2G Low voltage on ISC/IAC Pins on ECU

[gontijexx]

Hi guys this is my first thread here and I already have an issue with my 2g here in Brazil. my ISC was not working and my car was dying when release the clutch.

I decided to buy a new black one, all resistance across the pins are okay in the new one (40 ohms), and I already tested with a 6v battery and works. I got 12v on pin 2 and 5 on the ISC harness, but when I connect to the ISC nothing happens.

I decided to measure on the ECU harness as suggested in service manual and I only got 6.7v on pins 4,5,17,18 on B-53 connector. And now I just froze and don't know where to start looking for, any help?

 

[dwb]

1g had issues burning up the driver chip in the ECU if the ISC coils shorted out. I don't know personally if this is an issue that plagues the 2g too, but might be worth opening up your ECU and have a look.

driver01 [ECMTuning - wiki]

www.ecmtuning.com

 

[gontijexx]

But the voltage it's measure with the harness disconnected from ECU, the ECU it's not the problem right now

 

[DogWhistle]

But the voltage it's measure with the harness disconnected from ECU, the ECU it's not the problem right now

Make sure when you are measuring the voltage at the ISC that you have the ISC unplugged, otherwise you will get a false reading due to the internal resistance of the ISC.

Also carefully check for corroded pins at both ends of the harness connections, and if you find none, carefully trace the wires in the harness. It’s not going to be fun, but if the other two components are good, it needs to be done.

Please tell us what pins at the ECU are giving you 12 volts? Normally the ECU provides the grounds on specific ISC pins and the 12v power comes from the MPI/MFI relay circuit and outputs around 6 volts.

 

[gontijexx]

So I'm following the service manual, I measured the 2 and 5 pin on the ISC, I got 12v but now that you said I just have voltage 6.7v on the ecu harness/connector pin 4,5,17,18 with the ISC plugged, without connect just 0v

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[chrysler kid]

In the past I would leave the old sensor bolted into the throttle body, and have the new sensor plugged into the harness so I could watch the new sensor move in and out several times.

You want to be sure to reset the code for the check engine light before you plug in the new sensor, or reset the ECU to make sure it will command the new IAC. If the ECU still has the error code it won’t power the IAC

The IAC should have 3 different positions if I remember correctly. You should be able to watch the new IAC move in and out when you start the car

 

[gontijexx]

The problem is that the IAC doesn't even move when I turn the key ON, just static no vibration

 

[DogWhistle]

So I'm following the service manual, I measured the 2 and 5 pin on the ISC, I got 12v but now that you said I just have voltage 6.7v on the ecu harness/connector pin 4,5,17,18 with the ISC plugged, without connect just 0v

I think @steve could possibly shed some light on this.

 

[steve]

So the FSM diagnostic chart is having you check at two ends of the harness. First at the Harness to ISC/IAC end with the ISC disconnected to make sure that the stepper will receiving battery voltage from the MPI circuit to the two center pins with the ignition on. Sounds like you measured those correctly and saw 12v.

The second test measures at the Harness to ECU end with the ECU B-53 connector disconnected but the others connected and the ignition on. Since no current is going to be flowing here there isn't going to be any voltage drop due to internal resistances and you really should see battery voltage on each of the 4 ISC pins. This is just a round trip continuity check but it also checks that the ECU can activate the MPI relay.

This doesn't seem to be working correctly in your case. That could be not having a good ground for the measurement, something wrong with the harness, possibly measuring the wrong pins on the connector, and perhaps a much more that I can't think of before my first cup of coffee today.

I'm not sure what chrysler kid is trying to say in his second and third points, but I'm not aware that the ECU will stop trying to position the ISC if it has a code for it. If it did it could never recover from a temporary fault but I haven't tested that on a 2G ECU.

I do know that the ECU doesn't have any ability to power the ISC individually, that comes from the MPI relay and the ECU can only turn the whole MPI power circuit on or off. I also don't understand the comment about 3 different positions. The ISC on the turbo cars is a stepper motor, within it's range of the pintle it can move from one extreme to the other. When you turn the ECU on it attempts to rehome the ISC to a known position. The ECU doesn't have any feedback so it steps the pintle to it max out, then max in, followed by positioning it to it's initial steps for the current conditions.

So what I'd do is eliminate the question of is the harness good by measuring the continuity end to end from the engine bay to B-53 for each of the 4 signals on these Green/Red, Green/Black, Grey/Blue and Green/Yellow wires.

I'd double check to make sure I was using a good ground and the right pins on B-53 for the voltage tests again.

I'd inspect and measure the ECU drivers and their circuit traces to make sure they hadn't been damaged in the past.

Maybe I'll have more ideas later once my brain wakes up and my fingers stop missing the keys.

 

[gontijexx]

I will try the continuity between ECU and ISC, ECU was sent to @Lintu to burn a new chip with stock 1995 bin, he will check the drivers too, and maybe I need to search a new ground. I was using pin 26 on the ECU and a screw on the dash

 

[Lintu]

So the FSM diagnostic chart is having you check at two ends of the harness. First at the Harness to ISC/IAC end with the ISC disconnected to make sure that the stepper will receiving battery voltage from the MPI circuit to the two center pins with the ignition on. Sounds like you measured those correctly and saw 12v.

The second test measures at the Harness to ECU end with the ECU B-53 connector disconnected but the others connected and the ignition on. Since no current is going to be flowing here there isn't going to be any voltage drop due to internal resistances and you really should see battery voltage on each of the 4 ISC pins. This is just a round trip continuity check but it also checks that the ECU can activate the MPI relay.

This doesn't seem to be working correctly in your case. That could be not having a good ground for the measurement, something wrong with the harness, possibly measuring the wrong pins on the connector, and perhaps a much more that I can't think of before my first cup of coffee today.

I'm not sure what chrysler kid is trying to say in his second and third points but I'm not aware that the ECU will stop trying to position the ISC if it's has a code for it. If it did it could never recover from a temporary fault but I haven't tested that on a 2G ECU. I do know that the ECU doesn't have any ability to power the ISC individually, that comes from the MPI relay and the ECU can only turn the whole MPI power circuit on or off. I also don't understand the comment about 3 different positions. The ISC on the turbo cars is a stepper motor, within it's range of the pintle it can move from one extreme to the other. When you turn the ECU on it attempts to rehome the ISC to a known position. The ECU doesn't have any feedback so it steps the pintle to it max out, then max in, followed by positioning it to it's initial steps for the current conditions.

So what I'd do is eliminate the question of is the harness good by measuring the continuity end to end from the engine bay to B-53 for each of the 4 signals on these Green/Red, Green/Black, Grey/Blue and Green/Yellow wires.

I'd double check to make sure I was using a good ground and the right pins on B-53 for the voltage tests again.

I'd inspect and measure the ECU drivers and their circuit traces to make sure they hadn't been damaged in the past.

Maybe I'll have more ideas later once my brain wakes up and my fingers stop missing the keys.

Hey Steve!

Just a quick FYI: his ECU is the same “unknown” 305162 that I have.

His OBD is also non-functional, just like mine was. Hopefully flashing the BIN you kindly provided will fix it, as it did on my car.

I also found out that many Brazilian 2GAs ended up with their stock ECUs replaced by the 305162. Still not sure where those ECUs originally came from, though…

 

[gontijexx]

Its was bad ground