Intermittant Weak CMP Signal

Keith Browning · March 2, 2013

Here is a screenshot of the oscilloscope showing the CMP signal in red during cranking.

The truck is a 2005 E-450 with a 6.0L that has a long crank condition that varies from several seconds to as much as half a minute. It never stalls and otherwise runs perfectly.

No DTC's.

The harness was replaced about a year ago and who knows what else was done to correct this. The other dealer gave up and the customer has been living with it for at least a year.

We replaced the sensor, checked the block and sensor bore for rust and I am told it was good. The sensor circuits were load tested as were the CMP-O and CKP-O circuits between the IDM and PCM which was a Hot-Line recommendation which made no sense to me as we verified a signal issue from the sensor wiring pre-PCM.

Looking at the signal above with the low amplitude and dip of the leading wave, I sense and fear there may be an issue with the camshaft walking. Sometimes the signal flatlines completely and sometimes the signal is nice and strong.

Opinions?

Bruce Amacker · March 2, 2013

The amplitude of the CKP dips at the same time the CMP wave dips, suggesting a change in cranking speed. Can you give me peak-to-peak voltage measurements? Those are very important, too.

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Bruce Amacker · March 2, 2013

Here's some caps I have from quite a while ago. I "think" the voltage measurements on the sides are correct, but don't bet money on them. I remember having some troubles with scaling the voltages a while back.

On the cranking waveform I'm seeing about a volt P2P on the CMP and 20v on the CKP, which is bogus. I think the CKP decimal should have been one more place to the left giving 2v.

On the idling waveform, I'm seeing about 2.9v P2P on the CMP and 4v on the CKP which sounds right. By the overlay it looks like I "fixed" the capture on the CKP years back. I have the scope set on DC coupled, which means I am showing both the AC and DC portions of the waveform. I might have been better off selecting AC coupling for an AC waveform. The DTS website compresses the pics making the voltages harder to read, if you (or anyone) wants the originals e-mail me.

:grin:

Bruce Amacker · March 2, 2013

Is it an ambulance with battery shutoffs?

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Jim Warman · March 2, 2013

If there is ever any doubt about a circuit, I find it handy to load test the circuit normally and then check the volt drop across the load (being careful to check system voltage at the same time).

A lamp may appear to illuminate sufficiently but still hide a significant volt drop in the circuit - something that could render a VRS wave undecipherable - even in a biased application.

I am assuming that SYNC or FICM_SYNC is taking time to toggle "yes"? How about FUELPW?

Enquiring minds want to know...