So the AC compressor on my daughter's 1996 Maxima went out, and we decided to try and replace it. It seems to just have been the clutch that locked up,and the system was operating well prior to that. I installed the new compressor, which came pre-filled with PAG oil, and also installed a new dryer, as well as new o-rings for both. Vacuumed the system down, and it held vacuum well. I added about 1.5 cans of r-134 with the car not running, then let it sit a few minutes, then turned the compressor over by hand about 10 times. Replaced the drive belt & tensioned appropriately, and locked the idler pulley in place. Started the car and turned on the AC, but the compressor would not come on (clutch was spinning, but compressor not engaged). High and low sides were both reading ~110 psi. Checked power to compressor, and it was 0V. Found a blown 7.5A fuse in the fusebox under hood on drivers side. Replaced this, and still no power to compressor. Tried swapping horn and AC compressor relays under the passenger-side hood, and this made no difference (and the horn worked with both relays). Finally, I tested the compressor by removing the AC comp. relay, and jumping across the pins (I think 1 & 2, and 3 & 5, if I recall correctly). This did engage the compressor, but I only did it for a second to test, as I was afraid to jump it for long for fear of damaging it.

The r-134 refill capacity for the Maxima is listed at 1.4 lbs +/- 0.1 lbs, which should be right at two 12-oz cans. Now, I'm not very experienced with AC, but I would have thought that with 1.5 cans added in, that would give enough pressure to run the compressor. And the pressure readings seem to indicate that. I know that low-side pressure reading was high, but that was with the AC off, and I think it normally reads high without the compressor running, right? And I would not think that with only 1.5 cans added in, that it would be over-pressurized? What do you guys think?

I'm thinking maybe it could be the pressure switch on top of the dryer, which of course I had to remove from the old dryer and put on the new dryer. I had no difficulty with this, so I wouldn't necessarily have expected this switch to fail, and it seems very coincidental. And I'm also wondering why the 7.5a fuse blew to begin with. Given what I've described, where do you suggest I begin with the troubleshooting? The pressure switch? What's the best way to test that out, and if turns out it is bad, then I guess I have to start over with the evacuation and recharge of the system?

Any guidance you can give is appreciated, and I can fill in any details I might have left out as needed.

Thx!

 

No thoughts? I was thinking I could test the pressure switch by disconnecting it, and jumpering pin 4 of the plug connecting it to pin 1 of the same plug. If the compressor comes on then, I believe it tells me the pressure switch is bad. This is of course assuming that the pressure in the lines is really sufficient that the pressure switch should be giving continuity between those pins. As I mentioned, I was reading ~110 on both the low and high pressure lines with the compressor off. I'm not sure of the trigger levels on that switch. Since it's called a triple pressure switch, I'm guessing the pressure has to be in a range to enable continuity between pins 1 & 4 (i.e. xxx<pressure<yyy).

I know pin 2 is ground, and pin 4 is the AC on/off from the ECU, and pin 1 is the switch output back to the ECU. Does anyone know what pin 3 is?

I know this test won't definitively tell me the pressure switch is bad without knowing what the line pressures should be (maybe someone can educate me here?), but it will definitely confirm that all else in the system is working well, and the problem is either the pressure switch, or the refrigerant levels.

Anyone have better ideas on how to proceed here?

Thx for any help.

 

Pressure switch is fine. Charge weight is fine.

Jump the compressor at the harness plug with the car running. Leave it jumped and let the cabin cool down.
Report back with:

High-side pressure, engine warm at 2k rpm
Low-side pressure, engine warm at 2k rpm
Lowest dash vent discharge temp
Ambient temp and humidity during test

 

And there are two fuses, one in the under hood fuse block, and one in the fuse block under the steering wheel. Did you check both?

 

Thx for your input. I'll give this a try and let you know. My daughter has the car now, and it'll be next weekend before I can my hands back on it again. Yes, I did check both fuses. The one in the dash fuse box was fine, only the one under the hood was blown.

It's kinda tough to jump at the compressor harness plug, since it's not so accessible. Is it a problem for me to jump at either the relay on the passenger side under the hood, or by jumping across the pressure switch connector plug? The relay is the easiest to remove and jump, and this is what I did to momentarily test the compressor the 1st time. As I recall, I used 2 jumpers, each of which connected across the terminals in perindicular directions.

