Trouble Shooting A/c In R33

[PuppaDuck]

Stupid phone. That should have been part numbers 27675X or 27675XA.

(The Thermister/Thermostat inside evaporator box.)

Im attaching the related parts diagram for your reference.

Edited February 8, 2014 by PuppaDuck

[89CAL]

The number is actually 27675-15U10

$40.94 out of Japan

[PuppaDuck]

I'm attaching a couple of snippets of info I found, which I think will be useful to others wanting to know more about their A/C overall and what can go wrong with it. Basically the thermistor I mentioned above, takes temperature readings (of the evaporator core) for the thermo control amp, which interacts with the ECU to cycle your A/C clutch on and off. If the evaporator core is too cold, then it backs off till it warms back up to a safe operating temperature range.

So... if you're at the stage where all other easy-to-get-to components test ok, the problem may be with the thermo control amp or the thermistor.

Testing the thermistor while your A/C is cycling will help eliminate the thermo control amp as an issue if you hotwire the thermistor, at the white connector clip, and the thermo control amp responds accordingly with the correct behaviour. The thermo control amp appears in my earlier parts diagram as part 27761N. This part is easier to get to and you will see it as soon as you get the glove box off.

This guy Wulfgang explains the bigger picture a little better: (Ref: http://forums.nicoclub.com/keeping-the-ac-on-an-s14-t125307.html)

But first you need to understand how the basic A/C system works on Nissans. Inevitably, problems will arise, and you will be much better off when troubleshooting if you know how things are supposed to work.

I assume most people know about the evaporator, condenser, and dryer. So first off, a list of the major control parts and their functions:

1. A/C clutch. This just engages the drive belt on the compressor. It is actuated by a relay. When the clutch comes on, the A/C gets cold.

2. Triple (or dual) pressure switch. This switch has an overpressure switch, an underpressure switch, and in some cases (triple) another switch that can be used for fan control. This switch is normally closed. The dual pressure part is the important part. If your system pressure gets too high for whatever reason, the overpressure switch will open to protect from explosion or compressor failure. If your system pressure gets too low, usually from leaking refrigerant, the low pressure switch will open to protect the compressor. The dual pressure switch is usually wired directly in series to the A/C clutch relay. This means that no matter how the climate control is working, no matter what the ECU does, no matter what else is going on, if the pressure is too high or too low, the A/C WILL NOT COME ON. Now that you know about the triple pressure switch, forget about it. It is there only for safety, not for control. It DOES NOT cycle the compressor. Unless your system is broken, the triple pressure switch will always be closed.

3. Thermo control amp. This is basically a temperature controlled switch, much like the ones you use to control your radiator fans, but somewhat more sophistocated. This is what "cycles" your compressor on all systems (fully automatic climate control or manual on all S13/S14 and all R32/R33 -- I don't know about R34). The thermo control amp is necessary because in most cases, the compressor does not have a capacity control. It pumps refrigerant based on how fast it is spinning. For example, if you are driving along the interstate at 3000 rpm, then the compressor is pumping a huge amount of refrigerant. With this much refrigerant, it is easy for the evaporator to get too cold and ice up. So the thermo control amp senses the evaporator temperature and sends out a signal when the temperature gets too low. Likewise, when the evaporator gets warm again, the thermo control amp sends out a signal that it is ok for the compressor to switch back on. This is the "cycling" you have heard about. Some cars use a separate low pressure switch to achieve the same thing.

So the basic control scheme is this: You push your A/C button on your dash. The button powers up the thermo control amp. The thermo control amp switches on the A/C clutch and the compressor comes on. You feel cold air. Then, when the evaporator gets too cold, the thermo control amp switches off the A/C clutch. You may feel the air begin to warm up just a tad. Then the thermo control amp switches back on the compressor and the cycle repeats. If the refrigerant pressure gets really, really high or really, really low, then the dual pressure switch opens shutting off the compressor to protect it. Since the thermo control amp knows nothing about the pressure, it might still be trying to cycle the compressor, but nothing will happen because the dual pressure switch has the final say.

So where does the ECU come in? It is wired in series between the thermo control amp and the dual pressure switch (and A/C clutch relay). The ECU acts as a switch itself, intercepting the thermo control amp signal. If you stomp the gas pedal, then the ECU knows you want maximum power, and so it will switch off the A/C temporarily. If your engine overheats, then the ECU may also decide to switch off the A/C temporarily. In either case, the thermo control amp still keeps trying to cycle the compressor, but the ECU won't let it. However, in most cases, the ECU simply acts as a closed switch, allowing the thermo control amp to cycle the compressor.

