What could be causing the PCM to shut down after firing two times

H2OCOACH

MEMBER
2000 CHRYSLER TOWN AND COUNTRY
3.8L
V6
FWD
AUTOMATIC
206,000 MILES

Engine will crank and fire two to three times then shut down and only crank unless I turn the key off and try to start again then fires two to three times and shuts down. Engine will not fire up and stay running.
Symptoms: The van has been experiencing a lack of power and a reduction in MPG. Down to 18 – 19 mpg from mid-20’s. The oil pressure was in the low acceptable range. The serpentine belt began to whine so I replaced it. After the whine was gone you could hear a “worble” sound.
History: I took the belt back off and started the van in effort to discover if the “worble” was coming from any of the accessory equipment. The “worble” sound was still present which to me indicated a possible timing chain or oil pump issue. Upon investigation I found the timing chain had approx. 1.25’ total play from side to side on the rear portion of the chain. The oil pump was also out of tolerance. I replaced the timing chain complete with cam and crank sprockets and the oil pump. I made sure to line up the TDC arrows on both cam and crank sprockets as well as installing the chain so the colored links are in line with the dots at three o’clock on each sprocket. Since I was doing this work I also replaced the cam shaft position sensor, crank shaft sensor, water pump, tensioner pulley and idle pulley.
Now the van will not start
What I have investigated:
Battery is fully charged and at 12.4 volts.
Battery terminals are tight, clean and corrosion free.
Checked the wiring harness as best I could for chaffing and/or breaks.
Checked all electrical connections and grounds
The fuel injectors are grounded.
The ASD relay is good.
Fuel pump is working and fuel rail is pressurized.
All fuses are okay.
The cam shaft position sensor is good. It is getting power and signals five volts
The crank shaft sensor is new
The coil pack test out at 12,500 ohms across the contacts on the secondary side and.9 to 1 ohm on the primary side.
Cylinder compression for cylinders 2, 4, and 6 is 210, 215, and 210 psi.
When testing the cam and crank sensor 5v signal I tested all the way back to the PCM.
I tested for spark at the coil pack and all 6 contacts sparked 2x then shut off.
Question:
The coil pack resistance is out of tolerance on the primary side by approximately 0.3 ohms. Is this enough to cause the PCM to trigger the ASD? If so why is it doing it now when the van started right up and ran before doing this work?

5 Replies

CARADIODOC

MECHANIC
33,752 POSTS

Sounds like you do not understand the operation of the ASD relay. The Engine Computer turns that relay on for one second when you turn on the ignition switch, then it turns it back off. If you can hear the hum of the fuel pump, you'll hear it run for that one second. The computer turns the ASD relay on again during engine rotation, meaning cranking or running. It knows that by the pulses it receives from the crankshaft position sensor and the camshaft position sensor.

On older vehicles the ASD relay does not get turned on if the signal is missing from either sensor. By 2000, give or take a year, the engine would continue to run if one of those sensors failed, but once stopped, the engine would not restart. On still newer models, the engine will run in a back-up mode with one failed sensor.

The ASD relay sends current to the injectors, ignition coil pack, oxygen sensor heaters, alternator field, and the fuel pump or pump relay. You need to determine if the relay is turning on during cranking. If it is not, you may be getting those couple of sparks while the relay is still on during that initial one second.

Use a test light to check for the 12 volts. Most digital voltmeters do not respond fast enough for this test. Back-probe through the electrical connector for the coil pack, any injector, or either of the two smaller terminals on the back of the alternator. The correct wire is the one that is the same color at the coil and every injector. Typically that is the dark green / orange wire. You will see the test light turn on for one second when you turn on the ignition switch. What is important is if it turns on again during cranking.

How did you set the air gap for the crank sensor?

As for the ignition coil resistance, do not waste your time with that. Those are three individual coils, and you know all three did not fail at the same time. Also, no mechanic wastes customers' time and money testing parts like that. Values listed in the service manual are for reference, but no two parts will measure the same. If one of the coils is bad, you will be running on four or five cylinders, there will be diagnostic fault codes related to that, and the Check Engine light will turn on.

Do you know how to read the diagnostic fault codes? Chrysler made doing that yourself easier than any other manufacturer. See if there are any codes set. If you get one related to "cam and crank sync", double-check the timing chain. On interference engines that code will set if the timing belt jumps two teeth, and that will cause the Engine Computer to shut the engine down to protect the valves. Your engine is not an interference engine, but if the chain is off, I believe the same code can be set.

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Tuesday, October 18th, 2016 AT 8:33 PM

H2OCOACH

MEMBER
2 POSTS

CARADIODOC

Thanks for the response.

I will give it a try.

Your idea that I am turning the key fast enough to go through the first two positions (accessories and run) and into the start position within that first second is not accurate. I have always paused in the run position to listen for the fuel pump to cycle.

I set the air gap on both the cam and crank sensor using the supplied felt pad that came with the new sensor.

I will check for codes again but the last time I did there were none.

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Wednesday, October 19th, 2016 AT 2:35 AM

H2OCOACH

MEMBER
2 POSTS

Caradiodoc

Checked for codes - none

Back probed with a test light on the common wire for the coil pack and the injectors (it is the dark green with an orange stripe). On the coil and cycles 2, 4, and 6. The light came on for one second in the run position and stayed on during the crank cycle for all test points.

I also (just for grins) put the test light to the wires for the individual coils in the coil pack and got the same results. I thought the light would flash but it did not. It came on for one second during the run position and stayed on during the crank.

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Wednesday, October 19th, 2016 AT 6:24 AM

KEN L

MASTER CERTIFIED MECHANIC
42,973 POSTS

If I may butt in here it sounds like you have a bad PCM which is common on these cars. But to be sure do a pin to pin check here is a guide.

https://www.2carpros.com/articles/how-to-check-wiring

Also I found these wiring diagrams for you.

Please let us know what you find so it will help others.

Best, Ken

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Wednesday, October 19th, 2016 AT 9:46 PM

CARADIODOC

MECHANIC
33,752 POSTS

I'm missing something and need to think about this a minute. You did a test that I don't do, but it has provided a clue. The constant 12 volts on the dark green / orange wire proves the Engine Computer is indeed turning the ASD relay on during cranking, but that is normally in response to those two signals showing up from the two sensors. Those sensor signals are also what times and triggers the firing of the ignition coils and injectors. To get a steady light on the other sides of the ignition coils says those coils aren't being grounded in preparation for developing spark voltage. I wonder if it's possible your test light is just glowing so brightly that you can't see the pulsing. Check for spark on one of the spark plug wires to see if it really is missing.

If spark is missing but the ASD relay is being turned on suggests only one sensor signal is needed for the computer to turn the relay on, not both signals like in earlier years. If there is no fault code related to a missing signal, you'll need a scanner to view live data and see how those two sensors are listed. Each one will have a "No" or "Present" listed during cranking.

Setting the air gap for those sensors with those thick paper spacers is correct. Some aftermarket sensors come with a thin plastic rib molded on the end instead, but they do the same thing. When you remove and reinstall one of those, you're supposed to cut the remaining part of the rib off, then use a paper spacer.