[RESOLVED] Idle Is Low When Ac Is On ( Solution)

[razey]

Hey guys,

I am having some trouble with my AC, i am not to sure what the problem is so any help is greatly appreciated. Basically, when i turn my AC on and put the car in drive, my RPM's drop to about 625 and slowly rise up to 700ish RPM's. Some times when i put it into drive the the AC on it stays at around 625 RPM's. It seems to be intermittent mabey...
Also i have a good size boost leak in the car, i dont think this will cause my AC problems, but i am fixing it this weekend.
Also my car was shipped to Puerto Rico about 2 months back, i ended up having to replace the Battery that died on the way over here. Could something be jacked up with my alternator or AC Compressor?

Thanks,
Steve

 

[Locke]

Clean the ISC, and check the operation of the ac pressure switch. If these are good, check the compressor bearings.

 

[RayBan GST]

well, i have been around this problem for weeks now and i searched a lot and found no accurate answer for this. the car idles just perfect without AC , once the AC kicks in the idle is really low around 0.200 rpm and the car will almost die.

i tried everyhting i cleaned my throttle, i changed the idle control motor and my BOV is recurculated but nothing seemed to fix the problem , i changed plugs with proper gapping, changed cables, checked for boost leaks but nothing helped in this problem ...... at alll i am really very upset that i really can not figure it out

i checked my battery and my alternator and the readings were ok so no problem in them.

what is left .........????

thanks

 

[rush2252]

Do some minor adjustment on the SAS or BISS its the screw in the throttle body on the front side adjust it out to increase RPM in for decrease. I would assume the Idle Air motor is probably not functioning correctly or somebody has screwed the BISS all the way down trying to get the car to idle down at one point. At any rate unscrewing it should bump the idle up where you need it 850 w no accessories

 

[RayBan GST]

i changed the motor and my BISS screw is fine and adjusted , i am idleing fine without AC and there is no problem at all.. why only when the ac is on my rpm will drop. i tried to play with the screw and i managed to get the idle to 1000 RPM wiht the AC on but once i switch off the AC my idle will shoot to 2.2 K RPM ( there is something wrong and i really want to know) its not the screw its not the motor ? i checked almost everything what is left???

 

[rush2252]

Ahhh I almost mentioned this earlier but figured id let you try that first as to rule out the BISS. The only other thing I can think of is either overcharged A/C system or your expansion valve within the AC is plugged. This would cause excessive resistence within the compressor and put a huge load on the motor. Make sure your not over charged and also listen when the AC engages.. My expansion valve needs to be changed and it makes this funky noise like blowing through a straw. You can hear it when it comes on.

If the AC is overcharged or the compressor is tryin to go out this could cause the issue

 

[Killa_dsm]

Have you checked to see if your ISC is functioning properly?

 

[RayBan GST]

what do you mean by that : please elaborate

overcharged A/C system or your expansion valve within the AC is plugged. This would cause excessive load ?????


please explain what exactly you mean and where is this valve located ????

thanks

 

[reno23]

so far you have checked all the same area's i checked when it happened to my car, but the last thing i checked on mine was the Throttle Position Sensor (TPS). I removed it, cleaned it, and repositioned it better. Problem with Low idle with A/C gone. But you should also make sure your system is not over pressurized, like noted before.

http://members.shaw.ca/dsm.1000q/Engineprimer/2G/throttlebody.htm
-TPS is the Orange Dot-

 

[steve]

Have you checked to see if your ISC is functioning properly?

+1

The ISC is what the ECU uses to maintain the idle speed when the AC kicks in or the power steering pump drag the idle speed down. You also need the IPS inside the 2G TPS to be correctly set so that the ECU knows that it's idling.

 

[RayBan GST]

thanks steve but i really need to know why only when the AC is on ? my car idles and drives perfectly fine without AC ?


i will check what you have suggested today and will report back

cheers

 

[steve]

thanks steve but i really need to know why only when the AC is on ? my car idles and drives perfectly fine without AC ?

Your going to hit yourself in a minute for even asking.

What happens when you drive up an incline, you either have to open the throttle more or the rpms will drop. The AC compressor has to do work to compress the refrigerant so when it kicks in the load on the engine goes up just like when trying to climb an hill and the ECU needs to open the throttle to keep the engine speed up while the engine is doing work. It uses the ISC to allow more air into the engine rather than moving the throttle plate.

 

[RayBan GST]

hi steve,

i checked all the things that you suggested and i even adjusted the TPS as per the manual , now the car idles at 1100 K without AC and .600K with the ac on , however now the car is not dying on me while i am driving


there is still a huge drop in the RPM ,but at least now the car will not stall and Die


i will try to reset my ECU what do you think ?


100000000000 thanks

 

[steve]

Sure you can reset your ECU but I don't expect that to do anything.

Without knowing how you checked and what you found I really can't provide much more assistance.