In some cars, there will also be an FICD. This fast idle control device is just a bypass valve that opens to allow more air into the engine when the A/C kicks on so that the engine idles up. There are different ways to control this, but the simplest way is to just wire it up to the clutch relay. Cars that do not have the FICD (R32 for one) just use the AAC to control idle-up since the ECU already knows that the A/C is coming on.

And then there is climate control. Despite all of the fancy gizmos and additional temperature sensors, the climate control can only tell the A/C system that it is too hot in the car or too cold in the car, just like your house thermostat. If the climate control says that it is too hot, it will try to turn on the A/C. However, control of the compressor clutch still belongs to the thermo control amp, not the climate control unit. And in fact, your compressor will still cycle on and off even when your climate control says it is too hot. Once again, the cycling is necessary to limit the amount of cooling in the evaporator coil to keep it from icing up. Like I said before, if you really want the fancy climate control, you'll have to work that part out on your own. But once you get that figured out, there is only one connection to the rest of the A/C system you need to worry about: one wire goes from the climate control to the thermo control amp.

...and... (Ref: http://forums.nicoclub.com/some-air-conditioning-fix-thoughts-t93247.html)

A/C control is simple in our cars. We have a temperature switch. THAT IS ALL!!! No computer, nothing like that. Just a temperature switch (aka thermo control amplifier). The sole purpose of the temperature switch is to keep the evaporator core from icing up, which it will obviously do if the air coming in is cooled below the freezing point (32ºF).

It works like this: you turn the A/C switch on. The compressor kicks in and the air gets cool. Depending on load, the air may get too cold (like somewhere around 34ºF, with enough room for safety above 32ºF). So if that happens, then the thermo control amp turns off the compressor and waits until the air temperature climbs back up to say 36ºF. Then it kicks the compressor back on. Simple.

That's the basic operation. Everything else is just there for safety. For example, the pressure switches guard against refrigerant leaks or overcharging, which could destroy the compressor. The ECU control is there to shut off the A/C when you put the pedal to the metal or if the engine overheats. Any of these things will open the A/C clutch relay circuit and shut off the compressor, taking the thermo control amp out of the loop. After the problem is fixed (engine cools down, you lift your foot off of the gas pedal, recharge the system, whatever) the A/C clutch circuit is again closed and control is returned to the thermo control amp.

But in a properly charged and working A/C system, the compressor cycling is COMPLETELY controlled by the thermo control switch.

Finally, I found a solid forum link write-up showing the replacement of a thermal control amp on a Nissan Frontier.

http://www.clubfrontier.org/forums/f46/diy-guide-how-change-thermal-control-amp-1st-gen-frontier-72291/

The R33 will be different, but it gives you an idea of how the component is accessed and how it fits into the fins of the evaporator to take temperature measurements.

I'm planning on having a go to see how close I can get to component removal and conduct further testing if possible, without degassing and removing completely.

From there I can make a decision whether the whole thing needs to come out or not.

Others with non-Skylines were able to keep the evaporator box open with a torch or block of wood while they wiggled the sensor off and on with chopsticks or a screwdriver.

As I mentioned before, there's always the option of simply disconnecting the existing sensor and installing another one next to it.

Hopefully a quick peek to see how it's put together will make for a better informed decision as to which way to go.

I'll take a photo for you if I get through.

[PuppaDuck]

Just found a youtube vid of a guy opening up his non-Skyline thermo control amp to find bad solder joints.

I'll definitely check this out while I'm there.

http://www.youtube.com/watch?v=YbGHIaHGBaE

That'll be a nice easy thing to access/troubleshoot/fix! Especially since the temperature probe (thermistor) also attaches to it and may simply have bad solder joints.

[Mick]

Hey guys you all seem to know alot about aircons so i was wondering if you might be able to help me. I noticed that when driving the car my feet would get very hot after a while, i worked out that the climate control was slowly blowing hot air at my feet even tho the climate control was totaly off, it only blows very slowly and what i noticed was that if i press the blue down arrowa few times while it is still off that it actualy starts blowing cooler air instead. This is crazy cos the unit is totaly off but still blows air and it still changes the temp of the air when i press the up and down temperature arrows even tho it is off.

