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The AC on my 2008 Mazda3 fails to cool the car after I've been driving it for more than 10 minutes or so. It's able to produce cold air while I'm idling from a cold start, but it seems like the compressor refuses to turn back on once the car has been warmed up (even during a hot idle). If I let the car cool back down and start it up again, I can once again get cold air.

I already checked the pressure on the low refrigerant line using one of those refrigerant recharge kits with the built-in pressure gauge. When I can actually get the compressor running, it shows a normal refrigerant pressure. I do notice that when the compressor cycles off, or is refusing to run at all, the pressure gauge shoots up into the red "overcharged" zone. I did not add any additional refrigerant so I assume that this has something to do with the pressure feedback switches that internally control the pressure of the refrigerant. Am I correct in assuming that this is a normal behavior?

My next step (inspired by the Haynes manual) was to inspect the compressor belt and compressor clutch. I removed the front right wheel and the wheel well trim:

Exposing the compressor clutch on a 2008 Mazda 3

The belt looks fine and feels taut. I removed the clutch plate and inspected the inside surface:

Interior surface of the clutch plate showing heavy corrosion

Aha! The surface of the clutch plate appears to be heavily corroded. According to this source, this corrosion can cause the clutch plate to slip when it is supposed to be engaging the compressor pulley.

Is this the likely source of my problem? Why would it only be an issue when the car is hot (thermal expansion?) Can I just dremel off the rust or do I need to replace the entire clutch assembly?

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  • Possible duplicate of 2002 Ford ZX2 A/C compressor turns off when engine is warmed up, but works fine when engine is cold
    – GdD
    Jul 2, 2019 at 8:06
  • To me, this sounds like the clutch gap is too wide. Are there any shims between the hub and the shaft? If so, try removing one shim and then reassembling the clutch. Fine sand paper is all it takes to remove corrosion by the way. Leave these gauges alone, they're not professional gauges.
    – Al_
    Jul 2, 2019 at 12:13
  • @Al_ yes, there was one tiny shim that I can remove. I'll give it a shot!
    – alexw
    Jul 2, 2019 at 13:28
  • @alexw Check the gap width before reassembling. It's usually between 0.011811 and 0.023622 in with perfectly working clutches. It shouldn't ever slip.
    – Al_
    Jul 2, 2019 at 15:11
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    @Al_ would you consider writing a full answer with all of this information?
    – alexw
    Jul 3, 2019 at 18:00

3 Answers 3

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IF the gauge is in the red when this happens, it's no the clutch. The high pressure switch is preventing the compressor from engaging. Fix the high pressure problem first.

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    If the problem were to be that, it would struggle to produce cold air at idle (that is, stationary car) on a cold engine, and it would always reliably produce cold air while moving. The gauges found in kits aren't very reliable compared to professional ones. Also, the high pressure switch would repeatedly cycle the clutch, rather than keep it from engaging. This sounds like a clutch gap width issue.
    – Al_
    Jul 2, 2019 at 15:42
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I have the exact same car (make and model and year) and I just went thru this drill.

I'm hoping in your case the clutch air gap was just too big. You set that via small washer spacers beneath the clutch plate as its installed. Spec = 0.35—0.65 mm {0.014—0.025 in} Air gap reference is here.

In my case the A/C clutch magnetic coil was burned out. The wiring got too hot and melted. I had to replace the coil. In your case, it sounds like the thermal switch is opening, perhaps prematurely (or perhaps because of other causes of excessive heat...) The thermal switch is located on the end of the compressor.

A/C Compressor

Check out the wiring diagram:

A/C wiring diagram

With that said, the clutch replacement parts were pretty reasonable. Here is the part I purchased... The kit includes a new coil and pulley and clutch plate and bearings. I had to cut and resolder the electrical connector (hint: make the new one EXACTLY the same length as the old) The kit did NOT include a temp sensor.

Oh, and replacing that belt is a wicked hard task.. I struggled for hours until I saw this video. How to Replace Mazda 3 Serpentine and AC Drive Belts, by Oliver Porter Published on Jun 25, 2017 Oliver is my hero.

I ended up using five or six tie straps (and spinning the whole thing clockwise, not CCW as in the video)

Now with all that said. I'm still not totally happy with my A/C performance on this car. I haven't really turned the A/C on since the repair for more than a few minutes. It doesn't quite seem right. I need to pick up another set of gauges before I test it again.

If you have other questions on 2008 Mazda3 (or just want to compare notes) my email is available on my profile.

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I ended up buying a new clutch assembly and installing it as my first serious attempt to solve the problem. The thermal sensor is located on the other side of the compressor and there is no way to replace it without removing the compressor, which you can't remove without having the refrigerant professionally evacuated at a shop. So, I spliced the wires from the new clutch assembly into the old thermal sensor while leaving the compressor assembly in place.

Sadly this still did NOT solve the problem. We took it to the shop and they said the entire compressor needed to be replaced - we still don't know if it was just the thermal sensor or if the compressor assembly itself was damaged in some way (lubrication leak, worn cylinders, faulty valves, etc).

It turns out that you can DIY a compressor replacement, but you have to take it to a shop to drain the refrigerant first, drive it home, replace the components, and then take it back to the shop to refill the refrigerant. Since the cost of the drain/refill service is considerable (couple hundred bucks), it's worth it to just completely overhaul your AC system with new parts after the refrigerant is drained.

In our case we replaced the compressor assembly (which also comes with a new clutch), expansion valve, and drier/desiccator. This is a fairly involved process as it requires work both under the hood and inside the cabin. After replacing the parts and having the refrigerant refilled, the AC seems to work like new.

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