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Odyssey wiring Problem?

Old 07-13-2018, 01:28 PM
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Default Odyssey wiring Problem?

I'm hoping someone here will be able to point me the right direction with misfires on a 2006 Odyssey EX-L.

Background: I recently completed replacing all five motor mounts. After getting everything reassembled, the vehicle started and ran just fine. However, later that day it started idling very poorly and flashing the CEL. Codes show P0300 and just about every cylinder. The engine ran fine before the motor mounts.

In diagnosing, I noticed that holding the engine speed at 3,000RPM (while in park) the engine seems smooth but the tachometer flutters a couple times a second from 2800-3300 RPM, as though it were trying to catch my attention. The engine clearly isn't mechanically changing RPM this much, this frequently. An OBD tool also shows the RPM fluctuating (though mechanically the engine is not changing speed).

In keeping with the principle of "what did you last touch?", and combined with the strange behavior at 3,000RPM I'm thinking this is a sensor/wiring problem. Unless this fluctuation in RPM is a programming trick to catch the driver's attention?

Troubleshooting: I've physically looked at all the connectors in the vicinity of the five motor mounts and don't see anything obvious. I've pulled the PCV and can hear the check ball. I've checked the EGR and it looks fine. I've switched spark plugs and coils and at idle it shows the same two cylinders (1 and 5; at 3,000 the misfires show up on additional cylinders). All of this keeps bringing me back to thinking I broke something while replacing the mounts, and that the misfires are the result of the computer not knowing when to fire the cylinders—I'm assuming the crank position sensor drives the tachometer as well as the ignition timing. Fix the sensor and the misfires go away, goes the reasoning.

Question: Am I barking up the wrong tree thinking this is a bad sensor/wire? I appreciate help on this. Thanks!
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Old 07-14-2018, 09:52 AM
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Default Re: Odyssey wiring Problem?

EX-L and Touring trim models have the Active Control Engine - Mount System (ACM). Since the engine mounts were replaced and there is also an issue at idle, this may possibly be something related to the ACM system. Other than that, some other circuit's may have loss communication, etc. due to wiring pinched, cut, connector issue.

The system consist of the following components:
  • PCM
  • Engine Control Mount Unit
  • Front Engine Mount Control Actuator
  • Rear Engine Mount Control Actuator
  • ACM Control Relay
  • Active Noise Cancellation (ANC) Unit
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Old 07-14-2018, 10:57 AM
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Default Re: Odyssey wiring Problem?

Yes, this vehicle has the active mounts and cylinder deactivation, and it's certainly possible I've pinched or cut a wire somewhere.

What's caught my attention in particular is the tachometer's behavior. I'd love to hear from someone who's seen this before and knows the cause.
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Old 07-14-2018, 12:38 PM
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Default Re: Odyssey wiring Problem?

Check Connector C151 and the wiring along that harness that goes to the CKP Sensors (A and B).

The tachometer is controlled by the power supply circuit/controller area network controller built-into the gauge control module. The power supply circuit/controller area network controller receives engine speed information from the PCM through the fast controller area network transceiver via the multiplex integrated control system. The PCM receives engine speed information from the CKP sensors (A and B).

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Old 07-14-2018, 04:57 PM
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Default Re: Odyssey wiring Problem?

This is great information! I'll check it out. Thanks!
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Old 07-14-2018, 05:32 PM
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Default Re: Odyssey wiring Problem?

From what I can see, the connector looks intact, as do the wires leading into it from the CKP sensors:


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Old 07-14-2018, 05:37 PM
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Default Re: Odyssey wiring Problem?

I have a short video clip of the tachometer's behavior
. About 10 seconds in you can see the behavior and hear that the engine speed isn't changing to match what the tach shows.
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Old 07-14-2018, 06:01 PM
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Default Re: Odyssey wiring Problem?

Since the PCM, gauge control module, and multiplex integrated control system are involved, try a battery cable reset to reset the multiplex system.

See this other thread regarding how-to do the battery cable reset: https://honda-tech.com/forums/honda-...ction-3224166/
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Old 07-14-2018, 06:10 PM
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Default Re: Odyssey wiring Problem?

This article explains the battery cable reset.

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Old 07-14-2018, 06:32 PM
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Default Re: Odyssey wiring Problem?

Darn. I was hopeful, but the tach still does its dance around the 3K RPM mark. I disconnected both terminals and crossed the two wires for 15 minutes. I even turned the ignition on to two (though obviously there was no power).
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Old 07-28-2018, 06:02 PM
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Default Re: Odyssey wiring Problem?

I don't suppose anyone has access to a wiring diagram showing where the tachometer gets its signal? Does it come from the TDC sensor on the forward bank of cylinders? Or from the CKP? Or some other way?
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Old 08-07-2018, 01:42 PM
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Default Re: Odyssey wiring Problem?

To bring closure to this, I was able to wire up an oscilloscope to the cam and both crank sensors and was able to compare it to a known good. In my case, the camshaft pulse did not quite align with the crankshaft pulse (in the known-good waveform they align).

The waveform from my problem Odyssey:



An important data point that pointed me the right direction (and corroborated the scope's data) was a report I found for a completely different vehicle that had a "twitchy rev counter" and ultimately had a mechanical misalignment issue.

Partial disassembly in my case revealed the forward camshaft was off by one tooth. That explains both the misfires and the strange tachometer behavior.

Just in case anyone else runs across this issue.



Cam and crank alignment.
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Old 08-07-2018, 03:26 PM
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Default Re: Odyssey wiring Problem?

Thank you for posting your resolution.
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