Please any help you can give me would be helpful

[89CAL]

The part numbers that appear in the pictures are not the full part numbers. they are just an abreviated part number

Mick: Turn the A/C Unit to recirculate (the light should come on)

When the unit is off but it is still in fresh air mode, it allows some air to enter the car from outside the car. Because the air is still passing through the evaporator/heater and the mix door is still doing whatever your previous temperature was set at (but the compressor is not running to cool the evaporator down) you will get warm air through the heater. Turning the temp down closes the heater side of the mix door and the air should just be around ambient temp

[Jaredo]

I managed to fix my issue of rapid (instant on off, 1-2sec on to off, and long pauses between cycles) and random cycling of the AC even when the rest system checks out (right amount of gas, oil, dual pressure switch and climate control diag is ok),

There are a fair few mentions of the AC Thermo Amp being a possible culprit, I pulled mine apart to (they actually do come apart) to find a bulging capacitor. Replaced it and now it's back to awesome icy goodness and cycling normally.
You can get it out without degassing.
Your mileage may vary, this might not fix your issue, but it's worth a try if you have checked everything else.

Capacitor size: 33uf 25V (uf = microfards), get a 105deg rated one - 45cents at JayCar.
The temp rating of the stock part is 85deg.


1. If you pull the glove box out, then remove the panel behind it you can access the fan motor and evap core.
Directly in front of you, you should see a connector with 2 thick wires and a thin wire going into it (this is the FAN resistor).

If you look upwards from that to the top of the box you should see another connector and black small black box.

2. Remove the connector and then get a large headed flat screwdriver and pry it up from the right side towards the left-hand side of the car it should then come loose, if not stick your hand up there and pull it forward.
I already removed it by the time I took this photo and also didn't put it back. But it shows where its located.

Follow the cable up to the top of the evap box, use your sense of feel to find the little plastic holders from which you just slide the cable out of, do both so you can get some length.
Follow the cable some more and it'll go inside the box, use your fingers to remove some of the sealant if possible
Ref this picture to see how it's laid out.


3. Remove spring clip and 2 screws from the front of the unit, remove two large screws below the unit, there's one near the fan motor and one to the right of the sloping drain part.
3a. Remove the large screw to the left of where the thermo amp was.

4. Pull the bottom half box down a little, the seam should open up, you'll notice the evap core will also want to come down also.
Find something to keep the gap open unless you have strong hands.
It should look like this

5. Locate the temp probe on the right side of the evap core, the probe is on a clip that's just pushed into the fins
If you can get your hand in there, pull it out, else use a screwdriver try and lever it out. Careful you don't puncture the evap core


6. Once out, remove the clip so you can pull it out through the top.
Gradually pull the cable trough the top of the box till it comes out.


7. Once you have the control amp out crack it open, you'll probably have to break the outside case where the locking tabs are (locking tabs don't depress), use a really small or thin screwdriver.
Once that is done, slide the cover backwards a little.


8. Inspect the capacitor, the ends might be rounded instead of flat or it might look like it's buldging at one end and seeing as its going on 15+years it's probably at the end of it's life.
Desolder the old one, Solder the new one in.
Note the polarity, Negative is to the back of the board, positive at the front (there are also marks on the PCB). The capacitor will have it's negative leg clearly marked.

9. Test it before putting the system back together, leave the evap box open, and reconnect anything you disconnected. Leave the probe on the floor or something.
Run the car and AC, your AC should engage and should stay engaged.

If it worked then hurray! Put it back together.
if not, then the issue is somewhere else or the AC control amp is dead dead.

If you have a multimeter that can accept a temp probe (K Thermocouple) you'll be able to check what temps the AC control amp cycles between.
Mine cycles between 9.8 and 13deg C, that's plenty cold lol.

Ambient temp: 35.8deg C
AC compressor off at: 9.8deg C (Air vent temp, disengages around this temp)

AC compressor on at: 12-13deg C (Air vent temp, engages around this temp)

I really hope this helps others with the same issue, it's been a very tricky issue to resolve.

Edited September 27, 2014 by Jaredo

[benl1981]

Interesting thread.

I'm trying to diagnose a similar issue:

1. Air con has just been regassed and all good

2. Compressor or condensor fan do not kick in when AC button pushed.

3. By disconnecting the plug at the dryer (high/low pressure switch) and bridging the terminals on the plug the compressor kicks in and the cabin gets nice and cool

4. I have checked pin 9 and seems to be 12v, pin 46 seems to be 0v

Does this mean there are possibly 2 issues? I.e high/low pressure switch is not working correctly and for some reason the button is not triggering properly.

I can hear the relay kicking in when you press the air con button and also the engine revs go up and down..

