Auto Air Conditioning Diagnostics Question

[peterpilot9]

Well, it's that time of year here in the middle south; hot and humid so down goes the AC

The basics; 1990 300zx Z32 NA Auto Transmission 280K miles or so. Equipped with the Auto Air Conditioner.

Problem was easy to identify. The AC compressor clutch does not engage. So, using the great SM provided on this site, I have gone through the diagnostics as follows:

1. Reference the Trouble Diagnosis - Auto Air Conditioner page HA-115. Symptom - Magnetic clutch does not engage. Reviewed the Possible Causes column and the Diagnostic procedure column which indicates Preliminary Check 6, then Diagnostics procedure 19 if clutch is malfunctioning.

2. Went to Preliminary Check 6 page HA-131. Tested the sensor circuits in detail (Self Diagnosis Step 4 page HA-123)and all were in spec. Then set up for self diagnosis step 2, which is the actuator test. The magnetic clutch does engage when I select a first code of "5" or "6" per the chart on page HA-122. I did note that there appears to be a discrepancy with the table values on HA-131 which indicates compressor operation with first codes "3" or "4". Not sure what this is about but my clutch does engage while in Mode 2, first code number "5" or "6".

The result is therefore "OK" and the flowchart proceeds to indicate that the "Magnet clutch control system is normal. Refer to Magnet Clutch Control." I read this to mean the clutch is NOT malfunctioning and as such there is no need for Diagnostics Procedure 19.

Now it gets confusing for me. The only reference I find in the SM for "Magnet Clutch Control" is on page HA-106. There is a description of the Magnet Clutch Control that reads as follows:

"The auto amplifier controls clutch operation by the ambient temperature and signals from the ECCS unit. The auto amplifier will turn the compressor "on" or "off" as determined by a signal detected by the ambient temperature sensor."

I'm not sure what to do with this reference from Preliminary Check 6. Not sure if it means the auto amplifier is bad and should be replaced or if it means there is somehow further diagnostics that can be performed on the auto amplifier which might be detailed elsewhere in the manual. I don't see it in the SM so I'm at a loss.

Anyone have any experience here? Thanks for the help.

[Ace2cool]

The clutch will also not engage if the pressure is too low. Have you verified that your refrigerant is charged?

[peterpilot9]

You are quite right about that one. One of the possible causes is shown as low freon level on the Trouble Diagnosis page HA-115. However, when you follow the diagnostics instructions, the diagnostic system indicates sufficient charge if you are able to command the compressor to engage during the self diagnosis Step 2 - Actuator test. Looking at Preliminary Check 6 on page HA-131, it indicates you should check refrigerant level if you are unable to command the compressor to engage. Since my compressor will engage per Self Diagnosis Step 2, I believe the system is not seeing a low charge signal from the pressure limit switch.

As a side note, before I dug into the manual and went through all of the diagnostic procedures, I bypassed the AC relay and checked pressure with the compressor engaged and pressures were good. Wish that were the problem....

Still at a loss for where to go when test results are OK for Preliminary Check 6. I appreciate your taking time to look over my problem, Thanks!

[Ace2cool]

I'm not even gonna act like I know the A/C system that great, but if all the compressor and control tests are checking out all right, I'd look to a sensor. Possibly the interior temperature sensor in the overhead light panel. If I'm not mistaken, if you short the two leads together, it should read maximum temperature and engage the A/C.

[peterpilot9]

Another good suggestion but unfortunately, the sensors are also part of the testing flow. Before you complete the actuator test in Preliminary Check 6, you check sensor circuits in detail using Self Diagnosis Step 4. There are 7 sensor inputs tested in total, and all returned a no fault code of 50 in my case. Additionally, per the suggestion in Step 4, I ran through each sensor's readings using Self Diagnosis Step 1; Monitor Diagnosis. All of the sensors showed readings in line with actual temperatures and were also within the specs defined in the SM.

This is really kind of strange. The SM gives a really good flow of checks to perform for my particular problem, but in the end, the system doesn't show anything being out of order, except for maybe the Auto Amplifier. I can command the Auto Amplifier to send a signal to the magnetic clutch and engage the compressor, but in the Auto Mode, the system won't. Basically, it compares the temp selected to ambient temps in the cabin (with delays provided depending on input from the Sun Load sensor) and runs the compressor as needed. As far as the diagnostics go, all of the sensors are working within specs and sending valid info to the Auto Amplifier which will run the compressor when commanded manually but not automatically.

