Diagnosing A/C Compressor Clutch not engaging, what next?

[commodorehat]

So over the winter my A/C stopped blowing cold air on my 98 SE.  I'd like to fix this myself and wanted to bounce what I know off of y'all to validate where I'm at.

 

This is what I know for sure so far:

• Compressor is not getting the power it needs to kick the clutch on (< than 1 volt is going into the connector)
• A/C Relay is good (swapped it with a known working one, as well as checked the continuity, and made sure it was getting ~12 volts to two of the connections)
• A/C Fuse is good (swapped it with a working one)
• Dual pressure sensor is good (checked continuity and verified ~5 volts in/5 volts out)
• A/C system Pressure is good enough.  (I hooked a A/C pressure gauge up and got ~70-80 psi on both high and low on a 70-something degree F day)

 

I've got the nissan service manual (from NICO club) and looking at the troubleshooting steps, maybe the A/C switch is the next thing to check.  I also haven't checked the ECM and whether that is outputting power to the right terminals.  If I were to dig under the dash, which one would be the first to check?  The ECM (on the driver side) or the A/C switch (on the passenger)?  Or is there something else I should look at based on what we know?

 

Oh one more thing... Strangely (or not), by accident I was able to engage the compressor clutch from dual pressure sensor harness when I accidentally left the multimeter on continuity and put it in the sensor harness with ground.   Did this give it a higher voltage than the 5 I read going through there?   Anyways that at least showed me the compressor was capable.

Edited March 23 by commodorehat
Added vehicle year/trim

[commodorehat]

So I went the AC Switch route and I think there's something fishy between the dual pressure switch and the A/C switch. The switch itself has continuity, but there is not full continuity between the switch harness and the dual pressure harness (though it does read something, also there should be full continuity between the switch harness and the fan harness and there is not full continuity as well.  The A/C switch itself seems fine, I get continuity when the button is engaged and no continuity when not engaged.  Is there anything in between the two that could go bad?  I tried to get continuity by poking through the wires to eliminate the harnesses, and got the same readings.

[commodorehat]

Just following back up after I found the issue...  It was a bad wire between the dual pressure switch and the A/C switch.  By walking through the service manual diagnosis steps for a compressor clutch not engaging, I narrowed it down so that I knew there was something fishy between the dual pressure switch and the A/C switch.  From there I ended up getting a cheap short detector from amazon and ran it along the wire harness that runs between dual pressure back to the dash A/C switch, and found the spot where the signal drops.  Turns out the harness was damaged there.  I must've accidentally cut it on some previous work.

 

Here's the short detector... it worked pretty good for the price:  https://www.amazon.com/dp/B07BC4X28Y?psc=1&ref=ppx_yo2ov_dt_b_product_details

 

My advice for DIY A/C diagnosis is to follow the service manual steps.   They were systematic and worked pretty well for me in diagnosing the electrical problem.

[EricCR]

Thanks for taking the time to update us on the solution. It may help someone at some point.

 

If you have a multimeter you can also find open circuits by measuring resistance and/or continuity, just in case you already have one.

[commodorehat]

Hey Eric, I appreciate the reply.  Yes, agreed.  I didn't mention that specifically, but that was definitely part of it.  The service manual had me check continuity on various parts of the circuit as well as voltage being supplied when the ignition was on. 

 

Basically the compressor runs on 2 circuits, bound by a relay.  You have a circuit that has the dual-pressure sensor -> A/C switch -> Fan switch -> ground, and you have a circuit that runs to the compressor -> ground that is only complete when the first circuit completes.

 

The service manual has you systematically check that 1) each circuit has voltage, and 2) that there's continuity between each link.  It was only when I found that there was no continuity between the dual pressure sensor and the A/C switch, is when I broke out the short detector and ran it along the wire to find the breakage.

 

It was a good learning experience since I had never dove into electrical issue diagnosis.  I know its mentioned a bunch in this forum that has been so helpful to search, but NICO club has the service manuals for pathys.  https://www.nicoclub.com/nissan-service-manuals 

 

Between that in searching here I've been able to DIY pretty much everything since I bought my R50.