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Thread: Added A/C now ready to set truck on FIRE!!! Please Help

  1. #1

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    Added A/C now ready to set truck on FIRE!!! Please Help

    Ok to start off with it is a 94 MM that I just purchased with a 2.4 EFI M/T that did not come with factory A/C obviously and I am going crazy trying to fix this thing. I got all the parts from a donor truck that was near the same year, same motor and tranny. I also have a 91 MM that came with factory air (show truck) that I used as my go to so to say for help with the install. Ok now the problem is IDLE and gas mileage but I will get to that shortly. The only thing the donor truck did not come with is the coolant temperature sensor on the side of the thermo housing that the a/c harness plugs into. I did not realize this till I got everything installed and charged and the a/c would not ingage. Checked the show truck, noticed sensor, robbed sensor, installed and a/c worked like a champ but the idle went crazy as soon as I plugged the sensor in. Not long after the CEL came on. Checked code and it was the engine coolant temperature sensor. Replaced the one on top of the thermo housing and the one on the side that the a/c harness is hooked to and still no dice. I have either replaced or swapped for a known good part the... IAC, motor position sensor, idle control servo, EGR, PCV valve, both temp sensors, TPS sensor (also checked its setting and its voltage output to spec in manual and its in range), ECU from the show truck and the donor truck, MAF, a/c controller under dash, checked idle set screw. The CEL is now off but still idling like butt due to replacement of the temp sensor. When I purchased the truck it averaged 23 mpg or so and that has went to around 18 or worse.

    If I unplug the harness (the 5 wire one) on the throttle body that would be the IAC, it idles fine. If I unplug the temp sensor that is connected to the a/c it idles fine but no a/c. The idle is all over the place, just steady up and down and pretty high not sure of RPM but I would say a little above where it should be when the a/c is engaged judging from my show truck.

    The things I haven't gotten to are the O2 sensor and the cat converter. I also noticed a small crack where the pcv valve screws in, not sure but dont think thats my idle problem. Also checked all vaccum for leaks and also the head and throttle.

    This has got to be electrical somehow I think but I don't know. ANY help at all would be greatly appreciated. Thank you in advance!!!!

  2. #2

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    I just got all my parts to add a/c to my 94 also. I was wondering about the sensor too. It's possible the ECM doesn't recognize the extra coolant sensor. I noticed all the ECM does is detect a signal from the A/C control unit (on box) and use that to energize the relay. It may be doing the required software issue for the ECM by energizing the relay based on a signal from the a/c unit but it may not know how to adjust the idle with the a/c compressor engaged since it has never had a compressor. But then again you said you've swapped ECU's. Swap wiring harnesses next. There's not much to getting them out. I've seen a mitsu do this before. It was a 2.4 Eclipse but it was the IAC valve. The end fell off. But you swapped yours.. I wish I had mine swapped in already to tell you if it's something up with the swap or if it's an actual issue with your truck. I'll get my new dryer soon so hopefully I'll install it soon. Install takes no time to put it all in the truck. Couple hours.

  3. #3

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    Adding the a/c was a breeze and if you need any help please let me know. I read your post the other day about adding your a/c and I started to chime in and give you some pointers but I figured I would wait. As far as mine goes....... I'm still lost. The electronic portion of the idle is doing its job "technically".. it is idling the truck up and down but not in the correct fashion. If you have the a/c temp sensor unplugged (the one at the front of the thermo housing on the side) it idles fine but the a/c will not engage and function. In order for the a/c to work and get a regular idle I have to unplug the 5-wire harness on the IAC. I did replace the valve cover b/c there was a crack near the pcv valve port and I'm in the middle of replacing O2 sensor which is a pain. On removal of the old O2 the threads on both the manifold and the sensor itself decided to perform a vanishing act so now I have to re-tap the manifold and put a new sensor in. FUN FUN. BTW if anyone knows the exact tap size for that would be really helpful. As far as the harness goes there is not much there. Plugs in at drivers side fender, then you have a fuse, then a relay, then goes to clutch on a/c and back of compressor, temp sensor, around grill to low pressure switch, then plugs inside truck and goes to a/c unit under dash and a/c controller and a/c switch and a thermistor. All wiring is under dash already except a/c switch and the unit itself. Anywho, hopefully get O2 in soon and let you know how its going. Keep me updated on yours.