I do appreciate your suggestions. Guess my search will continue.

[NolimitZ32]

Are you testing actual voltage/resistance readings or running the tests through consult? Consult readings may not be accurate since they are processed through a controller which could be the fault point.

[peterpilot9]

The service manual indicates use of the Consult or the Self Diagnosis function. I am using the Self Diagnosis function which uses the Auto Amplifier to return data and actuate the doors, fan and compressor. If the controller (the Auto Amplifier) is bad that could make sense. The SM procedure just isn't clear where to go if you can manually command the Auto Amplifier to engage the compressor but the Auto Amplifier will not engage the compressor in the auto mode.

I'm not really sure where to start if the values returned by the Auto Amplifier are incorrect due to voltage issues. Maybe test each sensor for a short or open circuit even though the system diagnostics indicates they are all within specs? I can see how there could be an electrical issue given the overall condition of some of the wiring harnesses on the car. I'll look through the testing procedures and see what I can find.

Thanks for the advice!

[NolimitZ32]

Thats basically all you're left with is actually checking the sensor inputs/outputs for compliant values, it'll be very easy to tell once you come across a value that doesn't match whats in the book but it is a PITA and tedious work.

[peterpilot9]

You're right about that one! One thing is for sure, the self diagnostic system's display of sensor temps are very accurate for the ambient conditions. I appreciate the help. Just wish the SM were more clear on what to if the results of Preliminary Check 6 indicate that the magnet clutch control system is normal.

Oh well, the joys of driving an antique

[peterpilot9]

I haven't resolved the issue, but have set up a solution that gives me AC. Ice cold AC

If you read the thread, you will note that I went through each step in the service manual's diagnostic procedures for a situation where the magnetic clutch does not engage. Essentially, going through the steps will verify all sensor input values to the auto amplifier, the circuit status of each sensor and lastly a basic system pressure test and directing door motor via actuator tests. In my specific case, everything is in working order and I am able to engage the compressor by going into the actuator test in the Self Diagnosis Step 2 mode.

So, what I have done is to set the system up so that the mode door blows air through the face level vents, the intake door is in the recirculate mode, the air mix door is full cold and the compressor is on. Additionally, I can set the fan speed to one of four values as needed.

On a practical level, this is how it appears in the car;

The Set field will display a 30. Indicates you are in the Self Diagnosis Step 2 mode
The AMB field displays 65. The first value setting of 6 is the setting for the actuators to be positioned for vent, rec, cold air and compressor on. A first value setting of 5 alters the condition but basically can be used for defrost. The second value setting of 5 is the third highest fan speed but can be set anywhere from 3 through 6 as one might see fit.

So far, no problems. I think what is happening is that whatever logic the ECU uses to complete the switched ground to the AC relay when the system is in the AUTO mode is stopping the process. Not positive of this but I think this is the case. In this setup, the climate control system is still directing the commands to all the components, nothing has been bypassed in that system that I can tell. The AAV picks up the load and adjusts the idle and I have had no issues with heating driving in 90+ degree temps. Of course, the compressor will not cycle off when a desired temp is reached so I manually cycle it off after the cabin is comfortable.

I will continue to try and resolve this such that the AUTO mode functions, but for now, I'll be able to keep the sweat function down a bit. If there is anyone who reads this who is aware of a problem that might be caused by operating the system by direct commands in the diagnostics mode, I would greatly appreciate the input.

Thanks!

[amc49]

Be VERY aware that past a certain point leaving the clutch going without it being cycled off WILL tear up the compressor when it goes over pressure, your override may have bypassed the over pressure sensor that normally takes care of that. I do not know the specific system there but they ALL tear up compressors run solid long enough and BTDT. You do NOT want to go there. Faster the blower blows the less likely to happen, the faster speed pulls off more cooling from evaporator. Clutch on solid it freezes evaporator to block the compressor output and kablooey! Your last seconds before disaster warning will be ice crystals coming out of vents.

[peterpilot9]

Be VERY aware that past a certain point leaving the clutch going without it being cycled off WILL tear up the compressor when it goes over pressure, your override may have bypassed the over pressure sensor that normally takes care of that. I do not know the specific system there but they ALL tear up compressors run solid long enough and BTDT. You do NOT want to go there. Faster the blower blows the less likely to happen, the faster speed pulls off more cooling from evaporator. Clutch on solid it freezes evaporator to block the compressor output and kablooey! Your last seconds before disaster warning will be ice crystals coming out of vents.

