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PCM Shares Some 5-Volt RegulatorsPosted 3/12/2006By Mike Brown
You're looking at your scan tool data and the throttle position sensor voltage is at 5 volts. You pull out the trusty old digital volt/ohm meter (DVOM) to check the voltage on the signal wire at the throttle position sensor only to find that it's reading around 0.8 volts. When you open the throttle, the voltage sweeps up normally to about 4.0 volts ... how can this be? The logical assumption is that the power train control module (PCM) is bad and needs to be replaced. But after you replace it, you find the same symptoms! The P0121 or P0123 code that initially started this whole process has now become a big deal because the PCM has been replaced. Chrysler forgot to tell us that some of the 5-volt regulators in the PCM are shared with other circuits, often ones that seem to have no relation to each other. On the speed control circuit or cruise control, a 5-volt multiplex (MUX) circuit is shared with the throttle position sensor. Multiplex means that one wire can do the job of several circuits. The PCM puts out 5 volts to the speed control switches, and as you press the buttons, it monitors the voltage as it is grounded through the different-sized resistors in each switch. By seeing a different voltage for each switch at the PCM, the PCM knows which switch was pushed. If, for some reason, the speed control MUX wire is shorted to ground, it could pin the throttle position sensor voltage high. So the place to start looking is at the clockspring under the ignition switch cover on the steering column. Clocksprings have two connectors, one for the airbag and the other for the horn, radio and speed control functions (remember to disable the battery prior to working with airbag connectors). Find the cruise MUX wire and back probe it with your DVOM to monitor the voltage. It should be at 5 volts with just the key on. When the speed control buttons are pushed, the voltage should vary between 0 and 5 volts. If the voltage on the speed control MUX is low, try disconnecting the clockspring connector. If MUX wire voltage returns to 5 volts and the throttle position sensor and operation looks normal again on your scan tool, then the problem lies in the clockspring or in the speed control switches. Clocksprings are the most common failure on this circuit. If the voltage is low on the speed control MUX wire when unplugged from the clockspring, cut the MUX wire at the PCM connector to see if the voltage returns to normal at the PCM. If it does, then check for shorts to ground on the speed control MUX circuit between the PCM and the clockspring connectors. But what if the voltage at the speed control MUX wire is higher than 5 volts or if the only time the code sets is when the A/C is cycling on? This too can cause similar problems - most of the time it is caused by moisture in the wiring harness or connectors, especially at the power distribution center under the hood. Water can wick up through the wiring harness so don't think that higher-up components are safe. Pull the connectors apart to check for corrosion, water (antifreeze) or tracking. Common reasons for moisture in the wiring harness are power washing your motor, fording a river, or in the case of some Durangos with the auxiliary electric water pump for the rear passenger compartment, the heater will sometimes leak antifreeze into the wiring harness. Coolant has even been found in the PCM connectors. In light of this new information, the question is: Are there any other shared circuits in the JTEC power train control modules? Yes - the manifold absolute pressure sensor (MAP) and the upstream O2 sensor share the same 5-volt regulator on their signal wires. And the transmission's governor pressure sensor and the downstream O2 sensor share a circuit. The downstream O2 sensor fails often enough to see them setting a P1763 transmission code on a regular basis. I've even seen the connector torn out of the sensor and wrapped around the driveshaft. Now that Chrysler has introduced its new generation controllers, only time will tell what circuits are shared in those PCMs.
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