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ISCs test good but are not functioning correctly

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Kenneth Schlorf

Proven Member
90
9
Feb 15, 2015
Cottage Grove, Minnesota
Hi! I am considered a newb on this forum, but have been into 1G DSM since the mid-to-late 90s. I have a 92. I have tested three ISCs and they all read 30 Ohms correctly. I took out the ECU and everything looks great - no burnt chips, caps, etc... When I remove the ISC from the IM and hook it up to the wiring, while switching from ACC to RUN the ISCs just vibrate quickly - not out and then in smoothly. So I believe that it is something in the wiring from the ECU to the ISC. Does anyone have any common problem areas with this? I am thinking possible wires closer to the ECU since I have had that thing out a billion times since owning the car (Toddzmodz/TMO in the early days and now DSMLink). Any help would be appreciated! (perhaps something else I am not thinking of...)

Happy Sunday!
Ken
 
My ecu looked visually ok other then some brown staining on the bottom. I sent it to Ecmtuning and it came back as my Isc drivers were fried and my caps were leaking despite visually looking good for the most part.
 
If you do have link or something that can log/see ISC position, it should be at 120 with the key off.
 
Sounds like either a blown driver or an open in one of the ISC traces. It doesn't help that one runs right under the capacitor in the front of the ECU that leaks or that all the ISC traces run to the EMI filters that trap the leakage from the capacitors.

As a reference point about 25% of the ECUs that came in had ISC problems that weren't visible from inspection. I had a ISC dummy load with LEDs so I could see how well the circuit was working for bench testing.
 
Steve, isn't there a 90/91+ up thing here too? Not saying that's the OP's issue, but just for reference..

Not that would impact the behavior of the ISC rehoming on ECU boot. There will be the typical swap issues with 6 and 14 which effects the ECU seeing the Idle Position Switch. Since the car is a 91+ and assumably so is the engine there isn't a wiring issue with an ISC wiring swap.
 
Update for the archives and for those who are following: My ECU is fine according to ECMTuning. So while it gets shipped back, I checked the wiring and the four wires that go from the ISC (1, 3, 4 & 6) to the ECU (58, 59, 67 and 68) test OK while the connectors are just sitting there, but I am wondering if they will still have continuity when they are plugged into the ECU (thinking that the angles of the wires may cause them to lose continuity).

I am also planning to check the two red power wires to the ISC (2 & 4) once the ECU is plugged in (not sure if the ECU is necessary to see power to those wires, but I am not likely going to have time until then). I will find out in a few days. If good power is not being sent via those wires, my understanding is that it may be the MFI relay that may be causing bad readings...is that right...anyone know?
 
Did you visually and probe-check (dentist pick) the ECU and ISC connectors on the harness side for broken or bent-to-hell tabs? I had broken PTU contacts once.
 
I am also planning to check the two red power wires to the ISC (2 & 4) once the ECU is plugged in (not sure if the ECU is necessary to see power to those wires, but I am not likely going to have time until then). I will find out in a few days. If good power is not being sent via those wires, my understanding is that it may be the MFI relay that may be causing bad readings...is that right...anyone know?

The ISC power feed does come from the MPI relay. The ECU activates the MPI relay by pulling the pin (56?) to ground when the ECU sees the ignition switch turn on. The MPI relay output also then powers the rest of the ECU on as well the the various sensors and solenoids.

The coils in the ISC have to be powered in the correct order to make the ISC step.
 
Could be a loose or bad chassis grounding wire, also look for signs of corrosion in the wire connectors as well as any signs of kinked or damaged wires.

Also what comes to mind is did buy a brand new from the dealer [or parts house] isc and it's doing this out of the box, or are all the isc's you've tested been all used examples?
 
Also what comes to mind is did buy a brand new from the dealer [or parts house] isc and it's doing this out of the box, or are all the isc's you've tested been all used examples?

Thanks for jumping in to help out mitsubishikid. Two are new ISCs (revised versions showing ~40 ohms of resistance through the coils) and two are older ISCs (showing ~30 ohms of resistance through the coils), so I am very confident that it is not an ISC issue. And I'll double check the grounds, too.
 
Knowing that you have good ISC is a plus, and it narrows the issue down some, it has to be something reducing [creating resistance] to the supply of the proper voltage [or current] to the ISC. I know It's obvious, but make sure in advance your battery posts, and terminals are clean, and in good health, as well as verify your battery has the right voltage.

Maybe something in the wire loom is causing the issues, check all the wires individually you might end up having to pull the ISC wires out of the main loom, in order to fix the issue.

oh and thank you kenneth for keeping the post updated on here, this will no doubt help others in the long run.
 
