Seems one of the sensors in my ac/heater/fan unit don't want to work right.
After any amount of time, it blows very hot air. If I turn the fan down to
low, it will start blowing cold again. Could it be the hot/cold vent door?
(Forgot the name of it). The fan works fine. The temp control will make it
hotter if turned to heat. Also, the clutch does not kick on and off on the
compressor. Any ideas as to what it could be? Tanks.
It's most likely the fact that you neglected to include the year/make/model
of the vehicle in question. The operation of a mechanical blend door on,
say, a 91 Van will be very different than the electronic controls on, say,
an '05 pickup.
1999 2wd 318 PU
The AC works fine on fan speed 1 and 2. Fan speed 3 and 4 must close the
mech blend door because it will blow hot until I turn it back to fan speed 1
or 2. It will run all day on these fan speeds 1 and 2 without blowing hot
air. That should narrow it down to something other than the freon, clutch
Except the blend door on a '99 is servo-controlled, not mechanical. When it
turns warm, is there a decrease in velocity, as well? When you switch back
to the lower fan speeds, does it IMMEDIATELY turn cold, or do you have to
wait a while?
Okay - then it sounds like your problem is in the AC/heater control unit.
When in the two highest positions, the rotary switch isn't providing a
ground path for the PCM's A/C switch sense, and therefore, the PCM is
shutting off the A/C compressor.
To verify this, remove the dash bezel, and remove the heater control.
Disconnect the 7-pin flat electrical connector (note that there's also a
5-pin connector... we want the one with 7 pins). The two pins you're
interested in are the green/white (pin #2), and the black/orange (pin #7).
(These colors are from a 2001 manual - hopefully they didn't change them
from '99, but even if so, the pins are still the same). Now, you want to
test the connectors on the control module, not the wiring harness (just
using the wiring harness to identify the pins in question).
Using an ohm meter, you should not read any continuity between pins 2 and 7
with the controls off. With the controls in any of the A/C settings, you
should read continuity between those two pins. I think you'll find that on
the two low-speed settings, you have continuity, but on the two upper
positions, you lose continuity.
If that's the case, then you know for sure that your problem is in the
heater controls. You have a few options... obviously, you could get a
replacement control, from either the dealer, or maybe a salvage yard.
However, the real low-buck solution is to just install a switch in the dash,
connect one terminal to a ground, and connect the other terminal to an
in-line tap of pin #2 on the electrical harness. That switch then becomes
your manual "A/C activate" switch - just flip it on whenever you want the
A/C to run.
I had forgot about the randomness part of your symptoms. Yes, that's
strange. However, the only input the PCM has that controls the A/C
compressor is the input from the heater control switch, and the high and low
cut-off switches. The speed of the blower motor is immaterial to the
operation of the A/C system.
You say that if you turn the temp control to hot, the air gets hotter. This
tells me that the blend door is functioning properly, and allowing air to
pass through the heater core only when you dial the temp up.
It almost sounds like it could be the evaporator freezing up, which would
cause the low-pressure side to drop, and the low-pressure cut-out switch
would disengage the compressor clutch. However, if that happened, as soon
as the evap. thawed, the clutch would automatically re-engage. The fact
that when turn the blower knob down, the clutch immediately re-engages,
blows the freeze-up theory.
Let me ask you this... when it cuts out, if you flick the speed control
down to low, then back up to high (in other words, "fiddle" with the fan
switch), will the A/C still blow cold, or does it HAVE to go back to 1 or 2
in order to re-gain A/C function?
Were I in your place, I'd first try the bypass switch idea I detailed
previously. Barring that, I'd put a set of gauges on the system, and see
what the pressures looked like when it cuts out (assuming you can reproduce
the problem while idling). If the gauges don't indicate anything wrong
mechanically, but the clutch still cuts out, I'd replace the low-pressure
hmm, I'' try the switch changing .
also, the evaporator was soaking wet on the outside I remember when I was
trying to diagnose this problem. I just thought it was odd that it had
moisture all over it. I'm not an a/c guy. I'll take another look at the
moisture on the evaporator.
I wont be able to talk to u again till wed or thurs night. going to state
babseball tourney. Will let you know when I get back. I appreciate all
your time and assistance. oh, could it be a resistor on the blower motor???
How are you able to see the evaporator? It's inside the HVAC "box" behind
the dash. Are you referring to the accumulator, which is a cylindrical
container, about the size of a small thermos, on the passenger-side
No... the blower motor resistor simply controls the voltage supplied to the
blower motor. If it goes bad, then your blower motor doesn't work in
anything but the highest setting. As I said, the only thing the whole
heater control module does to affect the A/C is to provide a ground to the
PCM, which then in turn energizes the clutch relay, which engages the A/C
clutch (as long as neither the high or low pressure cutoff switch is
Okay - moisture on the accumulator is normal, especially on a very humid
day. The accumulator is on the low-pressure side, as it the suction line,
and therefore they get cold. Cold metal causes condensation of warm, humid
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