Mechanics

PCM PROBLEM??

2000 Dodge Neon

Computer problem
2000 Dodge Neon 4 cyl Front Wheel Drive Manual 184000 miles

My engine has two trouble codes: P0117 (ECT voltage input low) and P1193 (IAT voltage high). I checked the wiring for both ECT and IAT and noticed they both are connected together in a series circuit directly to the PCM. So I turned the ignition key on and tested to see if there was an electrical current going through the IAT adapter, but there's nothing; neither the ECT adapter had any electrical current going through it. So I'm assuming the PCM is not pumping out an electrical current through these two sensors. Should I replace the PCM or is there an underlying problem?
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Sunday_blitz
June 16, 2010.




Hi sunday_blitz. Welcome to the forum. You're looking at this circuit the wrong way. The inlet air temperature sensor, knock sensor, throttle position sensor, MAP sensor, camshaft position sensor, and coolant temperature sensor all have separate circuits, but they all share a common ground wire. That's the black wire with a light blue stripe.

Both of these temperature sensors have an extremely low failure rate. To have a fault code, and two of them at that, it is more likely they were set while other testing was being done. The 5.0 volt supply from the Engine Computer has to be working because it is needed to set a " voltage high" code that you got for the air temperature sensor. Unplugging that sensor while the ignition switch is on, a break in the wire to that sensor, or a loose connector will set that code.

To check that circuit, use a voltmeter to measure the voltage on that violet / light green wire. With the connector unplugged, you should measure 5.0 volts. With it plugged in and and by back-probing that wire, you must read between.5 and 4.5 volts. The higher the temperature, the lower will be the voltage. Expect to see around 2.5 volts.

To set the fault code for the coolant temperature sensor reading too low, the wire would have had to be grounded. You can measure that circuit the same way. There will be 5.0 volts on the tan / black wire when the sensor is disconnected, and when connected and back-probed, it must also read between.5 and 4.5 volts. Voltages outside those limits is what the computer uses to observe a problem and set a code.

I won't say the Engine Computer CAN'T cause your problem, but it is very unlikely because if the 5.0 volt supply was dead, there would be fault codes for the TPS and MAP sensors too. Of course the two sensors you're dealing with get their voltage through different pins in the computer's connector than the TPS and MAP, but to have two circuits with a problem is very unlikely. The camshaft position sensor wouldn't work either if the 5.0 volts was missing so the engine wouldn't run.

Besides the fault codes, what are the symptoms? What do you mean by IAT " adapter"? Do you mean the connector?

Caradiodoc

Caradiodoc
Jun 17, 2010.
First off, I guess I meant the connectors, not adapters. Second, the symptoms of the car besides the trouble codes include the temperature gauge on the dashboard not responding, the lighting on the left side of the dashboard is dark, the engine idles steadily for about 10-20 seconds after ignition and then it starts to make a sound like as if you were revving the engine at intervals of 1 second apart, but sounds more like the engine is going to die in-between the intervals. Once you rev up the engine with the gas pedal, this sound goes away. The smell coming from the exhaust fumes are very strong. All of this came about after repairing a broken timing belt along with replacing the damaged valves. I don't understand how repairing this can cause trouble codes P0117 and P1193 to appear.

Tiny
Sunday_blitz
Jun 17, 2010.
Oooooh. Makes a little more sense now. The codes could easily have been set after repalcing the timing belt if the engine was started and the mechanic forgot to plug in those sensors. Or, ... The sensors could have been monkeyed with while looking for other problems. Either way, disconnect the battery for a half a minute to clear the codes, then see if they come back. If they do, we can troubleshoot those circuits later.

The timing belt should be double-checked to be sure it is set correctly. One tooth off could make it run as you described, but I suspect there is some other cause related to too much fuel based on your comment about exhaust smell. The MAP sensor and its vacuum hose should be inspected.

The coolant temperature gauge uses a different sensor than the Engine Computer. The gauge sensor isn't monitored by the computer. Look for a sensor near the thermostat housing that isn't connected. It will have a single wire. Grounding that wire should make the gauge read full hot when the ignition switch is turned on.

The instrument cluster will have to be removed to check the light bulbs for the back lighting. If every one of them are out on one side, and the bulbs are ok, look for a microscopic break in the copper traces in the flexible circuit or printed circuit board. Also look for loose connector pins. They can be soldered to the copper circuits if necessary.

Caradiodoc
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Caradiodoc
Jun 17, 2010.

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