Thx again!

 

Yep, workin on cars is kinda tough sometimes.

See ya next week.

 

LOL. Sure, I got that- been there, done that, but I'm also not looking to make it harder than it needs to be. Let me rephrase: Is there any reason not to jump at either the relay or the pressure switch? I've already verified that the compressor does indeed cut on when I bypass the relay, so I've eliminated any potential issues between the relay and the compressor. Don't get me wrong- I have no problem in doing what has to be done, but I'd like to understand the logic behind, in case I'm missing something.

Thx again!

 

Here is the work flow according to the FSM. Start on page 18:

https://cardiagn.com/heater-air-cond...FPhOQNeaUk0Y1M

 

Hey, thx a lot for that, very useful! It took me a while to download as initially the link kept thinking I was some kind of bot, but I eventually worked around it. The diagrams gave me a better understanding of the interconnections, as well as things to look for. Since I already tested the compressor and verified the clutch will engage with power, I would start by removing the AC relay on the passenger side under hood. I would then disconnect the pressure switch, and jumper pins 1 & 4, and also 2 & 3 of the connector plugging into the pressure switch. With the AC turned on, this should effectively bypass the pressure switch, and tell the ECU to turn the compressor on. With the relay still removed, I would then test the voltage at Pin 2 of the AC relay, which should be set to ground by the ECU. Once I verified this to be the case, then that should tell me that everything is working as it should so far, and my only potential culprits would be the pressure switch and/or refrigerant levels.

I would then turn the AC off, and shut the car off, and then replace the AC relay, Start the car back up (with gauges connected to low and high side AC ports), turn the AC on, and verify the compressor is indeed engaged and running. If not, then I would need to test the AC relay/connections (although I don't expect this, since it was previously tested). Monitor and take readings on the pressures in the lines, as well as temps at vents, and also feel temps of high and low side lines. How long should I allow the compressor to run in this bypassed state, and what pressures on the low/high side should be cause for alarm and tell me to shut the compressor/car off? Sorry if I'm sounding overly cautious here, I just would hate to damage the compressor after having just gone through all the trouble to replace it.

From the FSM reference you provided, it seems I should be expecting pressures on the low side between 23-33 psi, and on the high side between 242-299 psi. If I should achieve those readings by "hotwiring" the compressor, then all should be well, and I should be able to reconnect the bypassed pressure switch. My AC should then come on and off according to the dash settings. If not, then I can assume the pressure switch is bad, right?

If I'm all wet here or you think this is not a reasonable or efficient approach, then please let me know. Thx again for your help!

 

The only way you will harm the compressor is running it dry, or slamming liquid into it. You don't need to worry about either. I mentioned previously to jump the compressor at the harness plug to eliminate the entire upstream system. The plug is easy to access from underneath the car, but I understand the pita of jacking up the car, removing splash guards, etc.. Supplying 12V to the compressor, however you decide to do it, will run the system indefinitely. Don't worry about harm to the system. All you're doing is manually what the car would be doing automatically. There is no risk of damage.

Don't speculate. You will auto-focus on non-contributing factors. Wait until you are in front of it to begin work flow.

Pressure switches don't fail. Unless you physically damaged the switch, it's fine. They are very simple and robust components. Did you drop it? throw it across the room out of frustration?

Nevermind the pressures in the FSM. They are a basic guide, as are all charts for AC. Use a T&P chart (which includes humidity if you live in a humid environment -this is VERY important, as an AC system's job initially is to work to remove moisture from the air before actually reducing the temp-) and compare with your ambient conditions. The fundamental principles of AC remain the same whether residential, commercial, or automotive. The engineers knew what they were doing when they designed the system. Automotive AC systems are heavily over-engineered (hence how they can drop a cabin from 110 to 55 in a matter of minutes, after doors open and close, etc.), and highly dynamic compared to the more exact science of residential design, the loads are completely different.

