control head and gain access to the chassis of vacuum lines . I'd have a buddy blow on the line under the hood that is supposed to operate the valve . I would then follow back whichever line had the air blowing back through it and try to find the blockage . If it's in the control head , there may be no fix other than a new one . But at least if ya get the air blowing back into the vacuum connector , at least you'll know that the line under the dash is ok ..

Damon

Generally on most 3rd gens of the fuel injected era it's on the passenger side near the rear of the intake manifold. There is one vacuum line coming off the manifold that goes to a check valve (a black cylinder with one side tapered in a cone shape). From there it goes back to the firewall behind it where it hits a vacuum T. One side of the T has a vacuum hardline that joins the wires in the wiring harness and goes up through the firewall, under the dash to the HVAC controls. The other side of the T goes all the way over to the driver's side inner fender to a black plastic "orb of death", also known as a vacuum reservoir, that holds some vacuum so the HVAC controls have some to operate with even under brief conditions of low manifold vacuum (heavy thottle).

That's basically it. The original connection point on the intake (source of vacuum) will vary depending on what type of engine it is (TBI or TPI). The rest, from the check valve on back, is pretty universal.

sofakingdom

iTrader: (1)

Each of the vac lines at the control head is color coded; yellow, red, blue, grey, etc. If memory serves, the heater control valve one isgrey, and the supply one purple. But I could be wrong... as they say, my memory was the second thing that started to deteriorate as I got older, but I forgot what the first thing was...

Maverick H1L

iTrader: (8)

Wrong, the heater control valve line is black...

There is a separate mode switch connected to the temp lever, on the left side, on the bottom of the control head. It only has 2 lines, an in and an out. It's either "on" or "off", and it's only "off" when the temp lever is at FULL COLD.