This is a classic test of the ISC functioning. The ECU looks at the IPS and AC switch and steps the ISC out to maintain the idle speed under load then back in when the AC controller disengages the AC clutch.

 

[RayBan GST]

ok i took off the TPS , and i checked it with the meter and followed the instruction i was reading 05 at closed throtlle ( not exactly 0.5 but very near)

did i mention that there is one screw in the TPS made me hate my LIFE it took me more than one hour to remove this damn screw on the TPS

after i did that that car is not dying now and the lowest it can read with the AC on is 0600 RPM


so i believe that i need to redo it again and have it 100% fixed , i am sure that i did something wrong


i will report back tomorrow

thanks

 

[rush2252]

steve what are you calling ISC? the Motor he already changed. I think its the A/C if its overcharged it can try to lock up a compressor or caused excessive drag. Check and make sure your not overcharged. you can get a cheap ass gauge from Oreillys make sure its in the green on the low side and not the HIGH. If the expansion valve isnt equalizing it can cause this as well.

 

[Killa_dsm]

steve what are you calling ISC?

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[rush2252]

okay well we were referring to it as the IAC idle air control.. not ISC.. but he said he changed that already Just making sure we were on the same page. What is the S for in that acronym never heard it. Like NA is naturally aspirated and people say NT for non turbo on this site..

 

[rush2252]

what do you mean by that : please elaborate

overcharged A/C system or your expansion valve within the AC is plugged. This would cause excessive load ?????


please explain what exactly you mean and where is this valve located ????

thanks

if the a/c is overcharged or the expansion valve isnt equalizing it can cause excessive resistence of the compressor wheel and excessive drag on the motor. Ive seen it quite a bit

 

[Killa_dsm]

ISC = Idle Speed Control

I have the same problem with my idle dropping when the AC is turned on and it was the ISC. My way around this is to not use my AC and only turn it on while the car is moving, then turn it off when I come to a stop.

 

[steve]

okay well we were referring to it as the IAC idle air control.. not ISC.. but he said he changed that already Just making sure we were on the same page. What is the S for in that acronym never heard it. Like NA is naturally aspirated and people say NT for non turbo on this site..

It's called a ISC (idle speed control) motor starting in the original 1G technical manual. They (Mitsubishi/Chrysler) started calling the same part an IAC in the 2G manuals. Either name is correct.

There is more to the ISC system than just the stepper. If any of the coils in it short there is a high chance they will blow the driver in the ECU used to step the pintle.

 

[RayBan GST]

yes ISC is changed but the mechanic that did the job did not know anything about tunning so i believe i need to tune the TPS again from inside as well



still i dont get it how come the AC system will be over pressure ( if the system is over pressure for sure it will blow a pipe or will cause a leak or maybe damage the vaporateur or the compressor


i will report back again tomorrow after tunning my TPS and again i changed the ISC motor because the stupid mechanic immediately start working on removing it without even checking if it was working or not, the replacment that i got was from 97 GST JDM scrap ( used ISC) i paid 10 USD for it


Thanks

 

[rush2252]

they run like 100normally so thats a steal even if its used. And when you hook your scan tool up make sure the TPS is at 0% at curb idle. I would still double check the A/C system

 

[steve]

ok i took off the TPS , and i checked it with the meter and followed the instruction i was reading 05 at closed throtlle (not exactly 0.5 but very near)

On a 2G, the TPS adjustment is not by voltage. It's by putting a feeler gauge between the throttle stop and the pulley to open the throttle a controlled amount and then adjusting the TPS based in the IPS inside it switching. I've posted the details on the site in the past for both normal and 6 bolt swaps using the various types of throttle bodys and TPS's.

yes ISC is changed but the mechanic that did the job did not know anything about tunning so i believe i need to tune the TPS again from inside as well. i changed the ISC motor because the stupid mechanic immediately start working on removing it without even checking if it was working or not, the replacement that i got was from 97 GST JDM scrap (used ISC)

The coils in both the old and new ISC/IAC need to be checked. There are two rows of three pins. The center pin in each row is common. You measure the resistance between the common center pin and the two pins on either side in the row. The old style metal cased model should have about 28 ohms resistance on each coil and the newer black plastic model about 40 ohms.

The O-ring between the ISC/IAC and the throttle body needs to be replaces when you change the motor so you don't have leaks. Once you change it the BISS will need to be set. On a 2G that requires a MUT, DSMLink, or some creative thinking.

 

[RayBan GST]

oh my god steve that mechanic screwed up big time then ....


the BISS screw was not set propelry i am 100% sure and these tunning you talked about i am sure my mechanic never heard of it

and also i have leak now as the car is not doing 14 psi anymore its doing now 12 psi


oh my god why i gave it to a mechanic ??? i should have done it myslef

i am off tomorrow , i will do it myself and will follow all the instructions here on the site and i am confident that i will not screw up ( i have all the tools and i have as well the old ISC) i am going check everything tomorrow

Thanks