MCU

From BluWiki

American Motors Master Control Unit used in many cars and Jeeps has been a bit of an enigma.

Sites like Jeep Adventures Under the Hood do a pretty good job explaining what, but not how.

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Main

There are two stages of retard that the MCU can preform. One is the idle retard, which sits somewhere between the base timing and vacuum advance.

Vacuum Connections

The Manifold switch has both a reservoir and a delay valve (Purple/Black) to delay it's signal change. The Delay valve slows vacuum application to the switch.

The Air Cleaner Door has a Yellow/White delay valve (Although any longer delay will work) to prevent it closing while the engine is at WOT. If this valve is defective or missing, you will get poor performance. If you are towing, it may be helpful to use a longer delay valve to prevent long low-vacuum conditions from closing the door. This door is an evaporative emissions device, and depending on your inspector must remain operative.

The Heater Door has a Gray/White delay valve (Gold may be substituted) to prevent it from closing at WOT, but allow the door to be modulated by a thermal switch. Gray appears universal for this application. If you are towing, it may be helpful to add a check valve in the supply line to the thermal switch to prevent long low-vacuum conditions from closing the door.

If your EGR vacuum line has a red & blue delay valve in it (cross reference 1974-80 Jeep models), the blue side should point toward the EGR valve and the red side should point toward the CTO switch.

Vacuum Mods

You may want to add a small (orange/black or purple/black) delay valve to the vacuum advance if you are getting detonation when you let off your accelerator pedal after heavy acceleration. This will bring the vacuum advance in slower and help eliminate the problem.

A 10-30 second forward delay valve on your EGR may improve cold acceleration and off-idle hesitation.

I/O Chart

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