  4. #4

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    I don't know the thread tap on the O2 but there's a special spring loaded tap socket most manufacturers use. Nissan had one. U know you can buy new manifolds from mitsu. I swear dude your truck is acting like it's got a bad IAC. Like there's a dead spot at the exact spot your truck needs to idle up. You sure the one you swapped wasn't bad?

  5. #5

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    book says to diagnose in this order:

    Cold:
    1 IAC
    2 Injectors
    3 IAC valve pos. sensor
    4 Closed throttle switch
    5 Fuel PSI
    6 ECT

    WARM truck:
    1 IAC
    2 Injectors
    3 IAC pos. sensor
    4 Closed throttle position switch
    5 Fuel PSI
    6 IAT
    7 ECT
    8 BARO
    9 Evap purge solenoid
    10 O2 sensor

    you didn't mention the closed throttle pos. switch. Does it go away if you hold the throttle just enough to get it off idle? That'll open the closed throtle switch. Also how old is the fuel filter? Could be clogged lowering the fuel PSI.

  6. #6

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    Im not sure about the idle going away just above idle. Im going to have to check as soon as I get the exhaust manifold back on and new O2 sensor back in. BTW the tap size for the O2 sensor is metric 18x1.50 on a 2.4 4G64. I just purchased this truck and I have no idea about fuel filter. I do remember the guy saying something about the IAC though, thinking he had it replaced. I actually swapped it out with 2 different ones, one off my show truck that I am positive is good.
    I have no idea about the closed throttle sensor, IAT????, ECT?????. Where are they and what are they.. info please.
    The truck runs and idles fine as long as one or both of the electronic things I mentioned earlier is not plugged in I would think ruling out injectors, fuel pressure. Basically what it does is surge... up and down and up and down at random speeds and intermitancy and as soon as you unplug the oh lets say a/c temp sensor.....idles down and purrs like a kitten. What I thought it was was the black box under the dash but I swapped it and still the problem remains. Im going crazy with this thing b/c I know its going to be something soooo stupid and easy. I sure appreciate all the help.

  7. #7



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    check the pcv valve to be sure it rattles and make sure it seals to the valve cover - it will cause the erratic idle
    Pennyman1
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    Living the D-50 lifestyle since 1980

  8. #8

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    Have tried 4 pcv's, one I know good off my show truck none of which rattled now that you mention it and that one is new.

  9. #9



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    now I have to find the new one I have and check it - I swear it rattles...
    Pennyman1
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  10. #10

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    I bought a new one and it didn't rattle either.
    This is electrical. It's always so stupid and simple but it'll take alot of proper diagnosis and checks to find it. I've got the book here so we should be able to figure it out.

    Closed throttle switch location: Look at where the cable meets the throttle body. Under it you'll see a sensor pointing up at a 45 degree angle. That's the closed throttle switch. It's built into the IAC motor. Check continuity across it from terminal 4. According to the book it should be open at idle (throttle closed at rest) and closed as soon as you get the throttle off idle. If you've got the mechanical throttle adjusted too high it could throw this sensor off unless u adjust it to compensate. You can also check ground signal at ECM 6. Check for ground or 12v depending on whether the throttle is open or closed. The sensor itself is grounded to the t-body. Make sure the case has continuity to ground.

    I got my dryer in yesterday so I'd like to put my a/c in my truck over the next week or so. It's supposed to be 90s all week so after working outside all day I may come home and crash instead of work on the truck. Once it passes 70s here it gets miserable since it doesnt know how to be hot and not 100% humidity
    Last edited by recian; 05-15-2013 at 05:24 AM.

  11. #11

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    You have any luck? I got my a/c in my truck and well the system is working. When it is working my idle works good. I've gota get my electrical problem fixed so I can make sure it works right.

  12. #12

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    Nope still nothing. Im now more lost than ever and completly clueless. New O2 sensor now and tried another TPS. I'm seriously thinking its that little black box under the dash but still clueless. A/C works great but idle sucks, just up and down constantly. Wish someone had an answer.

  13. #13

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    Did you ever figure out if it went away if you get just off idle (open the closed throttle switch) At least yours works. I'm still waiting on my connector so I can attempt to repair my harness but I'm starting to think the box is bad. It's like something inside can't keep the compressor on. It could be your problem also.

