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93 Civic clutch Cyling at Idle

rcklr on Fri July 09, 2010 6:53 AM User is offline

Year: 93
Make: Honda
Model: Civic
Engine Size: 1.5L
Refrigerant Type: R12
Ambient Temp: 95
Pressure Low: 30
Pressure High: 275
Country of Origin: United States

I have a 93 Del Sol S (no power steering) that the AC has started
acting up at idle. Sometimes the compressor clutch cycles on and off
every few seconds when idling. It happens when the engine has been
reving and drops quickly to idle, such as pulling up to a stop sign or
light. The idle will run up and down between 500 and 1000rpm as
theclutch engages/disengages. Eventually the clutch will remain
engaged and the idle will stabilize at 700rpm. The system works fine
at higher revs. The clutch cycling appears to be due to low pressure.

I've had the car for about a year, AC has worked great. The sight
glass is clear except a few bubbles when the AC is first turned on. I
hooked up gauges and pressures are normal according to the FSM
(1500rpm/ 275psi hi / 30psi low/ 95F outside / 54F vent ). Most of the
time at idle the clutch doesn't cycle (700rpm /220 hi /41 low / 60F
vent). There appears to be an ample charge of R12 in the system.

When the clutch starts kicking in and out the lowside pressure will
drop quickly down to 40 psi clutch engages, pressure drops to 25psi repeat repeat. This
doesn't happen all the time, typically when revving the motor and
allowing revs to quiclkly drop to idle. If the AC is turned on at idle
this doesn't happen, likewise if the revs are eased down to idle.

Car has 142k miles on the original AC. I assume it's been topped off
sometime during it's life (I've only owned it 1 year). I'm reluctant to add more R12 to the system
when pressures are normal at 1500rpm and the sight glass is clear.

Am I misreading this and it's just low on R12, or is it something
wonkier like a bad expansion valve?


mk378 on Fri July 09, 2010 7:58 AM User is offline

That's an engine problem. The computer is programmed to drop out the compressor should idle rpm drop too low. Remove and clean the idle air control valve. Also there is an air valve screw in the top of the throttle body which can be opened to admit extra air at idle.

rcklr on Fri July 09, 2010 2:14 PM User is offline

I'll give the IAC a look.


rcklr on Fri July 16, 2010 8:25 PM User is offline

Good call on the IAC. Ends up the base idle was too low. Spec for this car is 420 +/- 50rpm. Found out it was 300rpm. The car ran surprisingly smooth at 300. Raising it to 450rpm eliminated the cycling. IAC might need cleaning too, but it doesn't impact the AC.

Thanks for the tip. -RC

Edited: Sat July 17, 2010 at 7:56 AM by rcklr

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