A/C doesn't blow cold... sometimes

I started getting this interesting issue on my '91 NA1 where the AC does not blow cold, randomly. The blower fan seems to work just fine, I get a bunch of hot air coming at me where the climate control system is trying to cool the cabin to 60*F. I do not believe I hear/feel the compressor kick in when this happens.

The strange part is how intermittent the issue happens. It can be an entire week or month, no issue. The car will start up blowing cold AC and work all day throughout various trips (turning the car off and coming back to it). Then on the other hand sometimes it will be working and when I park and come back after 15 minutes, no more cold AC. A few days later when I drive again, it's back to working normally.

In a few cases where I had longer drives (30+ minutes) when it was not working, it suddenly began blowing cold. It does not always "get cold eventually" though. I have never had the opposite problem though, where it would stop blowing cold. If it's cold it will stay cold for the duration of the trip, it's just a matter of will it decide to work next trip.

Given this temperamental behavior, I don't think it's any of the main AC components themselves but likely an electrical/control module issue. Whatever is telling the parts of the system to work, isn't in those cases. I have a hunch it's the fan control system that's next to the ECU, which talks to the compressor. Perhaps some aging/bad capacitors or relays inside are causing the compressor to not be enabled when it should be.

I have been going through the service manual trying to identify how the system works. Earlier today when it was acting up, I tried the "self-diagnostics" for the climate control on Page 22-12. I am not sure if I was doing it right, but nothing seemed to illuminate to give hints as to what could be the issue.

Any ideas would be appreciated! (I do have the R-134a retrofit system)

Yeah its time to send the climate control out to get it fixed.

I think you are onto something with the electrical. I'd look at the triple pressure switch, compressor relay, compressor fuse and fan control unit. Your condenser fans could also be failing intermittently.

Awesome, thanks for the list of places to look. I'm pretty comfortable with electrical stuff, have my own little workbench with whole soldering station and scope setup. If it's just a re-cap or reflow job I'm up for it myself, the nice part about old tech is the components are much bigger than modern SMD stuff!

If 91 and the CCU was never refurbished, no chance of surviving without the capacitor leakage so start from there.

Not sure you are aware of this but it’s not the CCU that controls the A/C compressor CL.
In fact, it’s the ECU.

Here is how it works.

1. The CCU sends the A/C On ‘request’ to the FCU (Fan Control Unit).

2. The FCU switches R & L condenser fans On.

3. The FCU sends out the A/C Switch (ACS) On command to the ECU.

4. The ECU checks certain conditions and turns On the ACC (A/C CL) terminal.

5. The field coil at the A/C compressor CL is energised and pulls the CL against the pulley.

Kaz

Adding to Honcho's list of sources for intermittent electrical failures, inspect the electrical connection at the compressor for the compressor clutch. That has been known to fail.

You mentioned "I do not believe I hear/feel the compressor kick in when this happens". Take a cue from Kaz's list and also listen for the condenser fans when switching on the AC or if the AC stops working. If the AC compressor is not running and the condenser fans are not operating then the problem is likely somewhere between the fan control unit and the climate control unit. Access to the fan control unit is moderately easy. With access to the fan control unit it would theoretically be possible to probe the inputs terminals to find out whether the fan control unit is getting the AC on signal from the CCU and sending the compressor on signal to the ECU. Unfortunately, the service manual provides the FCU pin assignments; but, does not provide an input test so you would have to try and figure out the status of those signals by test. If the condenser fans are always running when the AC is switched on then the fan control unit might be OK. I say 'might' because the triple pressure switch connects through the FCU and if that connection is flakey it may cause the compressor to shut down; but, I believe the condenser fans continue to run. In that case the problem might be the FCU, the triple pressure switch or an actual problem with the refrigerant circuit.

The fan control unit has been known to suffer from terminal solder fractures just like the main EFI relay. With an intermittent failure the failure point may not be visually obvious so reflowing the connections might be the only way to confirm the problem by elimination. The FCU does have capacitors so you might have a capacitor failure; but, capacitor failures are not usually intermittent failures.

If the climate control unit seems to function normally, fan speed works, servo motors open and close and the heater control valve function seems to respond normally then the CCU might be OK. Even if it works, if your 1991 CCU still has the original capacitors removing and inspecting the circuit board for damage would be a good preventative measure.

Kaz-kzukNA1 said:

Here is how it works.

1. The CCU sends the A/C On ‘request’ to the FCU (Fan Control Unit).

In [AUTO] mode, the request is based on the outside temperature sensor.​

Click to expand...

