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Engine harness swap questions. '88 5-speed harness in '89 GT.
My original '89 GTs engine needs a rebuild due to stuck rings. While I have it out I still want to drive the car so I bought a used engine from an '88 5-speed S4. Then I can take my time rebuilding the GT engine and have as little summer down time as possible.
I found that my GT has the later 3-wire oil sender so I'm using the starter/alternator harness from the '89 GT and swapping the 3-wire sender to the '88 engine.
I also found a difference in the engine harnesses. In the footwell next to the ECUs The '89 harness has this device that looks like a relay. I don't know what it is.
The '88 harness has these loose pins in the same location on the harness.
My question is can I use the '88 harness without any issues in my '89 car?
It would be easy enough to swap harnesses except the hall sensor connector on the '88 harness is trapped behind the RH timing belt cover and I can't get the connector through the opening between the cover and the head without removing the timing belt to get the cover off. The timing belt was just replaced 3k miles ago on this engine so there is no need to remove it.
That relay is a bypass relay for the Ignition Control Monitor (ICM) system. That bypass works by connecting 12V power from the middle pin 87 to terminals A1 (red/green stripe) and A2 (red/yellow stripe) which supply voltage to cylinders 1-4-6-7 and 2-3-5-8 respectively.
In the schematic of the ICM relay, you can see that power comes in on pin 87 (from W13 on the CE panel), the temp sensors weigh in by doing whatever they do, and if there's a difference of greater than a couple of millivolts at terminals E1 and E2, the relay disrupts voltage to 1 or the other group of injectors.
The ICM relay also connects to the coding plug (pin 6) and pin 28 of the LH brain. So if you're going to try to use an '88 harness without any of the ICM stuff, I guess the issue is whether the '89 LH is expecting to see something on pin 28 that is no longer there. No idea whether it would be an issue or not.
No idea what the housing-less collection of pins on the '88 harness is.
I didn't want to run into any unexpected issues with the '88 harness so I removed the pins from the hall sensor plug so that I could pull the wires through and remove the harness. I'll run the engine with my '89 harness. I'll need to jumper the ICM relay due to the lack of temp sensors in the heads.
The temp sensors are in the exhaust manifolds, so keep the protection by fitting the GT exhaust manifolds and you'll be golden.
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The temperature sensors are matched type K thermocouples, devices that generate a small amount of voltage that varies depending on the heat applied. In this circuit, the two thermocouples are wired in series but with opposing polarity. So long as the temperatures are the same, there is net no voltage generated in the loop. The relay has a little comparitor circuit inside that looks for a generated voltage. Depending on the direction of any voltage detected, it triggers the appropriate contacts inside to isolate the injectors over the colder cylinders. There are additional features in the relay that disable the protection for a short while following engine start, and there's a little bit of deadband, the threshold of voltage that causes the protection to actually cut off fuel to one group of injectors.
I got the engine in and I'm using the original '89 harness from my car. I didn't swap the temp sensors over to the '88 engine's heads.
I tried to fire it today, but am getting no spark. Could the missing temp sensors in the ICM system cause no spark?
I also found this extra connector in the engine harness. I can't find a mate to it anywhere in the area of the fuse panel. Is there supposed to be a lose connector in the '89 harness that doesn't plug into anything? It's a 3-pin connector, but only 2 pins are populated.
That relay is a bypass relay for the Ignition Control Monitor (ICM) system. That bypass works by connecting 12V power from the middle pin 87 to terminals A1 (red/green stripe) and A2 (red/yellow stripe) which supply voltage to cylinders 1-4-6-7 and 2-3-5-8 respectively.
In the schematic of the ICM relay, you can see that power comes in on pin 87 (from W13 on the CE panel), the temp sensors weigh in by doing whatever they do, and if there's a difference of greater than a couple of millivolts at terminals E1 and E2, the relay disrupts voltage to 1 or the other group of injectors.
The ICM relay also connects to the coding plug (pin 6) and pin 28 of the LH brain. So if you're going to try to use an '88 harness without any of the ICM stuff, I guess the issue is whether the '89 LH is expecting to see something on pin 28 that is no longer there. No idea whether it would be an issue or not.
No idea what the housing-less collection of pins on the '88 harness is.
Rob, I think I misunderstood your post originally. Let me see if I have it correct now.
You're saying that this relay is a bypass that circumvents the function of the of the ICM system, correct? It simply connects pin 87 to pins A1 and A2?
I found the original ICM relay mounted on the LH/EZK bracket. It's in the background upper right of this pice. The clear box mounted to the bracket with a gold strap. There is nothing plugged into this relay on my car and the connector for the ICM fits it.
I'm still looking for the source of my no spark issue.
07-19-2015, 12:48 PM
Engine harness swap questions. '88 5-speed harness in '89 GT.
