A/C cycling on a '79
07-08-2003, 07:51 PM
#1
Racer
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A/C cycling on a '79
Can somebody tell me if this is the norm?
I think the thermostatic switch fitted to my A/C which cycles the compressor needs replacing.... it will switch the compressor on for about four seconds, then off for about eight.. then on for four etc....
.... is this right? .... shouldn't it stay on for alot longer??? :confused:
07-08-2003, 08:29 PM
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Racer
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Re: A/C cycling on a '79 (Skeets)
... it could be.. i haven't had it charged for a while.
Though if you by-pass the thermostatic switch and let the compressor run for longer, the system does seem to work (ok'ish) ... as well as it ever did, anyway!
.... if the freon was low, wouldn't the system just not cool very well?
.... i just thought that as it is a thermo-switch, it cuts the power when it reaches a certain (low) temperature? ... but if it's malfunctioning, it's cutting the power off too early??
... four seconds doesn't seem very long to run the compressor.
Though if you by-pass the thermostatic switch and let the compressor run for longer, the system does seem to work (ok'ish) ... as well as it ever did, anyway!
.... if the freon was low, wouldn't the system just not cool very well?
.... i just thought that as it is a thermo-switch, it cuts the power when it reaches a certain (low) temperature? ... but if it's malfunctioning, it's cutting the power off too early??
... four seconds doesn't seem very long to run the compressor.
07-08-2003, 09:05 PM
#4
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Re: A/C cycling on a '79 (Skeets)
Skeets
When this type of switch fails it usually either does not turn the comp on or it will not cycle the comp off. First I would check the freon charge to make sure it is not the problem. Then I would check the switch by placing the switch bulb in a glass of ice water while checking continuity across the switch with a meter. This should make the switch open so the comp clutch can’t pull in. It is possible for the coil temp to drop below 32 deg. F if the charge is a little low. These switches are sometimes adjustable and a small turn in both directions and placed back where it was preset can correct the problem. The switch should keep the coil temperature above 32 deg F in order to keep the condensate from freezing on the coil and blocking the air flow. The cycling time is dependant on coil temperature and coil temperature is dependant on many variables such as interior temperature, ambient temperature, air flow across the condensor coil, and A/C system condition, these are just a few variables that can effect the temperature that this thermistatic switch is sensing.
To check the charge the A/C needs to be placed on high Max the doors closed and the fan on high. With the car idling in neutral the suction pressure should be in the area of 30 to 40 psi if the system is in good working order. There are many variations due to interior and ambient temperature and the condition of the A/C system but these test conditions should get you close. Hope this helps.
Neal
:)
When this type of switch fails it usually either does not turn the comp on or it will not cycle the comp off. First I would check the freon charge to make sure it is not the problem. Then I would check the switch by placing the switch bulb in a glass of ice water while checking continuity across the switch with a meter. This should make the switch open so the comp clutch can’t pull in. It is possible for the coil temp to drop below 32 deg. F if the charge is a little low. These switches are sometimes adjustable and a small turn in both directions and placed back where it was preset can correct the problem. The switch should keep the coil temperature above 32 deg F in order to keep the condensate from freezing on the coil and blocking the air flow. The cycling time is dependant on coil temperature and coil temperature is dependant on many variables such as interior temperature, ambient temperature, air flow across the condensor coil, and A/C system condition, these are just a few variables that can effect the temperature that this thermistatic switch is sensing.
To check the charge the A/C needs to be placed on high Max the doors closed and the fan on high. With the car idling in neutral the suction pressure should be in the area of 30 to 40 psi if the system is in good working order. There are many variations due to interior and ambient temperature and the condition of the A/C system but these test conditions should get you close. Hope this helps.
Neal
:)
07-08-2003, 09:57 PM
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Racer
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Re: A/C cycling on a '79 (chevymans 77)
Thanks Neal,
that's really helpfull ..... i'll try those checks out.
Just hoping it's not low on freon though... as i'm still with R12 and i don't really wanna go to the expense of converting to R134 this summer!!
:cheers:
