# 90 sentra AAC valve question



## ahardb0dy (Nov 9, 2003)

Ok, long story but need some Nissan Tech help.

This all started when I went to flush my cooling system,

First the radiator petcock broke in the radiator, couldn't get the broken part out so I pulled the rad and of course it came right out in about 2 seconds.

Next no one stocked the petcock so I had to order one from rockauto.

Got the part and refilled the cooling system for another day.

Next drained the radiator, engine block and refilled and repeated a few times. last time I used radiator flush, let it run, drained and refilled a few times.


One time after refilling I started the engine and the idle was very high and wouldn't drop, and the electric fan never came on. verified fan worked by turning AC on and both fans did work, also un-plugged the thermo switch that is installed in the thermostat cover and jumped the two pins in the connector and the fan also worked.

I checked the codes on the computer expecting the thermo switch to show up and I got the code for the AAC valve (25).

I also noticed a buzzing noise from under the hood when the key was on and I traced the noise to the AAC valve.

unplugged the connector from the AAC valve and the noise stopped.

Removed the AAC valve and checked it per the FSM, ohms test passed, than checked the plug harness side for battery voltage and was getting 1 volt less than what I got at the battery.

I connected the AAC valve to my power supply and it worked fine, the little plunger would pull in when power was applied and pop out when power was removed from the pin. Thought maybe power supply having higher voltage was why the AAC valve was working so lowered the voltage down to 8 volts with amperage turned all the way down and AAC valve still worked fine. (my power supply has variable amperage and voltage)

reconnected the AAC valve to the harness under the hood and it would only buzz, plunger would not move. Turns out after troubleshooting the ground in the plug was not good. Added a ground to the firewall and tapped into the ground wire in the AAC valve harness and now plunger moves with no buzzing.

Re-installed the AAC valve and continued with coolant flush, tested thermo switch and found it was bad, had to order new one from rockauto so figured I would replace the thermostat at same time since I have never replaced it.

Replaced thermostat and thermo switch yesterday and let RTV dry overnight. today filled with water to check for leaks, no leaks, ran engine and idle very high, un plugged AAC valve connector and idle drops, let engine run until it got hot and fans came on like they are supposed too, thermostat opened as well.

So, after all this, have new thermostat, new thermo switch, new antifreeze (was running straight water before) and now I disconnected the new ground wire I made for the AAC valve.

So is the AAC valve designed to raise the idle when the engine is cold? and I guess once the engine warms up the little plunger would retract? What causes the AAC valve to move one way or the other? 

I guess next step would be to finish going through the FSM section to troubleshoot this.

Anything else I should check? thanks


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## dreamteam (Jul 29, 2005)

Ooh-boy, Not once did you bleed the cooling system?
with air in the head, the temp sensor cannot work correctly and causes
a high idle.


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## ahardb0dy (Nov 9, 2003)

No, I bled it every time I refilled it. Today I followed the troubleshooting steps from beginning to end for the AAC valve, the FSM is kind of vague as, for example in step "A" it says to unplug the connector on the AAC valve and check the resistance, it says it should be aprox. 10 ohms, but I was getting 11.7 I think, so does that mean it is bad? The other day when I checked it the reading was 10.7.

than in step "D" it says to re-connect the AAC valve and start the engine and warm it up sufficiently, than check voltage between ECU terminal 114 and ground, says should be 4-8 volts at idle speed but I was getting 10.54 I believe. Manual doesn't tell you if voltage is high to check this, or if it is low to check this ???

Anyway went thru the whole procedure, reset the codes, took it for a ride, engine seemed to run fine, got back home and re-checked codes and no code 25. So maybe it's ok now, will have to check the codes again tomorrow when engine is cold see if that effects it.

Was going to pull the AAC valve out of my pathfinder and swap it into the Sentra to see if it made a difference but didn't want to F--- up 2 vehicles, LOL next time I hit the junkyard I'll have to pick a few AAC valves up. Looked on rockauto and they don't list the AAC valve as being available separately. They only list the whole assembly and for $132 more than I would like to spend !!

this is what rockauto shows:


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## dreamteam (Jul 29, 2005)

*The spring thing*

whenever I work on the AAC valve, I seem to lose the spring.
Eventually I find it, so make sure it's in there.
Also, you need to clean the piston with some choke cleaner
to get rid of the carbon.
That's generally why they fail.
Now, I've noticed a clicking noise whenever the idle changes.
I always assumed that there was a relay somewhere and the voltage 
from the ECU was controlling it.


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## ahardb0dy (Nov 9, 2003)

A while back I removed the whole assembly that I posted the pic of and cleaned everything, took the 3 valves off of the metal base part and cleaned everything really good.

First thing I did today when I started to check the AAC valve was to remove it to make sure I had the plunger part in correctly as a drawing I saw in the FSM looked like it was in differently than I remembered I had mine installed. But after removing it I saw there was only one way the plunger could go in, one of the other valves the plunger part can go in backwards but not the AAC valve.

I did drop the spring when I first removed it but I found it and it is in there. Did some searching online and it seems you just can't buy the solenoid ( I guess that's basically what it is) separately. I'll have to get one from the junk yard next time I go to see if it makes any difference. I haven't been driving the car much lately since the AC is not working and it's been hot down here. 

Tomorrow going to start the car and see how it runs and than re-check to see if there are any codes.


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## bob89sentra (Sep 15, 2005)

AAC valve is used to controll idle speed. Thought it should not make the idle speed fast. 11 OHMs is a good reading. The solenoid has constant voltage to it ~12VDC and the computer grounds it to turn it on. The ground you added will short it on all the time. The likely thing that happened is your wax pellet fast idle cam is stuck or the port is plugged not letting coolant warm it up giving you a fast idle. There are 3 idle controll solenoids and one fast idle cam. The AAC and idle up solenoids help the engine at idle when under loads like power steering or AC. The FICD solenoid helps give extra fuel when the engine is cold and works along with the fast idle cam to help warm the engine up. Like stated before use choke cleaner to clean the parts on the solenoids. The computer will throw the same code for either of the 3 solenoids BTW so the AAC or the Idle up or the FICD could be the culprit too. Honestly you could leave them all unplugged and it would still run fine, except when AC loads or power steering loads lug the engine down at idle and could possibly cause it to stall. I suggest getting some spares from the bone yard and clean the set up in your car and verify the fast idle cam is adjusted and working properly.


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## ahardb0dy (Nov 9, 2003)

I disconnected the extra ground I had installed. The thermo element does work, I always thought when the coolant going thru it was cold the wax expanded pushing the idle up and as the coolant got hot it caused the wax to melt lowering the idle. But recently I noticed that as the engine gets warm the thermo element rod actually pushes out, so I know that is working. I don't have power steering and my AC is not working.

I did recheck the codes and am only getting the all clear code now. Idle seems fine now, I had a leak on one of the hoses I didn't even touch so I had to order a new one from the dealer (online) so I ordered the one that goes from the thermostat housing to the metal pipe on the back side of the engine, what a pain in the butt to get to the drivers side on both of these hoses!! Finally after about 4 hours got both old hoses off and the new ones on, but couldn't start the engine to check for leaks because I broke the tubing for my oil pressure gauge off at the fitting that screws into the block, and I didn't have another 1/8" ferrule so had to get one the next day. Got the oil pressure gauge line back on with no leaks but I'm getting a leak from one of the hoses behind the engine, under the intake manifold and I can't see whaere it is leaking from, so may have to jack the car up and look from the bottom, or I may try using a mirror to see what is going on. Going to do this on the week end. Thanks for the reply.


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