I have an ems 6860 computer. Could it be that pin46 needs to be programmed? Is pin9 from the climate control and pin 46 is an output from the computer?

Based on what pins 9 and 46 should be doing from some of the posts above I'm surprised the compressor kicks in at all.

I was thinking of grabbing a trigger straight from the climate control and using that for the compressor trigger but concerned that the evaporator sensor won't be incorporated or the high/low switch..

Many thanks

[89CAL]

I think Pin 9 should be grounded through the ECU I believe and Pin 46 goes to the cabin air temp sensor.

The A/C relay is Fed power from fuse #16, goes through the pressure switched and earthed through the ECU by the looks of it (this is what Pin 9 is doing) on the signal side

The switched side of the relay is Fed from the same power source as the signal side and engages the compressor clutch

Maybe worth swapping the relay to rule it out. Check that you have a circuit between pin 9 and chassis earth when the A/C is switched on, will probably have to back-probe the plug to do this (with it still plugged in)

It looks like the signal from the Climate control unit to the ECU to activate the compressor clutch is Pin 36 on the Engine ECU and Pin 27 on the Climate control unit (in the smaller plug). This should be the wire next to the only empty space on that connector

Thats about the circuit. Hopefully that helps

[benl1981]

Thanks 89CAL. I'll look into it further based off that. Did you mean 46 instead of 36 in your last paragraph.

Re. Swapping out the relay, doesn't the fact that when the terminals are bridged the compressor clutch works demonstrates it is ok?

I may have 2 issues.

To test the high low pressure switch on the dryer I checked each for the 4 pins for continuity. Only 1 set of diagonal pins show a closed circuit/continuity. Since this is meant to be for both the fan and the compressor clutch, shouldn't I have got 2 sets of pins that should continuity if the pressure switch is working as intended (with adequate gas in the system?

Is anyone with a working aircon on an R33 able to kindly set a multimeter to continuity check and rest one probe on one of the pins (unplug the 4 pin terminal at the top of the dryer unit near the power steer) and then move the other probe around to check the remaining 3 pins. And then do this to the other 3 pins.

That way I can decide if I start at this sensor.

Many thanks

Edited December 14, 2014 by benl1981

[89CAL]

Thanks 89CAL. I'll look into it further based off that. Did you mean 46 instead of 36 in your last paragraph.

Re. Swapping out the relay, doesn't the fact that when the terminals are bridged the compressor clutch works demonstrates it is ok?

I may have 2 issues.

To test the high low pressure switch on the dryer I checked each for the 4 pins for continuity. Only 1 set of diagonal pins show a closed circuit/continuity. Since this is meant to be for both the fan and the compressor clutch, shouldn't I have got 2 sets of pins that should continuity if the pressure switch is working as intended (with adequate gas in the system?

Is anyone with a working aircon on an R33 able to kindly set a multimeter to continuity check and rest one probe on one of the pins (unplug the 4 pin terminal at the top of the dryer unit near the power steer) and then move the other probe around to check the remaining 3 pins. And then do this to the other 3 pins.

That way I can decide if I start at this sensor.

Many thanks

I did mean 36, not sure what happened there.

And yes seems you are right about the relay, if you bridge the pressure switch and the clutch kicks in, then the relay must be fine and thinking about it a bit more it almost points to a simple answer - that your pressure switch is wrecked. You were testing the pressure switch on the reciever drier when you bridged it to get it to work yeah?

If so the part number is: 92137-17U00, my info says $41 from Japan

[benl1981]

Thanks for that.

I haven't got any further with this.

I searched that part number for the pressure switch and not much comes up. I was thinking of just buying one and getting it fitted to rule it out.

Might have to buy straight from Nissan.

Regarding my voltage at the pins which don't seem to match with what they should be I still think there may be a separate problem.

I was looking at the circuit diagram and think that since the revs go up when the AC button is pressed suggests that pin 46 / the thermo amp is Doing the right thing.

Does this mean that I could Use the signal from pin 46 (or convert it somehow) signal instead of trying to get the ecu to output pin 9 correctly.

Edited December 30, 2014 by benl1981

[benl1981]

Ok. I've cut into the pin 9 wire running into the pressure switch at the canister. If I ground the feed into the pressure switch the compressor kicks in.

This signifies the pressure switch is ok and pin 9 is not outputting the ground signal when it should.

Since its an aftermarket ecu I'm thinking it's easier to use pin46 as the trigger for the AC clutch instead of pin 9 (only drawback is the ecu won't cut aircon if full throttle etx).

