Air conditioning blowing only highest setting but HOT air

When engine braking is being used downhill the intake and cylinder depression or vacuum is at the highest so if there is a weakness it is more likely it will reveal itself under that condition - if there is a coolant leak in the intake manifold or gasket or even in a head gasket or alternatively high vacuum sustained for a time can draw oil down valve guides or up past piston rings.

Whatever is finding it's way into the cylinders and wherever it is arriving from it is likely temporarily fouling one or more spark plugs.
Examining the spark plugs immediately after a longish downhill run should provide a clue...

The first thing to do would be go to your local auto parts store and buy an air conditioning recharge kit. Provided your vehicle isn't too old, they sell recharge kits that require no tools.

If their is a leak anywhere while you recharge, you will know.

Try recharging the air conditioning first, it is very simple and the kits come with simple instructions.

If there are no leaks, your A\C will blow cold air again. As for the pressure, if it continues to slightly blow air you may be looking at a new air compressor unit or a vaccum line. A new unit will run you anywhere from $200-$2000 dollars plus labor depending on the vehicle.

If the recharge works, at least you will have slightly blowing cold air rather than hot and you can continue that way until the unit fails.

> With the control set 1 2 3 4 it does blow any air,
? does NOT?
A common problem with this control is the failure of a stepped resistor which causes only the highest speed to still function since this bypasses the resistor.
Tracing the leads from the fan speed switch may help locate it.

You may have air trapped in the lines to or from the heater core.
If you (still) have a real radiator cap on the radiator, remove it with a cold engine, start and run the engine allowing it to warm up while watching the water/coolant level. Make sure the heater control is set to its highest heat setting while doing this. If it is trapped air, you may see it 'belch' after the engine is warm and the engine thermostat is open. Refill as needed if this is the case.
It's more difficult without direct radiator access and may require several cycles of hot/cold to allow refilling through the overflow.

is the compressor turning? If not, replace the AC clutch relay. $11 at rockauto.com

That is the blower resistor. 4 is wide open, so, no resistance. I had 4 of them put into my truck. The wires were burnt and shorted the others until the problem was found. Check the plug if you do this yourself.

Your problem is a faulty heater control valve. The design used on these vehicles allows for hot water to flow constantly and compete with the evaporator as there is no blender door. You have to replace the valve and possibly the control head. Meanwhile, you can bypass the valve to have cold air

U have a AC refrigerant leak, that is why it worked for a week after the system was topped off, now it has leaked out again.

hi check the resistor pack located near the heater motor.this is by passed when heaters set to max so they work.

mine did the same thing,it would not shift into overdrive,when it did it kept downshifting. I had to replace the sensor that tells the transmission when to shift and not to. I think it was the shift sensor.