  14. #14


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    A low charge will also cause the compressor to kick on and off as the pressure switch gets an inaccurate reading. Even a few ounce's of pressure will cause this. To bad you're in the US. here I can go to the local parts store and get a can or R12a that works with both R12 and R134 and refill the system for $12.00 a can.

  15. #15

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    a/c works fine, kicks on and off normally and the charge is good. I have swapped what you are calling the "closed throttle switch" (my literature shows that as motor position sensor/idle speed servo and idle position switch which I think is what your referring to). I basically have three of everything...what came on the truck, the donor truck and my show truck which has a/c and everything works flawlessly on and no dice on anything. Even unplugging the a/c temp sensor now doesn't work. It will idle down then goes up and down just at a lower rpm. Plug it back in and it idles up and does the same thing just different rates. When I turn the a/c on and the clutch engages the idle then levels out to normal and will stay there but will still idle erratic every now and again. When its at it's worst if you give a little gas above idle it does stop and just stays at the rpm you have it at whether by foot or set screw. But like I said I swapped out the idle position switch twice and with one I know is good. Just played with it for 2 hours and I'm pooped, please someone help.

  16. #16

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    I'm not as sure about these trucks but I know newer mitsu's it's common but you sure you don't have carbon buildup in the t-body behind the IAC? the valve won't work right if the port in the t-body is built up and plugged or the mating surface of the plunger isn't clean.

  17. #17

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    Also on my truck as long as i hold the connector for the control unit itll work all day. I just cant hold it and drive lol

  18. #18

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    Yea thats no good. Got the new exhaust on this morning. Sounds good but dont run no better and now its starting to miss like crazy and stumbles real bad while driving. Sucks. Beginning to hate this truck with a passion and the a/c quit on the way to get the exhaust. Just touched the connector to the control unit and it started working. Gremlins I tell you.....gremlins

  19. #19

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    I think I am having the same problem as you with the a/c controller. When I move the harness the a/c cuts in and out or if I hit a bump or something running down the highway. Does this with both controllers I have. I would really like a new controller unit but I cannot find one anywhere. Most people have no idea what kind of truck it is to begin with. This is starting to get very annoying. I hope you get all yours working correctly b/c you need some a/c in the south. Geez

  20. #20

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    Working at mitsu dealers for 4 years I'm starting to realize if you see a problem more than once it's common since mitsu's have such off the wall problems. I just got my radio connector in today to fix mine. I noticed that the old connector is so swollen it barely comes out. The new one slides in and out freely and it's tight in the cocket but not too tight. The old one had to be forced into the socket and barely fit. First look at the connector. Is it a green/tan color? If so the plastic has aged and swollen and you've probably got the same problem. Try taking yours out and see if the connector will come out. If it's very hard to get out then you better go on ebay and order one of these:

    http://www.ebay.com/itm/Mitsubishi-O...7532f4&vxp=mtr

    I brought all my stuff from work but forgot solder -.- So I've got tickets for fast6 here soon so hopefully after I can hit up radio shack and get some solder to get started. Reinstalling this won't take me long so hopefully I"ll have an update by tomorrow once you've tested what i mentioned.

  21. #21



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    To bad you guys don't photograph the install you did with AC and then send them to each other to view. Maybe one will see something the other one missed?
    I also noticed that things on the trucks that came from factory are not as good as the parts that you get when you buy it for a repair or replacement. I had a dealership owner tell me once that the replacement dif that was put into my truck on a warranty issue was much better then the oem dif installed from factory. I asked why and he said, because the are not on an assembly line grabbing parts. The dif was worked on by one person and each step carefully teched to tolerances. Assembly line parts are hurry up stick it in, even if it doesn't quite fit.
    Last edited by BradMph; 05-24-2013 at 06:37 PM.

  22. #22

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    You are right. original parts are made to last just out of the 3/36k. OEM tires are meant to last no more than 30k miles to give the best ride quality and they were made to be cheap. tires you get on a new car if you replace em with the same brand and everything you'll get more out of the 2nd set.

    A/C swap is pretty straightforward as far as electrical. If there's an empty plug you forgot something. Diagnosing a faulty part is worse since mitsu gives little diagnostic info on it like they do EFI.

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