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1G BEFORE I SEND MY ECU BACK...

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Anthony Boni

5+ Year Contributor
213
23
May 13, 2018
Wrightstown, New_Jersey
So long story short, I’ve been chasing an ISC issue in my car for several months on and off. I remove the ISC from the car and turn on my power, the ISC just vibrates. This goes for all 3 of the ISC motors I’ve tried. With the car running, the ISC motor starts at 120, slowly descends to 0, then slowly ascends back to 120. It repeats this the entire time the car is running. I’ve also left one ISC motor installed, and plugged in another ISC and sat it on top of my valve cover. Link showed the ISC position changing, but nothing was happening to the ISC motor sitting on the valve cover. It was just vibrating ever so slightly. Unplugging the ISC with the car running makes no difference in the way it idles.

It’s a 1990 DSM with a 91+ TB. I’ve swapped the two wires required when running the newer throttle body and ISC. I’ve sent my ECU to ECMTuning and they fixed a few things and sent it back. I’ve replaced the ISC motor 3 times. I’ve checked for resistance in the wires from the ECU to the ISC plug. I’ve checked I was getting 12 volts to the two power wires at the ISC plug from the MPI relay. I’ve ohmed out every one of my ISC motors. All of this, and I’ve found nothing wrong.

The only thing I can think of that can possibly be wrong is my ECU! It’s not my ISC(s) and it’s not my wiring. I trust the guys down at ECMTuning, but I’m hoping they overlooked something because I am losing my freaking mind trying to figure this out.

Some other things I’ve done before I’m asked: Compression tested the engine, leak down tested, boost leak tested, smoked the intake manifold, all new timing components, Verified base timing. Everything else on this car is PERFECT. I just can’t figure out this ISC problem!

I’m running speed density with an Omni 4 bar straight off the intake manifold (no Ts) as well as a GM IAT sensor.

What else could there be? Please help me!
 
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It sounds like a rewire of an isc that isn't right.

The rewire from a 90 to a 91+ throttle body is just swapping two wires. I don’t think that would be the culprit.

It's also possible that whatever issue you're having damaged the ecu again in the process. What's the status on your fiav?

FIAV has been deleted with a block off plate.
 
Don't quote me but I believe the FIAV delete does away with the isc?

The block off plate blocks off the FIAV ports but allows full operation of the ISC.
 

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So long story short, I’ve been chasing an ISC issue in my car for several months on and off.

With the car running, the ISC motor starts at 120, slowly descends to 0, then slowly ascends back to 120. It repeats this the entire time the car is running.

The only thing I can think of that can possibly be wrong is my ECU! It’s not my ISC(s) and it’s not my wiring. I trust the guys down at ECMTuning, but I’m hoping they overlooked something because I am losing my freaking mind trying to figure this out.

I'm not sure if your saying that the ISC steps keep cycling from 120 to 0 over and over while the car is running or something else.

On cold start the ECU steps the ISC to 120 steps to aid in getting air for starting and fast idle. As the coolant temp rises the ECU slowly steps the ISC in to close the air bypass and normally would stabilize around 30 steps if idling. The fact that the ISC is being driven to 0 indicates that the idle speed is higher that what is programmed and the ECU can't get the ISC to reduce the speed. It's not normal for the value to go back up to 120 after hitting zero unless the engine is restarted.

I would suggest you contact ECMTuning ASAP to see what they want you to do. I'm guessing that they will offer to inspect your ECU and retest it but they may want you to perform additional diagnostics before sending it in.
 
I'm not sure if your saying that the ISC steps keep cycling from 120 to 0 over and over while the car is running or something else.

On cold start the ECU steps the ISC to 120 steps to aid in getting air for starting and fast idle. As the coolant temp rises the ECU slowly steps the ISC in to close the air bypass and normally would stabilize around 30 steps if idling. The fact that the ISC is being driven to 0 indicates that the idle speed is higher that what is programmed and the ECU can't get the ISC to reduce the speed. It's not normal for the value to go back up to 120 after hitting zero unless the engine is restarted.

I would suggest you contact ECMTuning ASAP to see what they want you to do. I'm guessing that they will offer to inspect your ECU and retest it but they may want you to perform additional diagnostics before sending it in.

Yes sir, the car starts at 120, then descends to 0. When the fans kick on it goes up a bit to 15-17 steps and hangs out for a minute. It then steadily climbs to 120. The process repeats.

The thing is though, the ISC is not actually moving as it should. It only vibrates. I know this because with two ISC motors, ive installed one like normal and plugged in another one and let it sit on the valve cover. It only vibrates.

This leads me to believe it is not anything mechanical, it has to be electrical. I’ve smoked out the intake manifold and throttle body, and boost leak tested. Zero leaks. I’ve played with target RPM but the ISC position is always fluctuating. Probably because my idle is fluctuating +/- 100RPM due to the ISC not functioning properly.
 
