This one’s for my grandfather’s 1999 Chrysler Intrepid, 2.7L V6, 100k miles.
He’s got a misfire problem, and I pulled codes P0304 (Cylinder 4 Misfire Detected) and P0354 (Ignition Coil 4 Control Circuit). Well, we swapped coils with another cylinder, cleared the codes, and started it up…same misfire “feel” and codes came back for the same cylinder. So I think it’s safe to say, it’s not the coil.
Today, I replaced all the spark plugs with the OEM plug since they hadn’t been changed and looked pretty worn. Nothing too crazy on any of the old plugs though.
Still have the misfire, same cylinder. I can unplug cylinder 4 and get no change in the engine, but unplugging the connector to any of the other coils (this is a Coil On Plug engine) causes a change in how the engine is running.
Now, I pulled the connector to the cylinder 4 coil and put my multimeter on it and got 13.4 volts (fluctuates between that and 13.3). On all the other connectors (which are firing properly) I get 9.5 volts.
Does any know what would cause this and what the voltage should be? I was expecting around 12 volts to each coil…could 13.4 volts be causing the coil to misfire? It’s pretty suspect to me since the rest are at 9.5 volts.
Thanks in advance for any suggestions.
Jad
Also, what controls the voltage to the coils?
Do I understand you correctly- the higher voltage was measured with it disconnected from the coil but the others were measured with them still connected to their coils? If so, the voltage was likely at full bus voltage because it was unloaded (no coil) and the alternator was outputting a full charge voltage. The coils are typically configured with full bus voltage on one side and the other side grounded through the ignition controller.
No, they were all measured the same way - disconnected from their coils, one at a time (i.e. test one, reconnect it, go to the next).
I think the reason you are seeing a voltage less than 12 volts is due to the duty cycle of the signal you are looking at. The working socket has a signal that changes voltage because it is turning on and off as it controls the coil and this makes the voltage seem to read low on a meter as it averages the voltage level. The bad circuit isn’t changing so it remains high.
Another test you can make is a resistance test on the connector while the engine is OFF. Check each contact of a good one and the bad one with the reference to ground, and compare the readings. You most likely will find out they are different. Since the ground side of the coil is controlled by the PCM you may have to change that out. You first need to check to see if the wire between the the PCM and coil is good. The data shows the wire color is TN/LB to #4 and goes to pin C1-#1 on the PCM. If that wire shows good connection with the ohmmeter then the PCM is at fault.
Based on your answer to my question, I agree with Cougar.
Thanks Cougar! I’m not great with a multimeter but I think you explained that simply enough that I can try that!
You are welcome for the help. There are a number of excellent advisors here that can help you further with this issue if you need it. Let us know what you find out.
IF YOU WERE CHECKING THE FUEL INJECTOR CIRCUITS…
You’ve done good so far. The way a fuel injector circuit works is: with the ignition key in RUN (engine off), 12 volts is always being supplied to all fuel injectors. The voltage continues, in the wiring, to each fuel injector control connection on the engine computer, and on into the engine computer.
With the engine NOT running, there should be 12 volts at each FI input on the engine computer. If there isn’t 12 volts at the #4 FI terminal on the PCM electrical connector, back to the FI, there’s a poor connection in that length of wire.
When the engine is running, the engine computer completes each fuel injector circuit by grounding each circuit each time it’s needed to open a fuel injector to spray fuel. With a back-probe pin connecting a digital voltmeter to each FI terminal on the engine computer, you should see a voltage drop as each circuit is grounded to cause each fuel injector to spray, each time. Most voltage test gauges are fast enough to show HOW low the voltage drops. That’s OK for this test. An oscilloscope could show the full voltage range, if you are interested.
If you saw 12 volts at the PCM, with engine OFF: but the voltage isn’t dropping like the others, when the engine is running, the PCM isn’t working properly. This is called the PCM (ECM, etc) FUEL INJECTOR DRIVER CIRCUIT.
IF YOU ARE CHECKING THE IGNITION COIL CIRCUIT…
Check for 12 volts at the engine computer terminal on the wire from #4 coil, ignition key RUN, engine NOT running. If there isn’t 12 volts, there is a wiring problem between the engine computer and the ignition coil.
With the engine running, use a back-probe pin and a digital voltmeter, at the engine computer connector, to see if the voltage varies as the others do. If it doesn’t, there may be a problem inside the engine computer (COIL DRIVER CIRCUIT).
My wiring diagram shows #4 ignition coil control wire at the PCM #1 terminal is “tan/light green”. It shows #6 ignition coil wire at the PCM #4 terminal as tan/light blue.
With ignition key in RUN, engine OFF, the voltage at #1 PCM terminal should be 12 volts. If it isn’t, there is a wiring problem between the PCM and the ignition coil.