Questions:

1. does pin46 supply a ground when air con is on and Could I use that to trigger the aircon clutch in place of pin 9? I found pin 46 and it seemed to give about 11.8v when air con off and 0.3v when on but it was hard to get a good connection at the back of the ecu.

2. Also, what triggers the AC fan? Is it engaged when the clutch is engaged? Ie always on if aircon compressor is running?

Edited December 31, 2014 by benl1981

[zoomzoom]

After tons of searching I still cant find a straightforward answer.

My understanding that activating the clutch compressor goes thusly:

(Battery +ve) ------> (low pressure switch) ------> (A/C relay coil) ------> (ECU pin 14) ------> (Battery ground)

Keep in mind this is a stagea s1, so pin number will be different. Its mainly the components that Im interested in. The problem I am having is that the air con compressor will not turn on. The ECU side seems to be fine.

I notice that when I probe the A/C relay terminals, the power terminals read battery voltage (normal), however the relay coil terminals only reads about 6.8 volts, whereas what I read in the R33 manual states it should be battery voltage. If I try to manually ground the relay coil, there isnt enough power to make the relay click, and the voltage drops to zero.

So, is my diagram above correct? If so then am I right in thinking that the low pressure switch is causing this? Is it an on/off state switch or some kind of resistor? Where is the switch?

Its odd that the air con did come as normal once a week or two ago, however I have not been able to replicate that.

[89CAL]

I would assume so

Is also triggered off water temp sensor at the bottom of the radiator

[89CAL]

IGN fuse # 16 ----> A/C relay (both coil and switch are fed from the one fuse) -------> Pressure switch -----> Connector (M?) -------> Pin 9 on ECU

The switched side of the relay goes to compressor then earth after the relay

Edit: had wrong pin on ECU

Edited January 1, 2015 by 89CAL

[89CAL]

After tons of searching I still cant find a straightforward answer.

My understanding that activating the clutch compressor goes thusly:

(Battery +ve) ------> (low pressure switch) ------> (A/C relay coil) ------> (ECU pin 14) ------> (Battery ground)

Keep in mind this is a stagea s1, so pin number will be different. Its mainly the components that Im interested in. The problem I am having is that the air con compressor will not turn on. The ECU side seems to be fine.

I notice that when I probe the A/C relay terminals, the power terminals read battery voltage (normal), however the relay coil terminals only reads about 6.8 volts, whereas what I read in the R33 manual states it should be battery voltage. If I try to manually ground the relay coil, there isnt enough power to make the relay click, and the voltage drops to zero.

So, is my diagram above correct? If so then am I right in thinking that the low pressure switch is causing this? Is it an on/off state switch or some kind of resistor? Where is the switch?

Its odd that the air con did come as normal once a week or two ago, however I have not been able to replicate that.

How are you checking the power here? Depending on how you are probing it depends on what reading you are getting

Remember that the 2 probes are checking potential difference (delta V) so if you measure accross a switch for example that is switched off with 12V on one side and grounded on the other, you will get 12V accross it (as there is 12V difference across the switch. If you then close the switch, you will get 0V as there is no difference across the switch

The best way is to put the black probe to a good chassis earth and then use the red probe to search for voltage. You may very well be doing this, but even if so have a think about how you are doing it. I have had similar things happen where voltage disappears when probing something but there isn't actually a problem

6.8V should be plenty to activate the coil, have you tried swapping relays? I'm guessing you have already checked for a good circuit to ground as well

The pressure switch should work as an on/off switch. If it is very old (and most of us are in this boat) then I have seen some with buildup on the switch so it doesn't make great contact and you will get a small resistance there.

[zoomzoom]

Firstly I used extension wires to simultaneously plug the relay in while using the multimeter. Voltage was measured between the +ve pin on the relay coil and battery ground, without the relay coil forming a circuit. It measured 6.8 volts, where I understand it should read battery voltage. I then completed the relay coil circuit by grounding it to the battery. It did not activate the relay and the voltage dropped to zero. So the one fuse can't directly be connected to both the coil and the switch, there must be something in between the fuse and the relay coil responsible for the voltage drop.

Where are the pressure sensors? I can only find the 4 pin plug on the dryer unit. Is that both a high and low pressure cutoff and how can I test it?

[zoomzoom]

BTW Ive tested the relay too and its fine.

[89CAL]

Yes I believe the 2 switches are in the one unit (4 pin, suggests 2 x switches)

Meter from fuse 16 to the relay and see if you get a circuit to 2 pins. I have found this is the case on other relays before, the power is fed from the same source, the ground is different.