I'm not sure of 4th gen system because I haven't really cared to research it, but my 5th gen system only has a high pressure switch for safety cut-off, and no low pressure switch. I have researched this in my 5th gen 7th gen swap endeavour and have not concluded as to whether or not the low side switch is built into the compressor, as I'm using a 7th gen compressor with my swap motor in my 5th gen car, nor do I really care, because I constantly monitor vent temps for indication of evap coil freezing. Remember, if your coil is freezing, flashing occurs late, and liquid advances towards the compressor via the suction line.

However unlikely, with the information you have provided, I have a feeling you have a console switch issue, but until you're in front of the car and have the opportunity to have a back-and-forth with the car present, I would rather not assume anything.

 

Thanks for the guidance, and happy to hear no worries of compressor damage! I'll dig into it this weekend, and report back what I find.

 

My daughter had the car home this weekend, and I had some time, so I did a little investigating this weekend. Hooked up the pressure gauges, and read ~60-65 psi on both high and low side at initial rest. Started the car, and “hotwired” the compressor by jumping the AC relay on passenger side pins 3 & 5. After running the compressor, the low side dropped towards 30, and high side went towards 350 psi! I I disconnected the jumper at that point, and noted that neither low or high side lines had noticeably cold or hot temps. I also noticed that one of the radiator fans (passenger side) was not running. I did a little quick troubleshooting on this. Diagram below (pin 4=high speed ground, pin 3=low speed ground, pin 1=high speed 12V, pin 2=low speed 12V). I tested driver side fan, and with either pin 3 or pin 4 grounded, and pin 1 or 2 to 12V, fan came on low speed. With pin 1 & 2 to 12V together, fan came on high speed. For the passenger side fan, I got the same thing, but only with pin 4 grounded. Nothing happened with pin 3 grounded. When the passenger fan did come on, it was noticeably noisy as well. Concluded this fan was bad- one more thing to do!

3---4
1---2
-[n]- <- plug lock position

After this, I reconnected the fans back up, and decided to test again the next day. Cranked the car up with AC off, and read temp in vents with fan blowing to be 70-71 degrees. AC pressure was again at ~65 psi for both high and low sides. Ambient temperature was 66 degrees, with humidity at 88% (according to weather.com). I jumped across pins 3 & 5 of the relay socket to turn the compressor on. At 2k rpm, the vent temperature dropped to 45-48 degrees, with the low-side line being noticeably cool, and the high-side line being noticeably warm. Great, it seems to be cooling! However, the low-side pressure remained around 60, while the high-side was moving up close to 200. I disconnected the jumper at this point and turned the car off. I replaced the relay, and then started the car again to see if the compressor would turn on with the front panel switch. It did not. ☹ I did note that the one working fan did go on and off with the AC switch, so the switch itself seems to be working.

Low-side pressure seems high, although it seemed to cool well enough when engaged. I only added ~1.5 cans of freon in, so I should not be overcharged. I'm stumped. What do you suggest to try next?

 

Are you getting voltage across your AC fuses with the system on? I just went out and checked mine. I have 2.2mV on dash fuse and 33.1mV under hood fuse with the system running.

 

Hello, should I be checking this with the fuses removed? And if I'm not mistaken, these are both 7.5a fuses, right?

 

Leave them in.

I don't know what yours are. Mine are 10 amp.

 

I went ahead and ordered a new fan assembly, since this was an obvious problem, and can impact the AC system performance. Once I get this in and replaced, I'll resume with the testing.

 

Cool.

 

Ok, so I got the new fan assembly in and installed, and resumed testing. Started the car and AC, and noted that both fans came on immediately- that's good! Jumpered the AC relay to start compressor, and let it run for a bit. The outside temp was 54 degrees, and humidity 96%. The low side pressure reading varied between, 15-30 psi, and the high side between 160-250. When the pressures got near the high end of the range, the fan would cut on high speed, and the pressures would drop to the low side of the range. This was encouraging, as before the high side pressures were much higher, and showing no signs of abating before I shut the compressor down. The low pressure line was cool to the touch, and the high side line warm, but the vent temperature was only about 52 degrees. My daughter had the fan was on high speed, and if I'm not mistaken, this really have been on the lowest fan speed. Still, obviously not cooling well.