If you did get 12 volts (engine not running); but, don’t get the varying voltage at PCM #1 terminal (by backprobing), with the engine running, there may be a problem within the PCM’s COP Driver Circuit. The “fix” would be to repair the PCM (Powerplant Control Module [aka: engine cmputer]), or replace it.
Thanks for all that, hellokit. That will be very helpful too. I will check everything that I can and post back, although the lazy part of me wants to just find a cheap PCM and replace it. 
When you say #1 terminal on the PCM, will it be obvious which one it is (are there numbers on the unit for each wire?)? Or in Cougar’s info, C1-#1 (same thing?). I know there are a ton of wires and a few different connectors coming out of that thing so I want to be sure I’m backprobing the right wires.
Looking again at the info on this it shows that there are 2 TN/LB wires so one of them may be a mistake and Hellokit’s info is probably correct. The numbers may be marked on the PCM connector but I’m not sure on that. You should be able to find the correct connector pretty easily. The wire going to coil 2 is on pin 3 and should be TN/PNK. By checking a known good wire you will be able to figure out which pin is #1 and may be TN/LG. Before getting a PCM you need to make sure the wire between the coil and PCM is ok.
Thanks for clarifying, Cougar. That’ll help for sure. And I won’t start throwing parts at it…goin’ down the diagnosis road first!
I may have it wrong about when the voltage goes high, and when it goes low. It may be opposite of what I described because an ignition coil sparks when the current through it is cut; then, the voltage goes high. Anyway, all of the ignition coil circuits should operate the same. Compare #4 ignition coil control circuit to the others, with engine ON and engine OFF.
FYI: the ignition coil to PCM terminals are: #1 coil, #11 PCM terminal; #2 coil, #3 PCM terminal; #3 coil, #2 PCM terminal; #4 coil, #1 PCM terminal; #5 coil, #21 PCM terminal; #6 coil, #4 PCM terminal.
Disconnect the battery BEFORE you disconnect the electrical connector from the PCM. The terminal numbers are on the connector, or socket. You may not be able to see the numbers until the plug is disconnected.
Thanks again hellokit. I’ll be doing this tomorrow so I’ll post back with my results.
The Results
Well I think I got somewhere today, even if I couldn’t/didn’t do all the tests.
I was able to get to the PCM after removing a number of things and figuring out how to remove the connector. The good news is that finding the right pin was easy and there is continuity in the coil driver wire from the #4 cylinder coil connector to the PCM. I also checked #2 for fun, and it has continuity too, of course. So the wire seems to be fine.
I couldn’t figure out how to do any backprobing with my multimeter, I couldn’t get to the contacts with the probes I have and the PCM is just not accessible while hooked up and the car running, so I didn’t do any of those tests. Are there special probes for backprobing?
And the Haynes manual says you have to remove the upper intake manifold to access or test the fuel injectors so I didn’t go there either.
I tried checking for voltage with the car off, key on RUN but couldn’t read any on any of the coil connectors. Weird.
Now, I have to apologize for the previous readings I gave for the coils while the engine is running. I realized today I had been reading them incorrectly (bad ground). Today, I was getting a reading of 13.4/13.5 volts on every coil connector (again, by disconnecting one connector at a time and probing the power feed wire). Here’s the interesting part. When I probed the coil driver wire in the same way, I get a reading of -0.06 volts at all the good, proper firing cylinders (by probing the connector after disconnecting it). On #4 where I’m getting the misfire, I’m reading (+)0.03 volts. And it’s a constant reading on all of them, it doesn’t fluctuate (I know it might be but it doesn’t show on the multimeter).
Anyone know what that means or why the “good” readings are negative voltage (reverse polarity?)? I’m not sure what the specs should be or if I should even being reading the coil driver wire with a voltmeter.
And, does this point the finger at the PCM or could it still be something else?
bump
Good job on the voltage and ohm readings. You have good connection on the non-working coil so the simple answer is (although the most expensive) to replace the PCM. You have proved it to be the trouble area. You may have to get the replacement unit programmed. As far as the different voltage on the bad lead goes it is hard to say what has failed inside the unit and causing the slight difference in readings. Another trick I like to use is to compare the meter readings in the diode check mode when checking similair circuits. The circuit output is most likely tied to a driver IC or transistor circuit, either way, the PCM needs to be repaired. You can get a remanufactured unit to help you save some money over a new one.
Thanks for the reply Cougar. With your help and hellokit’s I feel like I learned how to do some basic electrical troubleshooting/testing, which was previously over my head.
Before I replace the PCM, are there are any ground straps I should be checking, or is the PCM responsible for grounding each coil…I’m guessing if it was a major ground, all the coils would have a problem, so maybe I just answered my own question.
I found a PCM on eBay for $200 that is rebuilt/remanufactured and they will program it to your VIN so you don’t have to take it to the dealer for programming (and it has a lifetime warranty), I think that’ll be the best bet.
Thanks again!