AC cycling on and off, hot and cold

First off... thanks for all the great info. I have long been reading on the forum but this will be my first post. I have searched extensively on the subject but I cannot seem to fix my problem.

I live in AZ and I am about to go on a road trip in the morning. I have been working on this for weeks and finally decided to see if I could get some help here on the forums.

VEHICLE: 2003 4runner 2wd v6 auto

PROBLEM: The ac compressor is cycling on and off... thus causing the air to cool off and then heat up. They air does get cold, but then the compressor kicks off and the air warms up, and then it kicks back on again. There is no "set" run time. Sometimes it does this every 15 seconds and sometimes it will go a minute or two before shutting off.


I have already been to 2 different shops and they both just started throwing parts at it with no luck. Both shops evacuated the system and recharged to the proper pressure. FREON IS NOT LOW according to them. One shop said the condenser was plugged up and causing the high side to go too high so I had that replaced, that apparently did not do very much because I have the same problem. Both shops verified there are no leaks.

It is not the common relay problem. I have swapped it out and I think that only pertains to people who have issues with the compressor shutting off and never coming back on. Also my a/c light does not blink. Swapping out the relay did nothing.

I cleaned the ambient air temp sensor. No change. If I am right I believe this would only affect my situation if I was not on the lowest setting... i have the 4runner set to the lowest setting and not using AUTO so I don't think this would matter.

One shop said that what is happening is the A/C is getting too cold. The thermistor down by the blower is shutting off the a/c to keep from freezing up... but I pulled the thermistor and left it dangling to make sure it was not "getting too cold" and that did not do anything.



Anybody have any other good tips to check???

Based off the info you have provided sounds like the LPS is faulty or tripping.

Basskiller said:

Based off the info you have provided sounds like the LPS is faulty or tripping.

Click to expand...

LPS? What is that?

Low Pressure Switch. It shuts down the system to protect the compressor from self destruction.
I'm assuming that since 2 shops have done an evac and recharge that the compressor is ok and functioning properly.

Basskiller said:

Low Pressure Switch. It shuts down the system to protect the compressor from self destruction.
I'm assuming that since 2 shops have done an evac and recharge that the compressor is ok and functioning properly.

Click to expand...

I will have to ask... I am not sure how to test the low pressure switch to make sure it is working.

I am assuming it is "ok". No leaks, no weird noise, builds pressure just fine, makes ice cold A/C... it just shuts off and on all the time.

Anybody else have any ideas?

I feel like the ac compressor kicks on/off very often on my 07 V8 as well. I've done no troubleshooting of the issue though as it still regulates temp decently. Depending on what you find I may get the urge to dig into it so please let us know!

Well nothing yet on my end. We just got back from a 5 day trip. A/C cycled on and off the whole time. It was ICE cold when the compressor was running and then would blow a little warm, and then ice cold... THE WHOLE TRIP. GRRR... that was a little annoying.

As long as the 4runner stayed cool then it was not so bad because even though it was cycling it did manage to keep the car cool... but if it sat out in the sun and got hot then it was a lot more noticeable because of how long the 4runner would take to cool down.

According to both a/c mechanics the "pressures" looked good... but maybe the pressure sensor is bad and seeing faulty readings. I think that will be my next step. I just hate throwing parts at a vehicle one by one until it fixes it.



Anybody have any other ideas? Maybe a way to test the system and sensors?

is it really set to the lowest setting? i just set mine to LO & adjust the fan speed.

Expansion valve

http://www.amazon.com/Denso-475-0512-Expansion-Valve/dp/B000PWV9O4

something might be blocked somewhere, expansion valve?...orifice tube?...making the compressor to sht down and restart ........maybe a line freezing and blocking.

really interested to see how this pans out

Thermistor

I ran into the same issue working on a relatives 2005 4runner. AC cycles excessively and it does not get as cold as it should. Looking at pressures, the low just hits where it should be and it cycles off.

I was looking for a low pressure switch and discovered that this system does not use one. There is a thermistor (thermostatically variable resistor) that is used to tell the control unit when the evaporator is cold enough to cycle.

The unit goes into the evaporator case from the Right side and can be seen by removing the glove box.

If I plug in the new thermistor, the clutch stays engaged and it gets cold. The trouble is that I can't figure out how to remove the old one from the evaporator case. There is a black plastic plug right where the wire passes through, and I can't remove that. Without the new thermistor in the evaporator case it will run all the time.

Anyone actually change one that can tell me how to R&R?

Thanks!

2turboZ said:

I ran into the same issue working on a relatives 2005 4runner. AC cycles excessively and it does not get as cold as it should. Looking at pressures, the low just hits where it should be and it cycles off.

I was looking for a low pressure switch and discovered that this system does not use one. There is a thermistor (thermostatically variable resistor) that is used to tell the control unit when the evaporator is cold enough to cycle.

The unit goes into the evaporator case from the Right side and can be seen by removing the glove box.

If I plug in the new thermistor, the clutch stays engaged and it gets cold. The trouble is that I can't figure out how to remove the old one from the evaporator case. There is a black plastic plug right where the wire passes through, and I can't remove that. Without the new thermistor in the evaporator case it will run all the time.

Anyone actually change one that can tell me how to R&R?

Thanks!

Click to expand...

I just responded to your PM... but I was wrong about the part I was talking about. I am going to look into the thermistor and see if that is also my problem. I have not tried removing mine yet.

Where is the expansion valve located? Behind the glove box or the blower?

OK, I got this changed finally and it did solve the excessive cycling problem. The AC is colder than it has been in years, so the failure mode of this piece may be gradual.