Is this the outside temp. sensor in front of the radiator?
IF so, seems the plug (see right side in the pic) is not connected with the socket/sensor - at least in my case ...

John_Wick said:

https://i.ibb.co/K9YBPN6/AC-Sensor.jpg

Is this the outside temp. sensor in front of the radiator?
IF so, seems the plug (see right side in the pic) is not connected with the socket/sensor - at least in my case ...

Click to expand...

Correct- that is the outside temp sensor.

Keep in mind, I believe the outside temp sensor only affects operation in Full Auto mode. The fan speed has to be set to the Auto position. Also, if you set the temperature control to the lowest setting it forces the system into continuous operation with no temperature control.

Given that your outside temperature sensor plug is a figurative 'mile' away from the sensor, I don't think it is responsible for an intermittent electrical connection problem if an intermittent electrical connection is what is causing your problem. If you used the self diagnosis procedure listed on page 22-12 of the 1991 service manual the self diagnostic should have flagged a problem with the ambient air temperature sensor by lighting the defrost indicator. The CCU self diagnosis and operation check will not identify problems with the Fand Control Unit, condensers or the compressor.

the technical information provided below gives me some assurance that any hvac matter can be solved
my 91 and 02 cool perfectly but it would be an emergency situation otherwise

i never turn full/auto ac off; ever, and only adjust the temp slowly if at all but mostly leave on 66 (65, 66, 67)
no one ever drives it and in the rare case i have a passenger, they already know they cannot touch anything including the a/c controls lol
i will report my progress

I literally have the same issue with my ‘91. I checked the capacitor on the FCU and didn’t notice the typical budge. Took it in to SOS abt a week ago and they couldn’t figure out the problem other than I was low on Freon which they took care of but still same issue. I’ll also look into what has been shared in this post.

One thing I noticed with my AC is it’ll work when the car is “cold” but after abt 30 mins of driving something disengages and I get hot air. If I let the car sit overnight the AC works the next day. I’m starting to think whatever is affecting the AC system gets “hot” and shuts down.

Capacitors can fail without the bulge, it's just one of the warning signs. Can also be cold solder joints or other things that are harder to see. You'd have to desolder the capacitor and test it in an ESR meter, which at that point you might as well just replace it with a known-good one of equivalent capacitance (and voltage).

My AC is working fine again, so this is on the back burner at the moment but I did ask my tech about it. He said similar here, though he thinks it's not the triple pressure switch since that's mostly a freon level thing (too little/too much pressure). I still suspect the FCU, maybe in the coming weeks I'll get in there and take a peek.

I am happy to see I'm not the only one with this issue. Misery loves company!

RHD_NA1_NSX said:

I literally have the same issue with my ‘91. I checked the capacitor on the FCU and didn’t notice the typical budge. Took it in to SOS abt a week ago and they couldn’t figure out the problem other than I was low on Freon which they took care of but still same issue. I’ll also look into what has been shared in this post.

One thing I noticed with my AC is it’ll work when the car is “cold” but after abt 30 mins of driving something disengages and I get hot air. If I let the car sit overnight the AC works the next day. I’m starting to think whatever is affecting the AC system gets “hot” and shuts down.

Click to expand...

Your last comment first. Have you checked the condition of your condenser fins to make sure that the fins are not clogged with dead bugs? The system shutting down after several minutes of operation is a classic sign of clogged fins. The refrigerant does not cool off causing the system pressure to increase which then operates the triple pressure switch to shut everything down. A failed condenser fan motor(s) can also cause this problem. Do the listen test to make sure the fans are running. On my 2000 it is really easy to hear the fans running.

Capacitor leakage may or may not be an issue on the FCU; but, the failure that has been reported and diagnosed is fracturing of the solder joints where the male terminal plug solders to the circuit board. This is similar to the failures that the main fuel injection relay suffers from; but, so far not nearly as common. The solder fractures can be temperature sensitive; but, the FCU does not directly switch the condenser fans so it does not carry significant current load (unlike the main EFI relay) which heats up the fractures so this is not a common problem like the main relay failures. You may need a magnifying glass or inspection microscope to see the fractures. The tell tale for a FCU problem would be if the condenser fans stop running.

You could also have a problem with the AC triple pressure switch which Honcho mentioned. This would also affect the condenser fan operation (and the compressor) The switch might have an intermittent connection of the high pressure cut off may have gone out of spec. Unfortunately, the diagnosis for the triple switch is by replacement. The closest the SM comes to a test of the switch circuit is on page 22-59 of the 1991 SM.