94 rear heat?

[1987ss]

i have the rear heat behind the drivers seat and it blows cold i looked under van to see if it was hooked up and it is so im sure it needs the heater core back there can you replace the metal lines under van with rubber ines if so what size? another question on the dash it stays stuck on defrost no matter were you position the switch any suggestions? its gettin cold in minnesota

[old_master]

The two problems are quite possibly related. The HVAC system will default to the defrost position if no vacuum is available to the control head. Some rear heat units use a water valve that is controlled by vacuum also. Without vacuum, temperature cannot be regulated. The vacuum source is from a tee fitting on the PCV valve which is located on the driver side rocker arm cover. A 1/8" plastic line runs across the top of the engine to a check valve/tee fitting. One side of the tee goes to a vacuum reservoir near the evaporator housing, the other goes into the interior where it connects to the control head. The problem usually lies where the line crosses over the engine. The heat dries the line out, it becomes brittle and breaks. Check these lines and repair if necessary, post your results back here. Hope this helps.

[1987ss]

thank you, im a check it out tomarrow

[drew300]

I don't know if the '94 is the same as my '97.
I found an electric solenoid that turns vacuum on/ off to the vacuum operated water valve for the rear heat.
My solenoid quit. There is vacuum at 1 side of the solenoid valve, but the valve doesn't open with 12 volts applied. If I apply vacuum to the water valve, I get rear heat.
One day I'll get another solenoid.....

[1987ss]

Originally Posted by drew300

I don't know if the '94 is the same as my '97.
I found an electric solenoid that turns vacuum on/ off to the vacuum operated water valve for the rear heat.
My solenoid quit. There is vacuum at 1 side of the solenoid valve, but the valve doesn't open with 12 volts applied. If I apply vacuum to the water valve, I get rear heat.
One day I'll get another solenoid.....

were is solenoid located?

[drew300]

That's a tough one...
From the front, with the hood open.
Behind the A/C reservoir, down, above the first or 2nd cylinder on the passenger side, attached to the firewall, near where the doghouse attaches.
It's about 3" long, top half silvery metel, lower is black plastic, with 2 vacuum lines on it.
That's all I remember.
I found it by following up from that vacuum operated water valve.
Hope that helps.
I haven't replaced mine yet.

[old_master]

If the system uses a vacuum controlled water valve for the rear heat rather than a solenoid, the valve is usually located near the rear heater core. It may be in the rear heater housing, it may be under the vehicle, or it may be under the hood. In any case, it is always located in the coolant line that supplies the heater core, not the return line back to the engine.

[1987ss]

Originally Posted by old_master

The two problems are quite possibly related. The HVAC system will default to the defrost position if no vacuum is available to the control head. Some rear heat units use a water valve that is controlled by vacuum also. Without vacuum, temperature cannot be regulated. The vacuum source is from a tee fitting on the PCV valve which is located on the driver side rocker arm cover. A 1/8" plastic line runs across the top of the engine to a check valve/tee fitting. One side of the tee goes to a vacuum reservoir near the evaporator housing, the other goes into the interior where it connects to the control head. The problem usually lies where the line crosses over the engine. The heat dries the line out, it becomes brittle and breaks. Check these lines and repair if necessary, post your results back here. Hope this helps.

Thank you i did have a vacuum leak and now i can switch it from def to floor just not to vent.. as far as the rear heat goes it still blows cold.. but thanks for the advice