This is a new chapter in the hair-tearing-out saga here: https://www.brickboard.com/RWD/volvo/1594228/740/760/780/powerloss_heater_core_replacement_pulling_hair_power_loss.html

1990 740 with a 1992 -560 ECU turbo and w/egr

Sooooo... After replacing my heater core, I've found my car drops injector pulses, and I lose engine rpms as a result. The problem is pronounced when warm and between 1300 and 3200-3500rpm. Outside of those ranges, there is no noticeable problem.
I figured out that I'm losing injector pulses by backprobing an LED into an injector while the engine was acting up. I had an inline spark tester, and the spark was flawless.

So, my logic is that is must be either the ECU, ICU, Wiring between them, or a fault between the ECU and injectors (maybe the RSR wiring, but I just replaced the RSR and the wiring pigtail to it).

So. I've got the greenbooks, but I am getting confused on what pins on what control unit should be hot. I've tried 3 pairs and all of them show 0V, but if that were true, the car wouldn't be running. I must have the wrong pins.


Here are my burning questions:
What pins is the signal from the ICU to the ECU on?
What does the signal look like? Is it a steady voltage or is it pulsed?
What is the "signal generator" referenced in the ignition greenbook? (The 700 ignition book starting on page 61 in the pdf and 59 on the text says ICU pin 17 sends a signal to ECU pin 1, but gives no description of the signal.)

I'm getting close to figuring this out... I think.
Cheers