Knowing I had only added 1.5 cans in previously (not to mention bleeding some off after being worried pressures were too high), I decided to go ahead and add the remaining 1/2 can. I did so, and then tested again. This time the low side pressures read anywhere from 15-23, and the high side pressures anywhere from 110-160. The vent temp did drop down to 45, again with the fan on high speed. Still not cooling as well as it should, and given the pressure readings, I would say I still need to add some more freon.

Do you agree, or are there other places I should also be looking?

 

54 degrees ambient is too cold to be doing any testing. Throw some refrigerant in it till the vents get cold and call it a day.

96% humidity? Wow.

 

LOL, welcome to Atlanta. It actually rained all day today, but that humidity might not be that uncommon in the summer even on sunny days. Tomorrow will be sunny and warmer, I think with a high in the mid-70s, so I'll recheck again then. I forgot to test the voltages that you suggested, will try to do that tomorrow. Note that so far I have only been able to get the compressor running by jumping the relay, but I didn't try it without jumping after I added the remaining 1/2 can. I'll also try replacing the relay and seeing if the compressor responds with the AC once it's more fully charged.

 

Correct, we have made zero progress toward a solution to your original problem.

 

Was able to read the voltages at the fuses-- 3.1 mV for the dash AC fuse (7.5a fuse), and 0 mv for the driver side under the hood fuse by the battery (also 7.5a fuse)

The compressor still would not come on with the AC switch, so I jumpered it and added some more freon while monitoring the gauges. I ended up adding ~ 1/2 can freon. The ambient temperature was ~73 degrees, with humidity at 80%. With the car at 2k rpm and the vent fan on low, the vent temp was ~48 degrees. The low side pressure read from 25-27 psi, while the high side read 205-210 psi. As I recall, just prior to adding the freon, the vent temp was in the low 40's, so it almost seemed to cool better before, even if the low side pressure seemed too low.

After adding the freon and doing this testing, I tried re-installing the relay to see if the compressor would come on, but no go. I'm wondering what the cause was that blew that 7.5a fuse under the hood initially, before I replaced it, and if that's somehow related to my issue with the compressor not coming on now?

 

You should turn on the a/c at the control console and measure the voltage at the pigtail for the compressor clutch! See if you're getting 12vdc there...if not perform a continuity test on that wire back to the relay. Maybe the relay connectors/wires are damaged or the clutch wire is damaged!

 

If I'm able to jump at the relay and get the compressor to come on, wouldn't that eliminate these possibilities? Note that I had also previously swapped this relay with a horn relay, so a bad relay also seems unlikely.

 

Anyone agree or disagree? Seems to me the problem has to be upstream of the relay. Any thoughts on the pressures/temps- can we rule that out as a suspect? If so, then I can focus for now on tracking down the (electrical) reason for the compressor not coming on, and stop fiddling with the refrigerant charging, at least for now.

 

Pressures are fine. Not an issue right now. CMax has it right, time to test the harness.

 

Thx for weighing in. I'd just like to understand the thought process. If I can get the compressor to come on by jumping at the relay, how could anything between there and the compressor be faulty? I must be misunderstanding something in CMax's suggestion.

 

I just re-read the first paragraph of my opening post- Didn't I effectively already test for what was suggested?

 

No, you did not, unless you tested the entire circuit from console to compressor harness connector. There are a lot of components inbetween. Everything you need to know including testing the high-side switch you're so worried about is listed in a step-by-step procedure in the FSM in the link I sent you and it's pretty easy if you're handy with a multimeter. If you're not, then this probably isn't the job for you. This is a couple hour diagnostic at the most, including time to gain access to the ECU and amp unit (if you're so equipped). I'm not really sure what we are still going on about. I am referencing my own FSM for my '03 and you should be able to knock out the work flow pretty quick, but you have to read it. There really isn't much to it but you need to follow the procedure. Unless you can rattle off all the circuit component checks you've already preformed, which you haven't, so you can't, I'm not sure where to start other than re-stating and re-typing the FSM, which I'm too lazy to do.

You are at "B", and you've already checked clutch function. Not much left to do.

 

And the below is why you are unsure about relay function, despite swapping them out. There's more to it than the component itself.

