[mr3wan]

Hey Guys,

Ever since I had my engine replaced due to an idler failure and subsequent timing belt failure, I have had an odd issue with my A/C system. The AC temperature will fluctuate from Very Cold (Dry) to Warm (and Humid) and then back to Very Cold in a cycle of every 5-15 seconds.

I know this was not the normal operation for the first several years with this car. I spent 30 days in it for a 10K mile cross country trip prior to the engine issue and my A/C never fluctuated temps.

I took it into the dealer at the end of the summer but they said it was cooling fine since the cabin would eventually reach a cool state even through the fluctuations continued

I still know this is not the original operation so I am trying to do some debug and have a few questions I could use help with.

I assume in the engine replacement they had to disconnect the A/C so I expect the system was drained and recharged. However, I'm not sure if the Refrigerant is low or is I possibly have a clogged Expansion Valve.

Does anyone know the difference between what happens with a bad expansion valve vs low refrigerant?

My next question is specifically on Low Side/High Side pressure and the operating/switch conditions on the 3-way pressure switch. I have a a/c manifold gauge coming to me so I can check the High/Low side pressure. I rummaged through the Service Manual to get ideal pressures but I am either missing something or some of the data is wrong.

Here is the first section:
2. Refrigerant Pressure with Manifold Gauge Set
A: PROCEDURE
1) Place the vehicle in the shade and windless condition.
2) Connect the manifold gauge set.
3) Open the front windows and close all doors.
4) Open the front hood.
5) Increase the engine to 1,500 rpm.
6) Turn on the A/C switch.
7) Turn the temperature control switch to MAX COOL.
8) Put in RECIRC position.
9) Turn the blower control switch to HI.
10) Read the gauge.
Standard:
Low: 127 — 196 kPa (1.3 — 2.0 kg/cm 2, 18 — 28 psi)
High: 1,471 — 1,667 kPa (15 — 17 kg/cm2, 213 — 242 psi)
Ambient temperature: 30 — 35°C (86 — 95°F)
B: INSPECTION


Now looking at the details for the 3 way pressure Switch:


Triple switch
(Pressure switch)
Low-pressure switch
operating pressure
ON to OFF 28.4 +/-2.9 psi
OFF to ON 32.6 +3.6/–4.2 psi
High-pressure switch
operating pressure
ON to OFF 455.4 +7.25/–29.0psi
OFF to ON 369.8 +/-29.0 psi
Middle-pressure switch
operating pressure
ON to OFF 198.65+/-17.35 psi
OFF to ON 256.81+/-11.60 psi


So I'm a little confused on this process. Does this indicated if the LOW side is between 28.4 and 36.2 the Compressor will be ON (switch goes from OFF -> ON) And then if it drops Lower Than 24.6 it shuts the compressor off?

This seems to go against the earlier data that showed the 'NORMAL' range Pressure with the A/C Compressor running is 18-28PSI. it would seem to me the normal range would trigger the Low Switch and shutoff the compressor if these numbers are right.

Can someone help me understand this logic?

 

[1 Lucky Texan]

I know little about A/C, just enough to get my wife's 03 working better after changing a coupla o-rings.

but, If I understand those numbers, you do have a problem. the ratio of high to low should probably be around 1:5 or maybe 1:6 . Your ratio seems wide to me.

is the problem worse when stopped idling, then gets cooler when moving? maybe bad/intermittent fan.

 

[mr3wan]

I know little about A/C, just enough to get my wife's 03 working better after changing a coupla o-rings.

but, If I understand those numbers, you do have a problem. the ratio of high to low should probably be around 1:5 or maybe 1:6 . Your ratio seems wide to me.

is the problem worse when stopped idling, then gets cooler when moving? maybe bad/intermittent fan.

So I don't have any actual numbers from my car yet. (Gauge comes tomorrow ).

The numbers I've posted are all directly from the Factory Service Manual - Hence why I am a bit confused.

My problem is most notice when I drive but it also occurs while at idle. Essentially it happens at all speeds (The varying temperature)

 

[1 Lucky Texan]

likely no relation but, I have to ask, have you checked the cabin air filter for clogging?

 

[cardoc]

You have it backward in your understanding.

The compressor shuts off at 28.4 psi and kicks back on at 32.6.

The compressor pulls a vacuum on the low side. The low side pressure drops in operation. To keep the compressor from overworking, when the pressure drops below 28.4, the clutch is cycled off. When the compressor is off, the system isn't moving the refrigerent, so the pressure will increase.

On the opposite side, the high side builds pressure due to heat and expansion. When the system pressure rises, so does heat and the condenser can only remove so much. So when the pressure increases, the clutch is cycled off to keep the system from overheating and damaging the compressor or blowing a joint/hose/condenser.

When the system alters cold/hot as you posted, it is either a sticking expansion valve or moisture in the system. The humidity you feel is the water being blown off the evaporator which draws the moisture out of the air passing across it in normal operation.

 

[mr3wan]

Lucky Texan - I replaeced the Cabin Filter when I first started having this issue and it had no effect. However, I will probably give it another check this weekend

Cardoc - ok cool...this makes sense. But once piece still confuses me. Why does the FSM indicate a 'standard charge' on the low side with the compressor running is 18-28PSI at 95F when the Compressor shutoff due to the Low Pressure switch would be at 28? Could this be incorrect? Should it read 28-36PSI for the standard pressure while running on the low side?

This is where I am most confused

 

[cardoc]

I think there is a typo. Tell ya what I'll do. I will take a video of my AC in operation so you can see it and know what you need to look at. As long as I have time today or remember to at the end of the day.

 

[cardoc]

Here ya go.

Reading an AC Manifold Set - YouTube

 

[mr3wan]

Hey Cardoc

Thanks a bunch for the video it really helps me out!

I'll try to post a video of my current running.

At about 65C I was seeing a fairly fast cycle time ~ 10seconds on the Low pressure side from 40 down to 28.

On the High pressure side it was kicking up to 175-210 or so. I'll take a closer look today. My High does seem like it might be a bit high than normal but i'm not sure yet.

 

[cardoc]

Your in Austin?

Bring me the car.

Sent PM

 

[mr3wan]

Special thanks to Cardoc for helping me get this sorted out!

So my initial review with my gauges showed the Low side within the expect range at 65F but my high side was showing a bit higher than expected around 175-210PSI.

Once the system purged it turns out there was too much refrigerant in the system.My system should only have about .9lb but was at 1.2lb

After refilling only to 0.9lb everything seems to be running normal.

Since it's cold today ~60 I can't tell if the A/C is still going from freezing cold to cool(humid) and back to freezing cold. I'll update once it warms up.

 

[cardoc]

It'll be cold.

 

[mr3wan]

Overfilled A/C Fail Picture