The thermistor (thermal variable resistor) is used to tell the control unit that the evaporator is cold enough to cycle. Typically clutch cycling for temperature control is used in conjunction with a fixed orifice tube instead of an expansion valve.

The thermistor should be suspected if you have excessive cycling, but normal pressures as did the OP.

To test, remove the glove box and locate the connector on the left. It has two small wires. the wires to the thermistor are in a plastic sheath. On my subject vehicle the original sheath was green and the replacement was blue. If you disconnect the connector, the compressor won't run at all. If you jump the two wires, the clutch won't cycle and the AC will be too cold. If you can achieve cold air by jumping the wires, you probably have a bad thermistor.

The part # was 8862535050 and it listed for $16.39 at this time. Plugging the new part into the connector also made the AC run excessively, because the thermistor was not in position to sense evaporator temps.

To replace the part, Do NOT Pull on the wires! They will just ball up in the evaporator case and complicate things.

The thermistor is held into the evaporator case, and held in position by what I will call a Thermistor Carrier. I get to name it apparently, because Toyota does not list it in the parts diagram. It really should come with the new thermistor, but does not. The carrier is about 6 inches long and guides the thermistor to just the right spot inside the evaporator case.

From the outside, it looks like a black plastic plug right where the thermistor wire goes into the white evaporator case. It has two prongs that lock it into inside of the case. I don't know how it is supposed to come out, EDIT NOTE: Fuzzy fingers says to turn this carrier CCW 1/4 turn to release it.....but I just pried on this plug until one of the prongs finally broke.
Then I was able to snake out the carrier. The carrier has clips to hold the thermistor and wire in place and it will be obvious how it goes when you look at it. I put a wrap of electrical tape in a couple spots to make sure the thermistor stayed in place when reinstalling.

Sorry I did not get photos. If the OP wants to hit me up since we are locals maybe I can help you out and we can document better.

2turboZ said:

OK, I got this changed finally and it did solve the excessive cycling problem. The AC is colder than it has been in years, so the failure mode of this piece may be gradual.

The thermistor (thermal variable resistor) is used to tell the control unit that the evaporator is cold enough to cycle. Typically clutch cycling for temperature control is used in conjunction with a fixed orifice tube instead of an expansion valve.

The thermistor should be suspected if you have excessive cycling, but normal pressures as did the OP.

To test, remove the glove box and locate the connector on the left. It has two small wires. the wires to the thermistor are in a plastic sheath. On my subject vehicle the original sheath was green and the replacement was blue. If you disconnect the connector, the compressor won't run at all. If you jump the two wires, the clutch won't cycle and the AC will be too cold. If you can achieve cold air by jumping the wires, you probably have a bad thermistor.

The part # was 8862535050 and it listed for $16.39 at this time. Plugging the new part into the connector also made the AC run excessively, because the thermistor was not in position to sense evaporator temps.

To replace the part, Do NOT Pull on the wires! They will just ball up in the evaporator case and complicate things.

The thermistor is held into the evaporator case, and held in position by what I will call a Thermistor Carrier. I get to name it apparently, because Toyota does not list it in the parts diagram. It really should come with the new thermistor, but does not. The carrier is about 6 inches long and guides the thermistor to just the right spot inside the evaporator case.

From the outside, it looks like a black plastic plug right where the thermistor wire goes into the white evaporator case. It has two prongs that lock it into inside of the case. I don't know how it is supposed to come out, but I just pried on this plug until one of the prongs finally broke.
Then I was able to snake out the carrier. The carrier has clips to hold the thermistor and wire in place and it will be obvious how it goes when you look at it. I put a wrap of electrical tape in a couple spots to make sure the thermistor stayed in place when reinstalling.

Sorry I did not get photos. If the OP wants to hit me up since we are locals maybe I can help you out and we can document better.

Click to expand...

So you were able to change this without pulling the case apart?

ChaosStarter said:

So you were able to change this without pulling the case apart?

Click to expand...

Yes :dance:

2turboZ said:

OK, I got this changed finally and it did solve the excessive cycling problem. The AC is colder than it has been in years, so the failure mode of this piece may be gradual.

The thermistor (thermal variable resistor)..........

Click to expand...

Been pulling on my very short hair for quite some time puzzling over this. Been through this forum many times and finally happened on this gem. Jumpered the connector and boom, continuous cold air! Mine had been getting luke warm then getting cold off and on and it was intermittent so nothing completely failed.
I can add a photo but this gentleman has described it perfectly. Don't really need pics. Someone would have to tell me how to do it anyhow.

Any risk of leaving it jumpered for a short period of time?

Well I replaced the sensor. The black piece that holds the sensor twists counter clockwise a quarter of turn then comes out. Unfortunately, mine is still cycling to long and allowing the air to get pretty warm. When it was jumpered, ran very cold. This leads to me to whatever comes next up line from this sensor.

What does this sensor connect to?

Fuzzy Fingers said:

Well I replaced the sensor. The black piece that holds the sensor twists counter clockwise a quarter of turn then comes out. Unfortunately, mine is still cycling to long and allowing the air to get pretty warm. When it was jumpered, ran very cold. This leads to me to whatever comes next up line from this sensor.

What does this sensor connect to?

Click to expand...

Did you check your pressures?

Unfortunately the next item up the line is the AC control module.

If you pull the thermistor out of the evap case and let it hang, it should also get "too cold" and not cycle (as long as the pressures are normal) because the thermistor will not be sensing the cold air stream. If this works normal then I would hesitate to condemn the control module, which was $$$ IIRC.

I don't own a 4 runner and am never on here, so PM if you have a ?