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1G Low idle with a/c on

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benspilk09

10+ Year Contributor
128
68
Feb 16, 2010
Little Rock, Arkansas
I'm about at my wits end with this problem.

Car is a 91 TSi AWD, all stock except for ecmlink and a wideband.

The car makes no attempt to speed up the idle when you kick the a/c on. Normal idle is perfect at around 800rpm, it drops to about 500 with the a/c.

I put the car in diagnostic mode and set the BISS per the VFAQ.

The isc buzzes when you turn the key on, but doesn't visibly move. It is clean and the plunger moves freely when you take it apart. It tests perfect across all 4 coils. All the drivers in the ECM look fine. The wiring harness tests fine.

In ECMlink, when I log ISC position and learn idle adjust they're both flatlined at 120 and 176 respectively. No changes, ever. I assume this means the problem is with the ECU? Anyone had an issue like this before?

Any help is appreciated, Thanks!
 
I don't know... But here's what I do know:
- The A/C uses voltage
- The A/C uses horsepower

Your ecu is supposed to recognize when the A/C is on so that it can compensate for the use of power. It must not be recognizing it's on. Or somehow it doesn't have the means to compensate for it...

When you turn the A/C on in any car it drops the rpm's, just not usually by 300.

I know that seems really basic, but sometime people overlook the real problem because of overthinking...
 
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the ecu is supposed to idle up when a/c is activated.

have you tried checking the ecu pin to ensure its being energized or grounded? i dont recall off the top of my head which it doesn. wire should be pin 7, large connector, black with green stripe.
 
do you have it set in dsmlink to target idle at 800rpms... IIRC the stock or default target idle is 750rpms so thats what the ISC expects to see unless you set the it in dsmlink to target 800 rpms, I know this is obvious, but i'd start with this since you have it, and it's easy to do, also if you have a spare ISC try swapping it out.
 
My target idle is 850, just so it won't stall with the a/c on.

I did some more playing in ecmlink. If I play with the BISS I see the isc position changing and attempting to change the idle speed, but nothing happens. I'm assuming the ecu isn't actually getting a signal to the ISC and I either have a failed driver or a bad trace on the board.

The isc does have power at the plug, it tests fine per my FSM.

I'll have to yank the computer back out and do some testing. I can't seem to find a good walk-through on testing the drivers and traces with a multimeter though
 
You can also log the AC input to make sure the ECU is seeing it.

If your ISC isn't stepping in and out when you turn the ECU on, there's a problem. If your ISC is a original metal one, change it regardless.

You can't always see damaged ISC drivers. The fried ones are easy but not the ones that can't sink enough current anymore to make the coil step.
 
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Yep, I've read through those links. I have not played with anything outside of idle speed. I did set my idle switch and verified my throttle closes to 0% and calibrated the TPS. All is working fine

It's just that the ISC position seems to be doing nothing to the idle.

I know I don't have any significant vacuum leaks, as it will die if I screw the BISS in too far.

I suspect an ECU problem the more I read, and the more I play with it. I see the ECU trying to move the thing, but there is no resulting change of idle. It's just locked wherever I set it using the BISS.

Yes, it's the original ISC. I'll try and find an updated one if they're more reliable, I'd hate to repair my ecu only to have It fried when this ISC takes a dump.
 
Allright, I've had some time to mess with the car some more.

I bought a brand new black ISC and it does the same thing. It should be good, ~40 ohms across all coils.

I verified again that the wiring harness is good, there is no resistance between each pin in the ISC plug and the corresponding pin in the ECU plug. I'm almost sure it's an ECU problem at this point.

I pulled the ECU again and verified that all 4 coil pins have connectivity to the ISC drivers themselves. I'm not sure where to test after that. I can't find anything on testing the drivers or motherboard traces themselves. Anyone have experience with this? Or do I just need to call up ECMTuning?
 
Did you bother checking the AC wire/pin like some members said. The ECU needs to see the AC turning on for it to even bump idle. Check to make sure the wire is good from the compressor back to the ECU.
 
It's not just an a/c issue, the computer has zero control over idle. The ISC is completely inoperable. Whatever I set the BISS at, that's the idle speed

I have not checked the a/c pin, the ISC needs to function before I dive into anything else
 
It's not just an a/c issue, the computer has zero control over idle. The ISC is completely inoperable. Whatever I set the BISS at, that's the idle speed

I have not checked the a/c pin, the ISC needs to function before I dive into anything else

So with the new ISC does it rehome when you power the ECU up?
Do you have a working IPS?
 
So with the new ISC does it rehome when you power the ECU up?
Do you have a working IPS?

Nope, there is no movement when you turn the key on. Just a slight buzz then nothing.

The IPS is working correctly, I can see it switch on and off in ECMlink

As for disabling the ISC, just the diagnostic pin? No, that's not checked. I can see ISC position change like I'd expect when I play with the BISS, but the plunger isn't moving because the idle doesn't speed up/slow down when the computer tells it to.
 
Ahh, if it takes an o-scope I'm probably better off sending it somewhere. I was hoping I could rule them out with a simple multimeter.

I contacted ecm tuning a few days ago about diagnosing and repairing the ECU, but they haven't responded yet. Are you affiliated with them? Or do you offer a repair service? I'm ready to get this thing idling right
 
Problem is resolved, I got ahold of ECM tuning and sent my computer in for diagnosis and sure enough one of the drivers had failed. It showed no external signs of failure so they aren't always obvious when they go.

Car idles great now and speeds up like it should with the a/c on
 
It showed no external signs of failure so they aren't always obvious when they go.

That was my experience as well. About a quarter of the ECUs I saw had driver issues that weren't obvious. That's why I build tools to test them.
 
That was my experience as well. About a quarter of the ECUs I saw had driver issues that weren't obvious. That's why I build tools to test them.


Hey Steve, any info on what to look for on these circuits when using an oscilloscope? I have one but am still learning how to use it properly. Thanks
 
Hey Steve, any info on what to look for on these circuits when using an oscilloscope?

Sure look at each coil. They get 12v on one side and the ECU pulls the other to ground. The driver IC should be able to pull it's side down to at least 1v when it's stepping the ISC, if it can't it's bad. Remember that the ECU pulses the coils in sequence to step one direction or the other so without software to force the stepping you'll have to be quick to see it.
 
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