I understand that the ISC isn't actually moving...

Since the fans are coming on and the idle speed is dropping thet ECU would drive the steps up but it usually maxes them out rather than stopping at 120.

The suggestion stands.
 
I understand that the ISC isn't actually moving...

Since the fans are coming on and the idle speed is dropping thet ECU would drive the steps up but it usually maxes them out rather than stopping at 120.

The suggestion stands.
I wasn’t aware 120 was not maxed.

I’ve contacted ECMTuning. I’ll wait for a reply. I can also post a log so you can actually see what the ISC is doing rather than me trying to describe it if that helps. Thanks for the replies it really helps
 
Kurt, the OP states that he did swap the 2 wires to make the 91+ ISC work with the 90, but your question is a good one.....is it a 1990 ECU Anthony???
 
Hopping in for a bit.
When you say you’ve checked for resistance, HOW did you check for resistance? walk me through what you did. —> A LOT of people actually do this wrong...

What was the value of the resistances for all three motors.
The ISC is EDIT: stepper motor :EDIT
I would be quadruple checking the work before I pointed to an ecu.

For instance:
I’d have to look at a diagram, I apologize for going in blind on this suggestion.
But if it’s strictly simple circuitry to/from ecu One or both ways. Try bypassing all that with your own temporary setup and maybe check results.

Depinning and pinning your own temporary rig is valuable. I actually have pins that have a female / male crimp connectors I can run jumpers into when I need to triple check stuff.


I’m REALLY good with wiring diagrams, if you need help in that area diagnosis, feel free to supply me with one that applies to your car and I can comb through it with/for you.
 
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Hopping in for a bit.
When you say you’ve checked for resistance, HOW did you check for resistance? walk me through what you did. —> A LOT of people actually do this wrong...

What was the value of the resistances for all three motors.
The ISC is duty cycled right? To hold correct positioning. How have you verified actual functionality of the motors? Visually saw them working... simple 12 volts applied theoretically should produce full travel (wide open or completely closed —> again, apologize for not being well versed on the isc) but some duty cycled components are not meant to handle the current / heat of long durations at full on)
If you have verified full AND partial travel (say 30-60% DC) then yes —> right back to wiring / ecu. I would be quadruple checking the work before I pointed to an ecu.

For instance:
I’d have to look at a diagram, I apologize for going in blind on this suggestion.
But if it’s strictly simple circuitry to/from ecu One or both ways. Try bypassing all that with your own temporary setup and maybe check results.

Depinning and pinning your own temporary rig is valuable. I actually have pins that have a female / male crimp connectors I can run jumpers into when I need to triple check stuff.


I’m REALLY good with wiring diagrams, if you need help in that area diagnosis, feel free to supply me with one that applies to your car and I can comb through it with/for you.

Hello!

I checked resistance by simply attaching a wire to each separate pin on the ISC plug and touching the negative/positive (I don’t remember which I used) on my multimeter to it. With the other multimeter probe, I touched it to the applicable pin at the ECU and checked for resistance. On each one, I had continuity and .3 ohms of resistance. However, touching my multimeters positive and negative to each other yields the same results. So I can deduce from this that I have 0 ohms from each wire. If I did that right.

For all three motors I was getting 38.7 ohms of resistance across all 4 pins on the ISC motors. I’ve tried to test them with a 6 volt lantern battery. They were all vibrating as the guide I was watching stated they should, but they never actually moved. Only vibrated. I’m not sure you can use a 12 volt power supply. The VFAQ I read states you could damage the motor that way.

I want to say it is a pretty straightforward wiring set up from ECU to ISC. I’m not sure concerning the MPI relay wires. I know those power a lot throughout the car, and that may put a dampener in things. MAY, I don’t know. I am not the guy to talk to about wiring diagrams, LOL.

That is a really good idea to make my own jumper to the ECU... I just might have to give that a shot!
 
The isc in these cars is not a regular motor, it is a stepper motor that is controlled only by ecu processor logic commands that then send a square wave signal to the motors various winding's in a certain sequence and not in terms of simple dc voltage applied to the terminals, the only way to test it without using logic commands and a proper driver signal is with an ohm meter or to visually observe operation.
 
The isc in these cars is not a regular motor, it is a stepper motor that is controlled only by ecu processor logic commands that then send a square wave signal to the motors various winding's in a certain sequence and not in terms of simple dc voltage applied to the terminals, the only way to test it without using logic commands and a proper driver signal is with an ohm meter or to visually observe operation.
Would it still be possible to create a jumper harness as bleakley was saying? Attach it straight to the ECU and MPI relays, turn the key and observe operation?
 
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I’m barely awake, just going to bed. Will probably have better response later.
Not well versed on steppers except what I acquired in tech. it was pretty much your electrical diag mixed with the diagnosing the steppers controlling vents in the car. I’m out of my element and it’s obvious I’d have reading to do in order to help further.

