Re: 258 BBD MCU concern re: OEM
The yellow rubber splash boots contain two groups of easy to use Diagnostic Connector terminals . Per the Jeep shop manual , when a grounded 12 v test light probe is inserted into many of the terminal pins , the test light should glow . This would indicate that juice is originating from the MCU , travelling through the MCU wiring harness , splitting at a fork : one fork carries juice to a single Diagnostic Connector terminal , the other fork carries juice to a switch ( TAC TES , Engine Coolant Temperature Switch , Green Adaptive ten inch manifold vacuum switch , the creme colored 4 inch ported vacuum switch , the defunct PCV solenoid , the idle solenoid , the upstream and downstream solenoids , the WOT switch , 2 stepper motor metering pins ). Several of these end point devices are merely sensor type switches with calibrated trigger points to open /close their respective circuit according to either temperature or vacuum pressure acting upon the sensor switch . As the change in temp or pressure forces the sensor switch to either move to the open or closed circuit position , then the electrical signal either does not / does return through the sensor switch and back to the MCU . Then MCU gathers this electrical signal as input regarding operating conditions of the engine : temperature or manifold pressure . In response to this input data feeding into the MCU , the MCU then sends signals to either the stepper motor at the carb to change air/fuel mix or to the distrib to change timing or to pulse air solenoids to pulse open / closed . If the sensor switch circuits never get juice supplied to them ( an MCU issue or a short to ground issue or an open circuit issue ), then they can never send an electrical signal back to the MCU . Thus garbage in , garbage out . The TES and Engine coolant temp switches are key to operate open vs closed loop mode . The 4 inch ported vac switch / 10 inch manifold vac signal are key to telling MCU how far you are stepping on throttle and accelerating and decelerating . This leads to A/F changes and timing changes ( ??) . The WOT switch also tells MCU when you have floored the pedal. Lack of juice at the Diagnostic Connector Box causes me concern as it indicates that the systems are not functioning correctly . Those little discussed Diagnostic Connector boxes are great . Note that some terminals should not have juice : ground , idle solenoid after the relay if A/C not turned on .