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2G No Signal At ISC Connector

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randman2011

10+ Year Contributor
552
282
Feb 26, 2012
Indianapolis, Indiana
Is there anything in between the ECU connector and the ISC connector that could cause an open circuit? I'm measuring voltage at the ECU connector but all four signal pins in the ISC connector are 0V/open. As a sanity check, I have also confirmed that neither the current ISC nor two BNIB units respond when plugged in. I've found lots of threads about problems with the ISC valve itself but nothing that I have seen mentions problems with the harness.

I used to get CELs for ISC problems and had all sorts of cold start issues. I bought some spare valves, but for other reasons ended up going Megasquirt before I investigated further. Now Megasquirt gives me the ability to set the ISC1A/B and ISC2A/B lines manually and I can confirm 12V at the Megasquirt, in the adapter harness, and at the factory ECU 26 pin connector but I get nothing on pins 1, 3, 4, and 6 at the ISC.

This car has a history of harness problems; I've repaired the fuel injector wires three times since I got the car in 2018. Those wire breaks were all pretty easy to diagnose and were in the length of harness between the thermostat housing and the injector connectors themselves. Since it's a lot harder to jiggle the harness to identify the problem spot for the ISC, I'm hoping that there's an easy solution to this before I just run four new wires to the engine bay or start shopping for a 2gb engine harness or lay down the money to go mil spec on just a daily driver.
 
Is there anything in between the ECU connector and the ISC connector that could cause an open circuit? I'm measuring voltage at the ECU connector but all four signal pins in the ISC connector are 0V/open.

I think you might have the wrong idea, the ECU isn't providing voltages to the ISC.

The ISC gets battery voltage from the output of the MPI relay to it's center two pins (2 and 5). The voltage runs through the ISC coils and out each of the four coils via the other ISC pins (1, 3, 4, 6) to the ECU. Inside the ECU the ISC drivers switches one of those four coil pins to ground causing current to flow in the coil. The ECU then switches another coil pin to ground causing the ISC to step. The order of which pins decides which direction.

So without the ISC plugged in to the ECU and the MPI active there will be no voltages on ECU pins 4, 5, 17, 18.

There isn't anything in the wiring between the ISC and ECU. I assume you have verified that the wire colors match at both ends and that you've tried measuring the resistance of each of the four between the two points?
 
Consider doing a continuity check on each wire from the ISC to ECU. That will hopefully narrow down if you have a wire break somewhere.
 
All, thanks for the pointers for testing the correct ECU operation, but as mentioned in the original post I am running Megasquirt and I have the ability to set outputs high and low as I please, so I can set the ISC lines to +12V, confirm that they are set at the Megasquirt connector, then observe that they are floating/0V in the engine bay. Without some factory functionality interrupting the signal in the harness, the only possibilities at this point are a break in the harness, my harness doesn't use the same pins for the ISC connections as described in the 2g pinout diagrams commonly available online, or I have connected the Megasquirt output to the wrong pins in the ECU connector despite regularly confirming multiple times over the past week. I'll do one final pinout check tomorrow when it warms up a bit but it sounds to me like I'll just be running four new wires to the throttle body and crimping a new 6 pin ISC connector.

@steve your description does make me think that the Megasquirt might not be set up to properly drive the ISC even though it has a 6 pin ISC mode whose documentation matches that description. In software it only has IAC1 and IAC2 and hardware handles driving IACxA high and IACxB to ground, for example. But there are examples of this running on an EVO 8 and various Mitsubishi V6s with 6 pin steppers using the same pinout. If I can get continuity between the Megasquirt and the IAC it sounds like I will still have to do some fiddling to get it working.

@tunedbysaturnsl Continuity checks across the firewall are difficult to execute by myself but I'll see what I can do with the alligator clips that I have. But that's why I was setting the output at the ECU to 12v and testing for voltage under the hood. The husband will manage to break something even if he's just holding the flashlight, so I'm much better off fumbling with this by myself than I am bringing his hands into the equation.
 
Continuity checks across the firewall are difficult to execute by myself but I'll see what I can do with the alligator clips that I have.

Randy,
Get a long piece of wire and make an extension to reach. It will save you much frustration.
 
I thought I was being clever by crimping a D-sub pin onto the end of a wire to help with continuity checks but I forgot that the factory ECU pins are flat, so the D-sub pin didn't fit into the connector anyway. But the reason that I could read voltage at the ECU but not under the hood was simple: I just happened to be checking the two pins that were switched in my harness. One was GND when it was supposed to be 12V and vise versa, and I was only checking them when they were supposed to be set high.

By checking the labels of the wires at the ISC I found that Mitsubishi uses the letter to identify the coil and the number to identify the polarity, whereas MegaSquirt uses the number to identify the coil and letter to identify the polarity. This would probably explain why there are 48,000,000 threads on the msextra forum about idle control not working and finding that the valve had been wired incorrectly. Below is the correct pinout. I originally had M27 and M29 switched.
DSM pin Megasquirt pin
ISCA1 4 IAC1A M25
ISCB1 5 IAC2A M29
ISCA2 17 IAC1B M27
ISCB2 18 IAC2B M31

My old ISC was able to run through the full range of motion when I tested it but I replaced it anyway. I finally have control of the idle speed in software but I've got a lot more tuning to do to make it more stable.
 
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