A/C cycling while driving

[Crazy Drew]

So, as it starts to warm up down here in Texas I've decided to start using my a/c again. I bought this car late last year and only used the a/c for a couple weeks before winter hit. I've now noticed that while driving down the road (side streets and highway) that my a/c seems to be cycling on and off. Does anyone else have this problem? This is the first car I've had do this that wasn't over or under filled on freon.

I know it's normal for a compressor to cycle at idle in a car but this seems to do it all the time and the more load I put the a/c under the worse the temperature swings seem to get. As of right now it seems to cycle back and forth between really cold and warm about every 3-5 seconds.

I'm due for an oil change in a few hundred miles and I'll probably have the dealer look at it then. I'm mostly just curious if anyone else is experiencing this as a "known issue" or if I have an uncommon problem.

[PMPB]

If I remember correctly, I think mine cycles on-to-on about every 15 seconds. I can tell because I hear the clutch 'tweet' as it engages.

[N1rve]

Do you have automatic climate control? I've noticed this on my auto climate at any degree above 65 degrees.

[Crazy Drew]

Originally Posted by N1rve

Do you have automatic climate control? I've noticed this on my auto climate at any degree above 65 degrees.

I don't have auto climate control, just the standard FRS knobs.

@PMBP - Mine cycles at idle at a slower rate, maybe every 15 seconds. What I'm seeing though is a difference in air temperature coming out of the vents like the compressor is cycling while driving. My past experience with other cars makes me think I'm tripping either the high pressure or low pressure switches in the a/c system. The air never gets hot but while cycling I bet it changes about 10 degrees momentarily.

[t.chk]

I have the automatic climate control and it turns it on and off as needed. When it is off it slow warms up till the system turns the A/C back on again. Also noticed it when you apply...generous power...it turns it off till you are done.

[HapaHaole]

if you are at WOT, it will cycle off. If the temp in the vehicle is being effected then you need to get it checked for either a bad pressure switch or evaporator temp sensor.

[billwot]

It's perfectly normal for your A/C to cycle on and off even if you don't have "automatic" A/C.

[PMPB]

Originally Posted by Crazy Drew

I don't have auto climate control, just the standard FRS knobs.

@PMBP - Mine cycles at idle at a slower rate, maybe every 15 seconds. What I'm seeing though is a difference in air temperature coming out of the vents like the compressor is cycling while driving. My past experience with other cars makes me think I'm tripping either the high pressure or low pressure switches in the a/c system. The air never gets hot but while cycling I bet it changes about 10 degrees momentarily.

The A/C will always cycle, even when driving. It'd probably have to be driving through death valley on the hottest day to keep the A/C on 100% of the time. The fact that the air goes from back and forth between hot and cold at a rapid rate makes me think there is a problem with the interior sensor and it keeps switching between hot/cold. Because once the A/C cycles off, it's not like the evaporator core turns hot instantly, far far from it.

[DJCarbine]

Originally Posted by Crazy Drew

I don't have auto climate control, just the standard FRS knobs.

@PMBP - Mine cycles at idle at a slower rate, maybe every 15 seconds. What I'm seeing though is a difference in air temperature coming out of the vents like the compressor is cycling while driving. My past experience with other cars makes me think I'm tripping either the high pressure or low pressure switches in the a/c system. The air never gets hot but while cycling I bet it changes about 10 degrees momentarily.

I had a previous newer car that would cycle the AC excessively. Threw my gauges on and saw the pressure on the low side falling fast when the AC kicked on, but kept falling below the low limit cutoff. Added a bit more refrigerant to prevent it from falling below during AC operation (a VERY small amount) and the gauge read a steady low side pressure while on that corresponded to the ambient temperature. Never had it cycling like crazy again.

Throw a cheap gauge on the low side and see what sort of pressures you are reading when it cycles on/off. It MIGHT need more refrigerant, but cycling (not excessive) is normal from what I understand. Specs on the limit switches might be in the service manual

TL;DR for everyone else-Put gauges on to see exactly what its doing, don't add refrigerant willy-nilly or overfill the system

::edit

More expensive gauges than normally found on an autozone R134 can are required for any thorough diagnosis, is the car still under warranty?

[Crazy Drew]

Originally Posted by DJCarbine

I had a previous newer car that would cycle the AC excessively. Threw my gauges on and saw the pressure on the low side falling fast when the AC kicked on, but kept falling below the low limit cutoff. Added a bit more refrigerant to prevent it from falling below during AC operation (a VERY small amount) and the gauge read a steady low side pressure while on that corresponded to the ambient temperature. Never had it cycling like crazy again.

Throw a cheap gauge on the low side and see what sort of pressures you are reading when it cycles on/off. It MIGHT need more refrigerant, but cycling (not excessive) is normal from what I understand. Specs on the limit switches might be in the service manual

TL;DR for everyone else-Put gauges on to see exactly what its doing, don't add refrigerant willy-nilly or overfill the system

::edit

More expensive gauges than normally found on an autozone R134 can are required for any thorough diagnosis, is the car still under warranty?

Yeah, car is still under warranty. I'll have the dealer look at it this weekend when I get my oil changed.

I know just enough about a/c systems to get myself in trouble. That's what made me think it's slightly under or over filled. Much more likely to be under filled.

I'm working on a friend's a/c system this weekend. Autozone actually rents out full manifold gauge sets and vacuum pumps. Worst case if the dealer doesn't have time I'll throw some gauges on it myself. I considered last night wiring in some indicator lights real quick on the low pressure switch and compressor clutch to get a visual indication while driving. My main concern right now is burning up the compressor clutch if it really is cycling that often. As I've stated in my previous post it's about 3-5 secs between when I feel the air warming up and it getting cold again so I'm thinking it's cycling "excessively".

[HapaHaole]

Does anyone know if the AC system has a sight glass?

[DJCarbine]

Originally Posted by Crazy Drew

Yeah, car is still under warranty. I'll have the dealer look at it this weekend when I get my oil changed.

I know just enough about a/c systems to get myself in trouble. That's what made me think it's slightly under or over filled. Much more likely to be under filled.

I'm working on a friend's a/c system this weekend. Autozone actually rents out full manifold gauge sets and vacuum pumps. Worst case if the dealer doesn't have time I'll throw some gauges on it myself. I considered last night wiring in some indicator lights real quick on the low pressure switch and compressor clutch to get a visual indication while driving. My main concern right now is burning up the compressor clutch if it really is cycling that often. As I've stated in my previous post it's about 3-5 secs between when I feel the air warming up and it getting cold again so I'm thinking it's cycling "excessively".

def sounds like underfilled, throw a gauge on the low side and I bet its tripping the low limit