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Discussion Starter #1
Hi folks,

'03 MDX (no nav)

My A/C is just blowing hot air and compressor is not engaging. I read somewhere that this could be caused by low freon (among other possible causes). What are the normal L and H pressure readings with engine on or off, and compressor not engaged? I know it can vary a bit depending on temperature, but I haven't found definitive info on expected pressure readings. My gauge reads L=116psi, and H=135psi with engine running but compressor not engaged ... it was about 100F yesterday when I took the readings. The L reading seemed kind of high to me (since that is near the max of my gauge), but I read somewhere it should be high if compressor is not engaged Is this true? I have no reliable reference that lists actual pressure values to look for. "High" is a subjective term.

I have also verified no short in terminals 1 and 2 of a/c clutch relay when disconnected from the box. There is also about 13V on terminal 1 in the socket. It also did not help swapping with a known working relay, so I'm pretty sure the relay is ok.

Here are what I plan to do next:

- I'll get readings later with engine off... not sure what help this will give
- try to put a jumper on the A/c clutch relay terminals 1-2 to see if I can force the compressor to engage, but not sure if it's safe with the "high" pressure reading I got. I'm a little paranoid now.
 

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Discussion Starter #2
I quickly took the pressure reading this morning with engine off: L=95psi, H=100psi. Ambient temperature was only in the high 80's, though, so much cooler than yesterday.
 

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mDeeX2003, without the engine running and/or the compressor clutch engaged, you'll get some pretty meaningless pressure readings. I'm not really sure what the "static" pressures are in a working system, but the low pressure side should go down, and the high pressure side will go up once the compressor is running (my recollection from working on other vehicles - assume the MDX is the same in this regard).

I can't imagine that you'll end up with too much pressure by jumping the relay, unless you've added some gas to the system for some reason. An alternative option is to just monitor the voltage at the A/C compressor clutch contacts while you try to spin up the compressor - that way, if you DO have 12 volts there, you know the clutch is bad... if you don't have 12 volts there, you need to trace it backward to find out why not. The link in the post above by forbin404 has some more info on tracing the circuit.
 

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Those pressure readings (static) are above the set point of the low pressure switch, so the system should kick in if nothing else is wrong. You are OK to do the jumper test at terminals 1 & 2. (Step #5)

If the clutch does not engage:

Quick test to see if you have a bad compressor clutch. Refer to step #21 in the testing procedure link above. Take a 15 amp fuse and a length of wire and jump from the battery positive terminal to terminal #2 in the relay socket. I suggest you insert the wire into the terminal, then hold the other end of the wire against one blade of the fuse. Then push the other side of the fuse blade into the battery terminal.

If the fuse blows, you have a short to ground in the compressor clutch or the red wire that connects it to the relay socket.

If you hear the clutch engage, and the high side pressure starts getting higher and the low side starts getting lower, the clutch is good.

If the clutch does not engage, and you get no spark at the battery terminal when connecting the fuse, the red wire is not completing the circuit or there is an open circuit in the clutch coil. Some models also have a thermal fuse on the compressor that opens when the compressor overheats. That will give the same results.
 

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Discussion Starter #6
Thanks for all the input. I have not taken the car to the shop yet, nor have I had any more time to continue the diagnosis since I last posted, but I'll try to test the compressor directly, as people have suggested, when I get some free time.

Anyway, what I was thinking was that based on my pressure readings, there is at least no leak in the system. If there was a leak, I would expect the readings to be much lower (or it becomes lower over time) whether the compressor is running or not. Does this sound sensible? If this is the case, I'm thinking the problem is probably isolated to the compressor (or perhaps some other component in the high pressure side).

And please pardon my ignorance, but what do the "delivery" and "intake" temp/pressure mean in the chart from forbin404's link? Are those referring to the low-pressure and high-pressure lines, respectively?
 

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For simplicity, think of "intake" as the temperature in the footwell of your passenger compartment, and the "delivery" as the temperature at the vent.

Yes, if your system has pressure, chances are good it's not leaking (at least not much, and not quickly).

I would not assume there's anything wrong with the compressor - at least not yet. There are a host of other things that can keep it from engaging. And very quick, simple tests will tell you definitively whether your compressor is good or not (detailed in the posts already written, so I won't repeat them here). As with any troubleshooting process, don't start assuming anything, but figure out what you can determine for certain, and then work backward or forward from that point, until you find something that's not right.

The good news is, it's going to start cooling off soon, so you might be able to delay finding the problem for many months! ;-)
 

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Discussion Starter #8
Update... So I finally had some free time for this diagnosis over the weekend. I applied power directly to the a/c compressor as suggested, and still not engaging, so looks like the compressor has finally croaked after 160k mi. Took it to a shop this morning for a second diagnosis, and they concluded the same thing. It's still not full-time winter here in cali, and will still need the a/c for a couple of road trips coming up in the next 2 weeks, so I'll just have the shop replace the a/c compressor. Thanks for everyone's tips and suggestions.
 
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