I have an aem in my 1990 T Surpa and the a/c idle up has never worked properly. when the a/c comes on the ISC valve is supposed to open up more to idle up the engine to provide power to the compressor without the engine dying. The valve opens by the % you choose in the a/c load comp % in setup- a/c - a/c options menus.
The aem does this when you press low, medium, or high fan. the engine idles up and the car idles fine. But when the a/c compressor goes off ( your a/c is still blowing cold) because of the hi/lo pressure switch the engine does not need to idle as high now and the idle learned value decreases to lower back down the rpms. The idle learned value helps to manage the idle to set rpm but the /ac load comp % is needed to make the car idle at set rpm with the added load of the compressor. The compressor is off but the fan is still on so the a/c idle up % is still present to keep the idle up for the a/c but the compressor went off and the learned value has lowered the ISC amount.
When the compressor comes back on it will stall or kill the engine now because the a/c idle up comp is no longer involved. The idle learned value compensated for it when the compressor went off; so when the comp came back on it really needed more idle and did not have it. The learned value can not compensate fast enough so most likely the car dies.
The aem used pin 10D on the 89 to 92 supra 1130 box. this goes to the a/c amplifier called a/c request on the instructions. this sends a ground to 10D and switch 6 (switch 6 ground = on ) will turn on a/c idle up for the a/c load comp value. But this signal in the stock wiring harness going to 10D stays on when a fan button, low, medium, or high is pressed. If you turn the a/c compressor button off it still keeps the load comp value. So when the compressor is going on or off the load comp value is always the same. The load comp needs to be added only when the compressor is on and removed when the compressor is off. This wire from the control unit to the ecu does not do that. An input is needed that will ground out the switch 6 as the compressor comes on and then off as needed.
To do this i made a harness to splice in on the ac compressors clutch wire that is behind the battery. There is a single wire that supplies power to the compressor clutch. I unplugged the wire. The harness will have a female and male clip to be inserted/connected to each end of the oem wire i unplugged. From this harness i spliced in a single wire that will goto a relay. This wire will supply 12v to the relay coil switch and of course the other side of the coil/relay switch will goto ground. This will make the relay turn off and on with the clutch. The other points (this is the side that usually has a high amp appliction applied) will have one post going to chasis ground and the other working end to pin 10D on my aem 1130 or whatever pin it is on your appliction that is "a/c request".
Now when the clutch comes on and off as the a/c amplifier determines based on water temp, hi/lo a/c pressure, and cabin temp sensor dictates the relay will also be tripped to supply the ground the aem needs to make the a/c load comp % come on to correct the idle load for the demands of the compressor. On the mark 3 supra the a/c amplifier controls the clutch not the a/c button on the a/c control unit in the dash. Since this a/c load comp % is working on and off with the compressor it will not effect the idle learned value unless the % for the a/c load is too much or too little. I have about 4.5 to 5.5 % with a small delay of .6 seconds. It works so much better.
I do not know it the mk4 ( mark 4 ) has the same problem but monitor the idle % and idle learned value. If your Idle % set value does not go up and down by the set load % in coordination with the compressor not the dash a/c switch but as the compressor goes on and off while the a/c blower is consistently on then the ground request is not coming on and off with the compressor but with the a/c control unit. I would imagine this mod will fix this problem on any car.
I have posted some pics. The harness i made with the clips at the appropriate ends and a pic of the harness installed. I connected the relay to the battery shelf. It will be under the battery tray out of the way.
Having this problem fixed is soooo much better riding around town with the a/c. Although you will still have some stuttering low idle problems because the aem cannot control the ISC fast enough to keep the valve open enough. every time the ac clutch comes on or off the ISC has to move. It will have an error that will get bigger over time and when you come to a stop the valve wont be where the AEM ECU thinks. The valve wont be open enough and the engine will stall or idle very low for a few seconds till the ecu increases the idle % correct to the max. That is why i have in my map the same % number for idle target for any idle speed over 2000 rpm or so. That way when driving the isc does not go over this number once above 2000 rpm. That helps keep the isc still. why would the car need to know how to idle over 2000 rpm? I wont let it warm up at a higher rpm than that.
The aem ecu drivers for the ISC are not strong enough or accurate enough to move the isc valve correctly. When you cut the motor and ignition completely off the ecu parks the isc at a known position every time. Then when you crank the car again the isc works correctly because the isc has been reset and the ecu is now working with a correct known idle % value. But the error will continue to occur again.
