Climate Control

Rear AC Blows Cold but Front Doesn't

>I've got an A/C problem that I could use some help with - car is a '93.
>
>The Problem:
>Front A/C does not blow cold. It has always been extremely cold in past
>summers. It does cool slightly when compared it to the system off.
>Rear A/C is cold, as before, but the front is not. Compressor runs
>(obviously) and the radiator fan runs while the A/C is operating.
>
>What I've done:
>- - Replaced bulkhead valve and hot water valve.
>- - Checked all valves and they all function properly.
>- - Checked out the in-dash control unit per the shop manual and it checks
>out OK.
>- - The setting motor also tests out OK per the shop manuals.
>- - Checked the outside temp sensor for corrosion. Connection is clean.
>
>What I haven't done:
>- - I just saw an earlier post with resistance values across the outside
>sensor, will check.
>- - Test the inside sensor.
>- - Check the freon.
>
>Questions:
>- - If the rear A/C is nice and cold, could there be a problem with the freon
>for the front?

Yes, but not what you might think. The same compressor, drier, freon (134a in your case), and condenser are used for both frnt and rear. Air through the center vent ALWAYS passes therough the front evaporator, so if you have the water flow shut off you should only see the effects of the evaporator.

>- - Can one leak but not the other?

No.

>- - When checking the inside sensor, is the resistance the same as the
>outside sensor?

The resistance will be the same IF the temperatures are the same.

>- - Assuming that the hot water valve, setting motor, flap valves and control
>unit are all doing their jobs, what else could be wrong?

Big assumption but let's go with it for a minute. The parts that are separate betwen front and rear are the expansion valve and the evaporator. The evaporator is a heat exchanger, and the failure mode for these is "leak". You still have enough gas in the system to run at least the rear system, and some of that should be going through the front... Unless the expansion valve is plugged/defective.

The expansion valve is a little metering valve that bleeds liquid into the evaporator. As it passes through the valve, the liquid flashes to gas thanks to the lower pressure, and also (hopefully) due to the heat in
the evaporator. The mechanism opens and closes the little orifice inside by moving a little rod. The actuator can stick, but more frequently something gets in there and plugs or blocks it.

But wait, there's more--

The first thing you can look at is the little sight glass window in the top of the drier. With the system running, there should be maybe a few gas bubbles in the liquid stream visible through that window. It's hard to tell the difference between no flow at all and a full liquid flow in there, though. You will most probably see lots of bubbles, a sign that the gas charge is a little low.

Next, make sure the heater valve is actually closed. You can look at the little actuator arm while the motor is running to see the position.

And more food for thought--

The auto temp control moves the bulkhead flap and the heater valve to adjust temperature. With the temp slider all the way to the cold position, the heater valve should be held closed no matter what the temp sensors are doing. You can ignore the sensors for a while , and concentrate on the freon and expansion valves.


My suggestion:

Find a qualified technician who has a set of gauges for the car. Are you in the Pasadena or Long Beach area? You can use mine. The gauges will tell you quickly the status of the charge, reinforcing the visual reading you are getting at the sight glass window. If you are lucky, you've lost some of the charge in the system over the winter, and a pound or two of gas is all that's required.

Don't be afraid to use real professional help on AC problems. The parts and related pieces can get expensive, and the risk of significant damage to parts is high if you screw up. You'll save in the end.


dr bob