hakukamana Posted October 4, 2018 Share Posted October 4, 2018 OK, just finished the install of the AC system, in the eliminator. Things went well, charged, used a gauge set. My question is regarding the engine idle and how the idle is supposed to increase when the system is turned on? It idles normal with it off, it idles the same with the AC on. I was under the impression that when the AC system is engaged the idle is supposed to increase a little, to handle the increased load. I just don't know how it knows that the AC is on, and what tells the motor to increase the RPM's? The electric fan, engages when the AC is switched on, so that circuit is working correctly. The AC also engages when the switch is turned on and the clutch engages. I am betting its something in the ECU????? Link to comment Share on other sites More sharing options...
Minuit Posted October 4, 2018 Share Posted October 4, 2018 Don't think I've ever noticed this on my '91. Totally different system, but still. The idle doesn't really seem to change when the compressor kicks on. I've had other cars (my Thunderbird comes to mind) that noticeably bump up the idle, but never on my MJ. I'd say as long as it doesn't bog down, nothing to worry about. Link to comment Share on other sites More sharing options...
Ωhm Posted October 4, 2018 Share Posted October 4, 2018 True that engine idle will increase when AC in engaged, but with the AC load now on the engine, idle rpms will remain the same, that is, not drop in rpms with the load. Yes, ECU controls Air Stepper Motor (idle air control motor, etc whatever one calls it) using AC Select and AC Request inputs to the ECU. This will kick the idle air motor 50 steps (≈50 rpms) when AC is engaged. Link to comment Share on other sites More sharing options...
HOrnbrod Posted October 4, 2018 Share Posted October 4, 2018 ^^ This. Mine was bogging down a little when the A/C compressor kicked on. Pulled the IAC and douched / cleaned the valve itself and the passageways on the throttle body with fuel injection cleaner. No more bogging down. Link to comment Share on other sites More sharing options...
hakukamana Posted October 4, 2018 Author Share Posted October 4, 2018 Ok Ohm that makes sense, I just pulled the ECU diagram, (3) connections to the ECU relative to AC. Pin A12 A/C Relay Ground goes to the AC clutch relay, Pin C2 A/C Request to A/C thermo switch, Pin D2 A/C Select goes to the AC low pressure switch. I don't see anything that would indicate the ECU does anything with the stepper motor Pins B3,B4,B5,B6 all tie into the stepper motor, but does not tie into the AC circuit. Weird? Link to comment Share on other sites More sharing options...
hakukamana Posted October 4, 2018 Author Share Posted October 4, 2018 Hornbroad, no bogging down, just no real increase in the idle from no AC to AC on. Its just like it doesn't care that the AC is on. I guess I was expecting a little bump in the idle due to load. I can see the volt meter, sensing the blower draw. Link to comment Share on other sites More sharing options...
Ωhm Posted October 4, 2018 Share Posted October 4, 2018 Thats what the microprocessor (ECU) does with any and all inputs, it control the outputs. Link to comment Share on other sites More sharing options...
HOrnbrod Posted October 4, 2018 Share Posted October 4, 2018 1 minute ago, hakukamana said: Hornbroad, no bogging down, just no real increase in the idle from no AC to AC on. Its just like it doesn't care that the AC is on. I guess I was expecting a little bump in the idle due to load. I can see the volt meter, sensing the blower draw. Bogging down was a bad choice of words, better to say the idle RPMs dropped slightly when the compressor came on. After cleaning the IAC, the idle remained constant with no dip when the compressor kicked in. Link to comment Share on other sites More sharing options...
hakukamana Posted October 4, 2018 Author Share Posted October 4, 2018 I am going to plug in the Red Brick and see if I can see a difference in stepper motor steps. Link to comment Share on other sites More sharing options...
hakukamana Posted October 4, 2018 Author Share Posted October 4, 2018 OK, that makes sense. Probably a dumb question, but I like to know how things work. If Ohm is correct its got to be something internal to the ECU. Link to comment Share on other sites More sharing options...
Ωhm Posted October 4, 2018 Share Posted October 4, 2018 Idle Air Control (IAC) will also kick up when automatic transmission is engaged, with no noticeable change in rpms due to load. Link to comment Share on other sites More sharing options...
Minuit Posted October 4, 2018 Share Posted October 4, 2018 Perhaps it's more accurate to say that the ECU's "desired" RPM doesn't change, it just has to open the IAC a little more to compensate for the added load. Link to comment Share on other sites More sharing options...
hakukamana Posted October 4, 2018 Author Share Posted October 4, 2018 Ok, hooked the red brick up guess what can't read IAC steps with the Snap On Modis, MT2500, does not provide that function. I did check engine RPM, AC off, AC on and as everyone has said no noticeable difference in engine idle RPM. The Eliminator is an automatic and does idle higher than the Metric Ton, engines are the same. Just one is manual and one is an automatic. Different ECU's also. I have a 29 in one and a 28 in the automatic. Link to comment Share on other sites More sharing options...
omega_rugal Posted October 5, 2018 Share Posted October 5, 2018 it depends, sometimes you don´t notice any change in rpms when the ac turns on... Link to comment Share on other sites More sharing options...
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