If youre getting the random alarm for no reason, all guages good etc etc, its in the ICM, you can replace it for half a grand, throw money at stats, impellers, sensors or...First I verified its definitely initiated in the temp warning alarm circuit, I pulled the yellow wire out of connector at ICM and other end at temp sensor, good continuity(under 3 ohms) while pulling and shaking the s%i* out of the harness, never blinks. If I ground the yellow wire to t-stat housing (proved grounded, again under 3 ohms resistance) repeatedly I get the alarm maybe 1 out of 5 times, also alarm most time wont stop even after pulling the ground, just keeps on buzzing until I pull the tan/blue wire from other ICM connector to remove continuity to buzzer. Ah hah, its the relay (or whatever) in the ICM.
What I did is pull the yellow terminal out of ICM connector and tan/blue from other ICM connector and splice them together. Now alarm system functions as should. If I heat temp sensor to just under boiling (inexactly 190 degrees) it closes and sends ground to buzzer as designed (just not through ICM). IR thermometer says engine is running at 168 degrees, Now take all that money you were going to spend on t-stats, impellers, hoses, water pumps, sensors and put it in the gas tank.
The only thing Im not sure of is if ICM still retards/advances timing as designed even if relay thinks its sending ground to the alarm. Doesnt seem to affect the performance of my 5.0 at all, except peace from the alarm and peace of mind that nothing is wrong with my engine. Id love to get ahold of an ICM and open it up to further research. If anybody has a junk one laying around and would like to donate to science LMK.
Some of us with carbed engines have this frankenstein wire harness that has a yellow and black 2 wire temp sensor for the alarm, part MIE and part MCM, I believe when they were transitioning from carb to FI we got these "in between" builds while they used up stock or had afterthoughts at the factory and ours dont match any diagrams to a T. I believe the ICM is also in between designs and thats where our problems begin.
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