My intent is not to confuse the issue, but I wanted to bring something up:

I checked all the wires in the smaller of the three plugs (c59) that go into the ECU because I was having an issue whereby if the plug was pushed one way the engine would die. After following the directions in the shop manual regarding removing the individual plug wires and poking through a small hole to ensure the female end was tight, that particular problem was now resolved (though unrelated to the ISC). Here are the shortened directions below (the third being the most important and that which I believe solved the connection issue):

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So now on to the ISC issue:

I plugged the ECU in, disconnected the battery for 30 seconds, and then plugged the 40 ohm ISC in but it was not attached to the intake manifold. When the key was switched from ACC to RUN, the ISC this time moved out, but not in (this is not what I was expecting). When the key was switched from RUN to ACC, the ISC moved in, but not out (this is what I expected). I was expecting the plunger to move out then in when the key was moved from ACC to RUN.

I considered the fact that it moved "progress" and connected the ISC to the intake manifold and started the car. I left the car run for a while - long enough for the engine to get "warm" (it wasn't necessary to wait until the fans turned on because...). Unfortunately, it started to surge again like it did before (only a little slower than with the 30 ohm ISC).

I am stopping troubleshooting for the night and am thinking about looking at the MPI/MFI relay and the power wires (2 & 5) going to the ISC tomorrow. I feel like this is the only hope left at this point...
 
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...I know It's obvious, but make sure in advance your battery posts, and terminals are clean, and in good health, as well as verify your battery has the right voltage.

Maybe something in the wire loom is causing the issues, check all the wires individually you might end up having to pull the ISC wires out of the main loom, in order to fix the issue.

oh and thank you kenneth for keeping the post updated on here, this will no doubt help others in the long run.

Battery posts/terminals are clean and in good health with proper voltage - thanks for the suggestion, mitsubishikid. If the MPI/MFI relay is not the problem, then I will be going through the wires in the loop from the ISC to the ECU as a last resort (I believe).
 
maybe you need to clean your throttle body...and change the BISS screw o-ring?
 
maybe you need to clean your throttle body...and change the BISS screw o-ring?

This isn't a bad idea, but you still need to verify whether or not the ISC is moving in and out as it should be. Try this. Connect on of your ISCs to the throttle body(just to seal the hole). Then, take another ISC and connect it to the harness. Start the car. Move the RPMs up and down. Is the ISC moving in and out? If so, you may have another problem causing your surge. If not, continue with your ISC troubleshooting.
Make sure to move the RPMs around. If you don't, the ISC may stay fully extended if the idle can't be brought down.
 
Try this. Connect on of your ISCs to the throttle body(just to seal the hole). Then, take another ISC and connect it to the harness. Start the car. Move the RPMs up and down. Is the ISC moving in and out? If so, you may have another problem causing your surge. If not, continue with your ISC troubleshooting.
Make sure to move the RPMs around. If you don't, the ISC may stay fully extended if the idle can't be brought down.

I will give that a shot, too - thanks for the idea edged1g!

AllanL: Thanks for helping with you suggestion. When the idle surge was happening, I was putting my fingers all over the place to see if that would stop the surge. The BISS was one of those places - even after I replaced the o-ring - but it didn't help to stop the surge. I have gone through pressure testing the intake, too, but as edged1g stated, this has been isolated to an issue impacting the ISC (at least) due to the lack of normal ISC movement.
 
So I left one 40 ohm ISC in the intake manifold and then plugged in another 40 ohm ISC not connected to the intake manifold. After starting the car, the ISC "plunger" moves out about 3-5 times sequentially further about .25 to .50 mm each time, then moves out again sequentially, but this time about 1.0 mm each time until it, I am guessing, is maxed out. The car does idle faster than typical throughout this setup. When I rev'd the car like this by moving the throttle by hand, the plunger did move about 1-2 mm inwards when stepping on it and back outwards 1-2 mm when I let off the throttle. I also probed the two red wires going to the ISC (2 & 4) and they are lighting the test probe.

Based on this...I am not sure if this is normal ISC action or not. One thing that I am noticing is that the idle surge is slower relative to what it was doing a prior (it's pretty slow now - about one cycle every 3-5 seconds). I think I want to check for a boost leak again...perhaps it has been the ISC o-ring that isn't sealing correctly? Perhaps I should take another look at the throttle body where the ISC goes in...maybe that isn't sealing well. I may check the voltage going through the red wires to the ISC...tomorrow or over the weekend.

Any further help or thoughts would be appreciated. Thanks for staying in tune!
 
I might just store this thing for the winter... This is after a boost leak test past 20psi. First log shows me starting it, showing the ECU and ISC playing nice to try to stop air from going past the throttle body and the second log was after it was running a little bit - ISC at zero because I am assuming it cannot stop the extra air from going past. Thoughts?
 

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