If you or someone has a wiring diagram o can tell you yes or not. Without looking at the circuit or being familiar with it (or knowing exactly what you’re doing period) no one can responsibly tell you to just start plugging sh*t in here or there.
I did say IF it’s simple to/from ecu. Meaning straight from ecu, to isc, then the answer is yes.

Edited out the irrelevant info I posted earlier.
 
You have verified the basic continuity for all 4 of the ISC coil to ECU wires and that the ISC power feeds give you 12v.

The only additional testing of the wiring I normally look at is for shorts to ground, shorts to +12, and shorts to another wire. There shouldn't be any continuity to ground or any of the other ISC wires while both ends of the harness are disconnected. You also shouldn't see any voltage with respect to ground at the ISC end when the ECU is on and running except for the two center power feeds.

As mentioned, the ISC is a unipolar stepper motor. https://en.wikipedia.org/wiki/Stepper_motor

The ECU pulses the coil windings in sequence to move the armature one direction or another. Applying a voltage across a coil winding attracts a pole on the armature. If you don't energize the next coil in the sequence the motor doesn't do anything.

The drivers in the ECU act as current sinks, they pull one side of the ISC coils low to cause current to flow in the coil. If the resistance of the coil is too low it causes the driver to pass too much current damaging it or burning the IC up.

I've seen cases where the solder on pins has melted and the pins welded to the PCB, those are easy to detect but I've also see cases where the drive is damaged and no longer able to pull the coil close to ground resulting in weak or no step and externally the IC looks fine.

The other typical failure mode is damage to the circuit traces between the ECU connector and the ISC pins from capacitor leakage or over current.

The ECU doesn't have any direct feedback that the ISC is steppping. It needs to start out in a known position so at power on the ECU steps the ISC out and in to force it to it's home position. This routine is what causes the pintle movement shown in the ISC videos and what many places use as a test of the ISC drivers when checking ECUs with a known good ISC.
 
You have verified the basic continuity for all 4 of the ISC coil to ECU wires and that the ISC power feeds give you 12v.

The only additional testing of the wiring I normally look at is for shorts to ground, shorts to +12, and shorts to another wire. There shouldn't be any continuity to ground or any of the other ISC wires while both ends of the harness are disconnected. You also shouldn't see any voltage with respect to ground at the ISC end when the ECU is on and running except for the two center power feeds.

As mentioned, the ISC is a unipolar stepper motor. https://en.wikipedia.org/wiki/Stepper_motor

The ECU pulses the coil windings in sequence to move the armature one direction or another. Applying a voltage across a coil winding attracts a pole on the armature. If you don't energize the next coil in the sequence the motor doesn't do anything.

The drivers in the ECU act as current sinks, they pull one side of the ISC coils low to cause current to flow in the coil. If the resistance of the coil is too low it causes the driver to pass too much current damaging it or burning the IC up.

I've seen cases where the solder on pins has melted and the pins welded to the PCB, those are easy to detect but I've also see cases where the drive is damaged and no longer able to pull the coil close to ground resulting in weak or no step and externally the IC looks fine.

The other typical failure mode is damage to the circuit traces between the ECU connector and the ISC pins from capacitor leakage or over current.

The ECU doesn't have any direct feedback that the ISC is steppping. It needs to start out in a known position so at power on the ECU steps the ISC out and in to force it to it's home position. This routine is what causes the pintle movement shown in the ISC videos and what many places use as a test of the ISC drivers when checking ECUs with a known good ISC.
I’m not sure how to check for shorts honestly.
 
Thanks Steve! :)
Anthony,
Short to ground is EASY to test man. Isolate your circuit (disconnect at both ends).
Set multimeter to test resistance.
One lead on circuit.
One lead to ground.
Anything less than ~10,000 ohms —> possible short. Common reading should be a REAALLYY high resistance reading or an Out of Limit reading (OL).
Short to another circuit is done the same way. If the diagram shows they are separate circuits, when isolated, multimeter should read OL.
Short to 12V is pretty self explanatory.
 
Thanks Steve! :)
Anthony,
Short to ground is EASY to test man. Isolate your circuit (disconnect at both ends).
Set multimeter to test resistance.
One lead on circuit.
One lead to ground.
Anything less than ~10,000 ohms —> possible short. Common reading should be a REAALLYY high resistance reading or an Out of Limit reading (OL).
Short to another circuit is done the same way. If the diagram shows they are separate circuits, when isolated, multimeter should read OL.
Short to 12V is pretty self explanatory.
Awesome I'll go ahead and give this a shot. I've contacted ECMTuners and they've agreed to re inspect the ECU. Cause they're awesome. Before I do send it back I am going to ensure I troubleshoot everything.


Just to ensure I understand this correctly;

You're saying to de-pin the ISC at the ECU, disconnect the ISC plug at the ISC, place one lead on the wire on one end and the other on ground?
 
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