It's getting Hot.. Diagnosing the AC System

So last week I was driving to C+C and noticed that when I turned the fan switch it would run, but the AC button did not cool things. So.. I thought a refill of R-134 coolant was in order. Only, when I went to put some in, the compressor didn't kick on and the output line didn't get cold. So naturally I went to check the connections to the unit and lo-and behold the fan stopped running altogether when I monkeyed with connectors on the system. So.. something didn't work and needed replacement.

When the AC system (HVAC system in general) isn't working, it's either the compressor isn't compressing the coolant or the evaporator unit under the dash and its electrics aren't working.

You can hear the compressor turn on with a click and appreciate the slight hum when its running. If you hear it click on and the Stop button is NOT light and is "out" (NOT pressed in), and you get warm air, the compressor isn't compressing or the gas is low. You can fill it with a little coolant and when you're filling it in the front trunk you DON'T see the outlet pipe getting frosty or water condensate on it.. there's something with the compressor. You can "overfill" with coolant in which case the dial on the fill system will be in the red or on the high side of things.

If this is the case, you got's to bring it in and they can vacuum out the old coolant and replace the compressor.

So: Fan works, AC button out, hear it whirring, still hot air with coolant in the system.. compressor is suspect. Bring it in to get evaluated by someone with good gauges and a HVAC filling unit.

If you are asking "where do I even buy AC coolant or what IS an AC compressor".. just stop now and bring it in. Rule #1 of the wallet: Don't learn to wrench on a Ferrari.

 

So what if you have issues with your AC system and it's not simply blowing warm air?

In my case, everything worked.. but the AC compressor wouldn't turn on. Then while monkeying with it, suddenly the interior fan/blower doesn't turn on? Well. That means there's something with the evaporator unit located under the dashboard. And this means its probably electrical.

When I had my 928, the very first thing you learn about electrical issues is you start with the fuse. Always start with the fuse! Is it blown, or is it normal?

With the HVAC system you have 3 fuse locations. The first is on the right side of the battery in US models. There is a fuse block with BIG 50 amp fuses located right next to the battery. The AC system fuses are at the rear of the fuse panel. unscrew the footrest and lay it down. Then pull out the fuses and look at them. If they are still intact, it means there was no major meltdown of the blower motor or major component. (For exact location.. please open your owners manual). If they are good.. next step:

The second set of fuses is located above the passengers feet on the AC unit itself. There is a green fuse and a grey fuse amidst a forest of wires. The grey fuse gives electricity to the AC Unit ECU (we'll talk about this later) and the green fuse with the cover on it gives electricity to the motors and blower on the unit. You can pull them out or check visually. If they are good next step:

The AC system has some other fuses behind the drivers seat. Open the panel with a phillips screwdriver and look at the fuses in the center fusebox, the bottom of the center fusebox. (for exact location, please open your owners manual). If they care good then it's probably a unit on the evaporator.

So: no interior blower/fan, no AC unit going on, then it's probably electrical. If all fuses check out then it's probably a component.

 

If all the fuses are good, but lets say the blower doesn't run or runs at one speed.. or the AC unit isn't cycling but it was working fine before.. it's probably a component of the evaporator unit.

The evaporator unit consists of a big blower to move air into the cabin, some motors to move flaps to direct the air to your feet or the defrost, a couple temperature sensors, a fan resistor/transistor unit, and an main ECU.

If the motors burn out to the various flaps, you'll have a function that won't work when you turn the dial on the dashboard to operate the defrost, etc. You may even burn out a fuse in your search.

If the temperature sensors don't work that measure air into and out of the unit, then the car won't regulate air temp or maintain cold or hot in the cabin. You can measure the resistance of these. One is black connector on the drivers footwell. One is on passenger side footwell. Difference will tell you one is toast. Buy a replacement and then compare the resistance to the new one. One matching it at room temp is the good one. Replace the other.

If your fan is stuck at one speed or won't go on at all when you turn the turn the fan switch, it's going to be one of two components that are bad, the blower resistor or the AC HVAC ECU.

Per user RUSTYBITS You can test the blower resistor by going under the passenger footwell and shorting the grey and blue big wires. If the fan operates, the motor isn't toast for blowing air. These are a common cause for problems in the 360 HVAC system. I bought one and have it on hand. Used they're about $400 USD (2017). Also used in Rover models apparently. Discontinued for Rover.

In my case however I suspected the ECU. Since the blower worked fine, but the AC didn't work when it worked last fall.. and then suddenly nothing, no blower fan at all.. I was thinking its probably the ECU. The ECU is located in the drivers footwell. It is a little box about 4 inches by 7 inches held onto the evaporator unit with a phillips head screw. It has two connectors on it, a clear/white connector and a yellow connector. Removal is easy. Unscrew the philips screw, remove both connectors. The yellow connector has a tab that pops out that helps you pull it out. Suspect this unit when things are being "wonky" with the HVAC system.

Of note when I was diagnosing the system, I had the ignition turned on and pressed in the connectors to the HVAC ECU. When I had a certain amount of pressure on the yellow connector, my blower would go to full blast and the AC would kick on. Sideways pressure on the connector did the same. This let me know the blower worked and the AC compressor was compressing.
I suspected it went bad and lo and behold.. with a used unit I bought online.. connecting it resulted in everything working normally again. Note: install with ignition key in off position.

There have been several revisions to the HVAC ECU part. It's expensive.. I got mine for a good price of $400 but they're on for about 700. Seems they have some similarity to the 456 unit. There are rebuilders that do it for $500 with a warranty but I didn't try it.

So: No fan or all fan, constant speed: consider Fan resistor or ECU

Odd behavior in the system, more than one function to working correctly suspect ECU.

If you are thinking "what's an ECU".. just stop and bring the car in. Total diagnosis for me was About an hour. The ECU installs in literally 5 minutes. Parts for above are expensive, as far as aI can see the fan resistor/transistor and ECU are the only two components that can fail and affect the running of the system. But.. I never say never and never say always so who knows.

 

I'm sharing my experience with mine for those that might be so inclined. And how is it incorrect BTW? Found my problem and fixed it..?

 

Sorry for breaking things up... easier to type thoughts out in parts.