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I have a 2001 with 150,xxx on it, and the alternator died (well, burnt itself up) while we were on a camping trip (AAA was a total lifesaver then!). Replaced with a reman and it works fine. However, now the a/c compressor won't come on. The condenser fan works, as does the blower motor. The clutch relay works as well; I switched the relay with the condenser fan and the fan kept working. There are no blown fuses, and all the plugs look connected. I also checked the blower motor resistor, and the fusable link is ok. Prior to the alt. failing, the a/c worked beautifully, so I would hesitate to blame it on freon.

Any suggestions of where to look next would be greatly appreciated!
 

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Given the proximity of the alternator and the A/C compressor (essentially right on top of each other), I'm betting it's a near-certainty that somehow the connection to the A/C clutch was broken somehow. If you're lucky, it's just a loose plug (sounds unlikely if you've checked all the plugs). Or, maybe when the alternator was pulled up the tech got his fingers around the A/C clutch line and pulled something loose.

I'd pull the splash shield and see if you can get a meter on the lead going to the A/C clutch. If you've got 12V DC there when you turn on the A/C, and your clutch isn't engaging, you know for certain that the problem is between your meter lead and the A/C clutch (probably the wire or maybe connector). Let's hope it's something simple and cheap and maybe easy to fix (I've never worked on my MDX in that area - looks a little tight, but you can probably drop the compressor to check it out easily enough).
 

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It was unplugged! However, plugged it back in and still nothing. Ran a jumper wire to the clutch wire from the battery and it worked fine. Traced it to the relay is not getting the grounding signal it needs from either the high pressure switch or the pcm. I saw a lot of helpful chatter talking about circuit A17 and A27. there most have been the actual shop wiring diagram to go along with this. Does anybody have a shop diagram for the plugs on the PCM dealing with the A/C circuit?

Thanks, Brian
 

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Very frustrating! I have checked everything I could think of and/or find (fuses; relays etc.) and everything seems to be fine; however the clutch won't engage the compressor unless I use a jumper wire for the relay. I jumped the circuit at the pressure switch (its a 4 prong, oval shaped. Jumped the 2 wires in the small part of the oval and the cooling fans come on, yay; jumped the 2 wires in the large part of the oval and the clutch does not engage, boo), to no avail. checked the switch with my dmm, and had resistance, so I don't think its that. Checked the relay circuit in the fuse box under the hood. It is a typical 4 prong relay (all the relays are the same on this car, so I switched out the relay on the clutch circuit with the cooling fan relay, which is working, and still nothing), 2 for the low voltage to engage the "switch" part and 2 for engaging the clutch. The hot side for the clutch has 12v, and I jump that to the other plug and the clutch engages. Checking volts on the other 2 with the ac off, one side has 12v and the other 0; with the ac on and the motor running, i get 12v on one and 5v from the other. At this point, how I understand it, the 5v is supposed to trigger the relay and the 12v on the otherside is supposed to not be there (go to ground), mine is not doing that.

Any suggestions at this point would be greatly appreciated!

Thanks.
 

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I take that last part back. The 5 volts is not supposed to be there. I hooked wired up to the relay and put the terminating ends in their appropriate spots except for the one where I was getting the 5v, started the engine and with the ac "on" I grounded the other side where the 12v comes into the relay and the compressor comes on. So I am getting this 5v ghost voltage, or its supposed to be there when the ac is off.

Thoughts?
 

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Addendum; i've read the 5v is supposed to be there for the pcm as part of a feedback loop. I've also read that there is a thermo protection switch for the compressor, that if not working, will have the same effect that I am having. This is the only thing I haven't checked. Does anyone know if this year had one of those?

Thanks.
 

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It's really just a matter of figuring out what the inputs and outputs at the PCM are. To over-simplify it, if you've got 12V on the hot side of the relay coil (and it sounds like you do), the PCM should be grounding the other side of the compressor relay coil (at the B11 output).

I'm thinking there's some sort of problem with the pressure switch (which may be that the refrigerant pressure is wrong). You'll need to have an closed circuit (very low resistance - certainly less than 1 ohm) between terminals 1 and 4, and a closed circuit (less than 1 ohm) between terminals 2 and 3 for the compressor to engage. To me, it sounds like maybe one side of the pressure switch is in the "off" position. If the fan comes on when you jump those terminals (1 and 4), but doesn't run when the A/C is turned on (or does it?). That means either the switch is bad (could be) or that you have refrigerant pressure that's too low (most likely) or too high (unlikely unless you just got it recharged and too much refrigerant was added).

But to make it easier, I just PM'd you the troubleshooting procedure for that circuit - I'm thinking that should let you figure out if you've got a bad pressure switch, PCM, or maybe just low gas in your A/C system.
 

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Discussion Starter #10
I was leaning toward changing the pressure switch. I jumper wired the compressor and then tested the ohms across the posts and the fan circuit was closed, and the pressure circuit was closed as well (however it did take a moment to register being closed, so maybe the switch is going bad or is). However I did the jumper wire trick in the socket that plugs into the switch; the fan came on, but the compressor did not. Given that, I was also leaning towards a bad circuit in the PCM. Replacing the switch is a lot less expensive, so I may start there.

Thanks for all your advice.
 

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Did this get resolved? This is an old thread but I have similar issue as the OP described. I replaced my alternator and starter but now the AC will not blow cold air, when it was working perfectly fine before the alternator and starter went. Blower works but AC clutch does not seem to engage at all. I've checked the fuses, relay and they all seem good. I'm wondering if I inadvertently yanked and disconnected a connection when replacing the alternator or starter. What is the easiest way to access the AC compressor to check to see if the plug is connected?
 

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FWIW, do you know if the shop disconnected the battery for the alt swap and after, put the PCM in relearn? Would be a simple thing to try.

Make sure the X is warmed up. Turn off all power acc, AC, radio, headlights. Make sure you have your navi and radio codes. Disconnect the battery for a few mins. Reconnect, put the key to acc II for 2 secs, start, and let it idle for about 7 mins.
 

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FWIW, do you know if the shop disconnected the battery for the alt swap and after, put the PCM in relearn? Would be a simple thing to try.

Make sure the X is warmed up. Turn off all power acc, AC, radio, headlights. Make sure you have your navi and radio codes. Disconnect the battery for a few mins. Reconnect, put the key to acc II for 2 secs, start, and let it idle for about 7 mins.
I did the relearn but still nothing from the AC clutch. It's strange because the AC was working just fine before I pulled the alternator. Would I go under, removing the splash shield, to access the compressor and check the connections? Anything else I should try to get the AC working?
 

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A few more suggestions:

Sounds like you're assuming system pressure is OK. If the compressor plug was disconnected, makes me wonder if the alt was dropped and caught the clutch wire or hit an AC line. You might want to check the metal AC lines, fittings, and hoses for damage. If there's an Autozone or O'Reilly nearby, you can borrow the AC gage for a refundable deposit. Note that one or both stores might have a time limit to return the tool in order to get your deposit back. If the pressure is correct, your suspicion of the bad pressure switch might be correct.

The Climate Control unit has a self diagnostic for opens and shorts. See this thread. http://www.mdxers.org/forums/73-first-generation-mdx-2001-2006/128761-2004-mdx-c-problem-help.html If the link doesn't work, you can search this forum for, climate control DTC. The DTC legend is in the attachment.
 

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