Hal
The aem does this when you press low, medium, or high fan. the engine idles up and the car idles fine. But when the a/c compressor goes off ( your a/c is still blowing cold) because of the hi/lo pressure switch the engine does not need to idle as high now and the idle learned value decreases to lower back down the rpms. The idle learned value helps to manage the idle to set rpm but the /ac load comp % is needed to make the car idle at set rpm with the added load of the compressor. The compressor is off but the fan is still on so the a/c idle up % is still present to keep the idle up for the a/c but the compressor went off and the learned value has lowered the ISC amount.
When the compressor comes back on it will stall or kill the engine now because the a/c idle up comp is no longer involved. The idle learned value compensated for it when the compressor went off; so when the comp came back on it really needed more idle and did not have it. The learned value can not compensate fast enough so most likely the car dies.
The aem used pin 10D on the 89 to 92 supra 1130 box. this goes to the a/c amplifier called a/c request on the instructions. this sends a ground to 10D and switch 6 (switch 6 ground = on ) will turn on a/c idle up for the a/c load comp value. But this signal in the stock wiring harness going to 10D stays on when a fan button, low, medium, or high is pressed. If you turn the a/c compressor button off it still keeps the load comp value. So when the compressor is going on or off the load comp value is always the same. The load comp needs to be added only when the compressor is on and removed when the compressor is off. This wire from the control unit to the ecu does not do that. An input is needed that will ground out the switch 6 as the compressor comes on and then off as needed.
To do this i made a harness to splice in on the ac compressors clutch wire that is behind the battery. There is a single wire that supplies power to the compressor clutch. I unplugged the wire. The harness will have a female and male clip to be inserted/connected to each end of the oem wire i unplugged. From this harness i spliced in a single wire that will goto a relay. This wire will supply 12v to the relay coil switch and of course the other side of the coil/relay switch will goto ground. This will make the relay turn off and on with the clutch. The other points (this is the side that usually has a high amp appliction applied) will have one post going to chasis ground and the other working end to pin 10D on my aem 1130 or whatever pin it is on your appliction that is "a/c request".
Now when the clutch comes on and off as the a/c amplifier determines based on water temp, hi/lo a/c pressure, and cabin temp sensor dictates the relay will also be tripped to supply the ground the aem needs to make the a/c load comp % come on to correct the idle load for the demands of the compressor. On the mark 3 supra the a/c amplifier controls the clutch not the a/c button on the a/c control unit in the dash. Since this a/c load comp % is working on and off with the compressor it will not effect the idle learned value unless the % for the a/c load is too much or too little. I have about 4.5 to 5.5 % with a small delay of .6 seconds. It works so much better.
I do not know it the mk4 ( mark 4 ) has the same problem but monitor the idle % and idle learned value. If your Idle % set value does not go up and down by the set load % in coordination with the compressor not the dash a/c switch but as the compressor goes on and off while the a/c blower is consistently on then the ground request is not coming on and off with the compressor but with the a/c control unit. I would imagine this mod will fix this problem on any car.
I have posted some pics. The harness i made with the clips at the appropriate ends and a pic of the harness installed. I connected the relay to the battery shelf. It will be under the battery tray out of the way.
Having this problem fixed is soooo much better riding around town with the a/c. Although you will still have some stuttering low idle problems because the aem cannot control the ISC fast enough to keep the valve open enough. every time the ac clutch comes on or off the ISC has to move. It will have an error that will get bigger over time and when you come to a stop the valve wont be where the AEM ECU thinks. The valve wont be open enough and the engine will stall or idle very low for a few seconds till the ecu increases the idle % correct to the max. That is why i have in my map the same % number for idle target for any idle speed over 2000 rpm or so. That way when driving the isc does not go over this number once above 2000 rpm. That helps keep the isc still. why would the car need to know how to idle over 2000 rpm? I wont let it warm up at a higher rpm than that.
The aem ecu drivers for the ISC are not strong enough or accurate enough to move the isc valve correctly. When you cut the motor and ignition completely off the ecu parks the isc at a known position every time. Then when you crank the car again the isc works correctly because the isc has been reset and the ecu is now working with a correct known idle % value. But the error will continue to occur again.
Hal
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