94 Civic Coupe Air conditioning issue

Hello all. We are having trouble with our air conditioning in our 94 honda
civic coupe. Here are the symptoms:

It will run well and cool for a while, then cut off and blow outside air
temp. Just as quickly, it can come back on. Sometimes when the car starts it
won't run, but then will come on. There seems to be no rhyme or reason to
when it comes on or cuts off.

Any ideas?
Is this enough info?

Thanks in advance.
aaron

 

As you probably know, there are two fans side by side under the hood,
one behind the radiator and one behind the A/C condenser. The radiator
is on the passenger side, the condenser is on the driver side. Whenever
the A/C compressor is engaged and running, the condenser fan should run
as well. Check under the hood to see if the condenser fan is ever
running when the A/C gets in the mode of blowing warm air, and report
back to news group.

 

I checked it and this is what I found. When the a/c is on but NOT blowing
cold air, the fan is NOT running. When the a/c is on and blowing cold air,
the fan is on. Does this lead to a possible diagnosis?

 

By fan, I mean the fan behind the A/C condenser that is next to the radiator
fan.

 

This narrows it down. A few things that is *not* the problem:

1. The compressor thermal protection device.

2. The ECU A/C circuit (The ECU reads the thermostat and switch
information, and when the A/C is to be turned on the ECU first increases
air supply to the engine, then activates the A/C compressor clutch).

3. The refrigerant high pressure switch (shuts down the compressor if
system pressure goes to high).

If either of these were at fault, the compressor would be shut down but
the condenser fan would be left running.

That leaves:

1. Fuse #13. Unlikely to cause an intermittent fault like yours, but try
removing and reseating it.

2. The steering diode. Again very unlikely to cause an intermittent
fault.

3. The condenser fan relay. It is not entirely clear from the wiring
diagram if a faulty condesner fan relay would prevent the A/C clutch
from engaging, but it is worth removing and reseating the relay. The
only fault that could conceivably cause your symptoms would be an
intermittent connection in the relay coil or the connections to it.

4. The A/C thermostat (attached to the evaporator in the air box behind
the glove box).

5. The A/C switch on the dash.

6. The ventilation fan switch on the dash.

When the A/C cuts out and you get hot air, is the A/C indicator light in
the A/C switch still on, or does the light turn off also? If the light
turns off, it is overwhelmingly likely that the ventilation fan switch
is intermittent. This switch has a number of outputs for the different
fan speeds plus one output for the A/C circuit. This output should be
grounded any time the fan is not off. If this output is flaky, the A/C
operation would be flaky as well.

If the light stays on, some more testing is needed. You need to get
access to the A/C thermostat connector behind the golove box. To remove
the glove box, first open it. On the passenger side of it is mounted a
spring loaded cylindrical damper. Remove one screw where it is mounted
to the glove box, then pull it off its boss. On both the left and right
hand sides of the glove bos are catches. There are normally facing
towards the front of the car, rotate them upwards and towards you as far
as they go, then close the glove box. Underneath the glove box you will
find one hinge on each side of the box. One bolt holds each hinge to the
dash; Remove these bolts. Then open the glove box and remove. (When you
reinstall the glove box, try to bias it a little to the passenger side
rather than trying to center it in the opening, mine rattles a little if
centered).

You will now see the evaporator housing. On right hand side of the top
half you will see a 3 terminal connector, with wire colors YEL/WHT,
BLU/RED and BLK/YEL (in that order, left to right). Unplug the connector
and measure continuity between the BLU/RED and body ground with the A/C
turned on and the ventilation fan on. There should be continuity. Drive
around for a while (the A/C will not work with this connector unplugged)
and see if there is ever a lack of continuity. Wiggle the A/C and fan
switches to see if there is ever a lack of continuity. If there is a
rock solid conection between BLU/RED and ground at all times, it would
indicate a problem with the thermostat. If you find that continuity to
ground is at all intermittent, it would point to the A/C switch or the
fan switch.

As stated earlier, if the A/C light turns off when the A/C starts
blowing hot air, the fan switch is likely the problem. If the light
stays on, then the A/C switch might be the problem.

I can think of one more failure mechanism. If the condenser fan or
condenser fan relay + connections would be intermittent, then the
condenser fan could stop running on occasion. The A/C compressor would
still be engaged and you would still get cold air for some time, but
eventually the refrigerant pressure would get too high and the A/C
compressor would be shut down. If this were the case, the A/C indicator
light would remain on when the system is blowing hot air.

 

Thanks Randolf. To further narrow it down. The light does not go off when
the a/c is blowing hot/warm air. It stays on regardless of the a/c's
performance. I turned it on and off several times with the hood up so I
could watch the fan and check the a/c. Each time the light came on whether
or not the a/c was blowing cold.

 

So we have pretty much eliminated all the simple fixes :-(

As I am sure you know, it is normal for the A/C to cycle on and off. It
turns off when the temperature at the evaporator gets close to freezing
and turns back on as soon as the temperature increases a few degrees.
Specifically, for the '94 Civic it should turn the A/C off at 3°C and
back on at 4°C. So even when it shuts off it should still blow cool air.

About the last thing I can think of to test would be to see if the
condenser fan and the A/C compressor clutch engage and disengage at the
same time. If you look under the hood, you can see the fan and easily
hear the A/C engaging and disengaging.

If the condenser fans shuts of first, and then the A/C clutch disengages
quite a bit later, I would say there is a problem with the condenser fan
circuit. If the clutch and fan always operate synchronously, my guess
would be the evaporator thermostat (actually a thermo switch). These
switches are usually mechanical devices with a bi-metal spring and
another steel spring for hysteresis. They do fail from time to time.

When doing this test, it might be difficult to distinguish between the
normal cycling of the A/C and the failure mode where it start blowing
warm air. Hard to feel the vents and look under the hood at the same
time.