 

Thanks again for the information, it is appreciated. But to be clear, what you state above was not exactly what was suggested by CMax, which is why I asked the question I did. Specifically, here is what CMax stated:

"You should turn on the a/c at the control console and measure the voltage at the pigtail for the compressor clutch! See if you're getting 12vdc there...if not perform a continuity test on that wire back to the relay. Maybe the relay connectors/wires are damaged or the clutch wire is damaged!"

You seemed to agree and confirm that I should focus on this, which puzzled me, and is why I asked for clarification since I believed I had already tested this. I still believe that to be the case. I never stated there couldn't be a problem in the circuit from the control panel to the compressor. Specifically, I stated that I could not see where there could be a problem between the relay and the compressor, which unless I misinterpreted, is what Cmax suggested. Despite what's implied, I'm not too lazy to go through the FSM step-by-step process myself (in fact, I stated that if the pressures seemed fine, my next step would be to focus on tracking down the electrical issue), and I will, but I also thought the purpose of these forums was to discuss & benefit from the experience of others. I will admit to being unsure about what I'd done so far as I'd never replaced an AC compressor before, and I was looking to get confirmation that I hadn't done/missed something obviously stupid in the process. Since the AC worked fine just before the compressor locked up, I thought the likelihood of this being a consequence of my error was high, and I wanted to eliminate the obvious before launching into full-scale investigation. That's where I thought others here could help.

Sorry for boring you with this long-winded explanation, as it's obvious you've already lost patience, but I did want to explain where I was coming from.

 

I understand all that was said and meant. I concede that I misspoke/should have stated differently to check the harness as a whole and that I agreed with CMax not specifically with his suggestion but rather his approach. Sorry about that. I'm not very good at troubleshooting over the phone. AC is a tough system and I'm not sure in the handful of threads that I have read through while doing my own research for this car during my engine swap that I saw any that were resolved that were anything more difficult than replacing a blown fuse or something similarly simple or obvious. I have a bit of a history with AC systems (EPA Universal certified, experience in the service industry with commercial, residential etc.) but I'm a horrible teacher. When there's a problem, my brain just takes over and I start working it. I don't really think about it until I'm at a complete loss, but all systems function on the same basic principles, so it's usually only the nuances of different systems that may not be common or familiar that I get stuck on.

So forget everything I said supra, and forget about continuity from the compressor harness connector to the IPDM (not sure why we even went there in the first place) and lets start with the basics here:

Hook your multimeter up to the compressor harness connector with a long enough lead to be able to read it while inside the car and jacking around with the console switch (rationale for this is old car, maybe it works intermittently, probably a waste of time but it's an easy peasy test and I don't know anything about the history of the car. If you're certain it is not the console switch, disregard this step. Also, this does not rule out the switch, it just reduces the probability to me that it is indeed the switch). If no go, bypass the pressure switch. You have verified that fuses are good, relays are good, clutch is good. You can't really f*ck up and short anything out at the harness connector, and you mentioned a possibly faulty pressure switch.

With the info you've given thus far, the above would be everything I address before referencing a manual (wiring diagram).

All you did was replace the compressor, the drier, and a blown fuse. It's always Occam's razor when things like this happen after a repair. It's always something you touched/screwed up/missed (by you, I mean anyone who makes a repair) and I don't even want to look at a wiring diagram until the blatantly obvious is addressed.

 

And after all that, we need to know why that fuse popped. Something downstream shorted. Time to break out the wiring diagram.

 

Ok guys, sorry for leaving this fallow for so long. The weather got cold and then when it warmed up my daughter wasn't home for me to test the car further, and then I got busy... But finally, I resumed troubleshooting. I read the FSM and tried to understand how things worked as best I could. First, I checked the fuses in the driver side panel and under the hood fuse box (I'm assuming these are fuses 16 & 61 that are referred to in the FSM?). Both were 7.5a fuses, and both were fine. Next, I checked the voltage at pin 2 of the AC relay under hood, which I believe comes from the ECU, and should toggle with the AC console switch. This read ~14.2V constant, and never toggled with the switch. Next, I read the voltage at pin 4 of pressure switch (which I believe should come as an input from the AC console switch?). This read ~0V regardless of toggling the AC console switch. Assuming the pressures were fine and the console switch were working, this signal should be output on pin 1 of pressure switch, to be routed to ECU pin 21 input. The ECU should then ground pin 12 which goes out to the AC relay to turn on compressor. Obviously, i'm not even getting to that point, as the input to the pressure switch is not good. It seems like either the AC console switch is bad, or some wiring inbetween there and the pressure switch is bad. Does the AC console switch output go directly to the pressure switch, or is there something else inbetween? I'll try to look at the FSM a little more closely as I did do hurriedly, but I wanted to go ahead and get this out there to see if someone might have insight or suggestions to offer up.