Thanks for the info! I'll be using this bypass sparingly because as you note, the compressor has no input signal to cycle, at least none of which I am aware.

Your note about ice crystals brings back some memories. Back in the day, I flew F model Cobras in the Army. It was equipped with an ECU (Environmental Control Unit) that operated off bleed air. That set up of course doesn't relate the use of a refrigerant compressor, but ice crystals were still part of the experience. We spent many a flight hour in very hot conditions in that era,and if you set the temp low enough, you could generate lots of ice on the air outlet valves in the cockpit. Then, if you had need to pull in lots of power, the pressure at the vents would spike and blow big chunks of ice into you face if that is where you aimed the vent. If you were good, you could pump the collective as you flew and grab a big bite of ice in your mouth

Anyway, going to visit someone who is better with all things electronic towards the end of the month. We'll spend time diagnosing and I'll get back to the board if I can find the resolution.

[peterpilot9]

Be VERY aware that past a certain point leaving the clutch going without it being cycled off WILL tear up the compressor when it goes over pressure, your override may have bypassed the over pressure sensor that normally takes care of that. I do not know the specific system there but they ALL tear up compressors run solid long enough and BTDT. You do NOT want to go there. Faster the blower blows the less likely to happen, the faster speed pulls off more cooling from evaporator. Clutch on solid it freezes evaporator to block the compressor output and kablooey! Your last seconds before disaster warning will be ice crystals coming out of vents.

So you got me thinking about the system and how it might prevent freeze ups, so I took a look in the service manual and found this tidbit on page HA-39:

Freeze Protection (For Auto Air Conditioner)
When the AC is switched on, the compressor runs continuously, and the evaporator pressure is maintained by a suction throttle valve (S.T.V.) to prevent freeze up.

The manual air conditioner configuration does cycle the compressor on and off to prevent freezing.

So it appears that when the system operates in the auto mode, the compressor is not cycled. So maybe the risk of running the compressor continuously outside of the Auto mode doesn't introduce a different variable in regards to system pressures and freezing. The S.T.V is located just upstream from the expansion valve.

In any case, I believe your advice to be prudent and will manually cycle the compressor as a precaution.

[amc49]

You are correct, some systems idle the compressor at low load while the valve setup lets things simply bypass. I simply was playing the numbers as far as system design. My bad.

[DCaff300ZX]

Good stuff here, I love it when a solid approach is used and includes the fsm that while a bit clumsy at times, does have more info than it may seem and takes some hunting and thought.
NOT an AC pro here, but I have had times where I've researched the AC system and have had to deal with a heater core replacement...anyway, during this research the cabin ambient temp sensor has caught my attention due to it's odd service life and placement in the right side of dash in the vent area- a VERY bad spot for long-term life expectancy, IMO. This seems to be a part of your issue in that I believe it controls or is part of the auto loop where cabin temp is concerned, and possibly it is not correctly informing the ECU that the correct temp has been reached.
Due to semi-common electronic discrepancies with the Z32 and it's very complicated and poorly-sealed electrical system, plus info from my Z guy after having to replace the starter solenoid (he said that ALL years of Z's are starting to show up in larger numbers with sealed electrical unit failures, in units of all years and types) I'd suspect that something may be up in that area, or arena...so hopefully that gets worked out with your electrical buddy as you go over your AC system. Continuity and correct connection with ECU and related items would be paramount obviously, as well as removing any corrosion found.
Good Luck!

[peterpilot9]

I apologize for this extremely delayed response! I though I had updated the forum on this issue, but looks like that's not the case.

To kind of summarize, I had a non functioning Auto Climate Control that I could manipulate with the diagnostics feature in the unit to command cold AC and defrost, along with all desired fan speeds. In addition, and ultimately related, I had a "ghost" alternator light and the "BRAKE" warning light indicator with no apparent cause.

Well, there was a member here who advised I check all the grounds as they are a known issue. Well, turns out that all of those issues were resolved when I pulled and cleaned one ground wire connection' a connection that from all appearances was clean and clear. So kudos to all those who had that insight!

I do not know an identifier for the ground, but it is the single bolt ground in the driver's side front near the intake runner. The reason I ended up pulling it wasn't related to this issue, but it ended up resolving the issues noted above.

[itsa300zx]

Thanks for the update! Got a picture of the ground point?