Oh, I will also just mention that with the compressor hotwired, I ran the self-diagnostics in the FSM, which checks the sensors, mode actuators, etc… All checked out fine there. Also, the system seemed to be cooling well. Here are some readings I took:

Outside temp 84 degrees, humidity=71%, pressure=30.01.

After hotwiring AC compressor, I took readings with the car at idle, fan on low, and no recirculation Low side=25-30, high side=175-200, vent temp=42-47 degrees. Low pressure line was nice and cold & sweating, but not freezing up. High side line was warm/hot to touch. I read later in the FSM that I should have had the car at 1500 rpm, fan on high, and recirculation on. Nonetheless, I feel the charge is ok and not the main problem in any case, if at all.

Any suggestions on the quickest way to track this problem down further? Do I need to disassemble the dash to check the control switch?

 

Oh NOW you want help

 

So I'm not quite sure what I did to **** you off, since I've tried to be respectful from the beginning of this thread here, but you seem hell-bent on jumping down my throat and finding fault in my approach, whether justified or not. I thought the forum was supposed to be about helping each other, not about badgering, which certainly won't improve participation. Now I could understand if I was being an ***, or if I was just being lazy and not making any attempt to help myself. I personally think I've done a whole lot more and given a lot more information than the average person posting questions on such forums. I'm not claiming to be an expert by any means, but I'd venture to say I've done more research than most, and my questions and troubleshooting have gone far beyond most noobs. Maybe you're just an unhappy person, arrogant, or both?

I gave an explanation as to why I left off my previous troubleshooting attempts. Sorry if that didn't satisfy you. You yourself said it was too cold to be doing any testing in your prior message here:

Thanks, and have a nice day.

 

Did you bypass the pressure switch yet?

 

Yes, I had bypassed the pressure switch, but the problem is that the input to the pressure switch (pin 4, I believe) is never being enabled. It remained at ~0V regardless of A/C on or off. Unless I'm mistaken, this input comes from pin 5 of the A/C Auto Amp. I'm not sure exactly where that is located (in the A/C control panel?), or how to test that in relation to the A/C on/off, but it seems pretty clear to me that the problem is somewhere between the A/C on/off controls and the input wire to the pressure switch. Reflecting back on my earlier posts, I can see where the fans were coming on/off with the A/C switch, so it would seem safe to conclude that the A/C switch itself is good. I note in the FSM troubleshooting it says to test the ECM pin 21 for 8-9 V. This pin should be fed by the output of the triple-pressure switch pin 1, which assuming the switch is just a pass-through switch when proper pressure is achieved, should be the same voltage as pin 4 on the input of the triple-pressure switch. So I'm guessing something in the A/C Auto Amp should put out that 8-9V on pin 5 when the A/C is on, but is not doing that for me.

 

Maybe a picture would help? From page HA-94 of the Heating & Air FSM...

 

Fred I'll look at the schematic and try to assist you most relay have 4 wires
1. The power that being sent from the battery via the fuse (large wire) 12 volts here at all times
2. The Load in this case the compressor clutch (large wire) continuity to the clutch wire.
3. The input solenoid wire (small wire) is the trigger or signal wire being used to close the relay solenoid ( this side is from the a/c switch or a/c ecu)
4. The ground wire that maybe controlled by the a/c ecu or is a permanent ground that allows a path for the solenoid circuit to be completed and the solenoid actuates completing the load circuits path feed power to the compressor clutch.
So a relay is really 2 circuits the load and the trigger. This allow small (amp) switches to be used to control larger (amp) components such as lights, motors, clutches, etc....