05 Ultra engine light came on but goes out. code is PO374-CKP sensor synch error.

Engine used to start on first revolution now it cranks over a severeal times before it starts. What does code really mean? What needs to be done to correct it? Take CKP sensor out and clean it, replace it? Is there an ohm reading for the sensor to see if it is the problem?

 

before replacing it, I would clean it, put it back in, coat the pins in the electrical connector with a good coat of dieletric grease as well as the ECM pins. Clear your codes then try it again.

 

To test this sensor...
The CKP sensor has a B+ power supply, a ground, and a signal circuit.

As the crankshaft rotates, the wheel teeth interrupt a magnetic field produced by a magnet within the sensor. The sensor's internal circuitry detects this and produces a signal which the ECM reads. The ECM uses this signal to accurately measure crankshaft velocity which is a variable used in order to detect misfire, spark and, fueling.

Looking at your service manual find the wire that goes to the B+ (battery) Measure from this wire to ground for the DC supply voltage. Measurement should be taken on the sensor side of the connector. Ground connection as well.

Do this test at rest and also with the engine spinning. A loose connection could break as the motor shakes. There was an upgrade to the sensor in January 2004 available . As P&A inventory is depleted all sensor part numbers will be superseded to an "A"
suffix.
If you have voltage present with the key on and with the engine spinning move the positive meter lead to the Signal wire. This is usually marked R for Red . With the engine spinning you should see a voltage signal on that wire.

You should measure the signal at the sensor connector and the ECM to verify it is reaching the ECM.

 

Peppi thank you for the info. Will do the checks you suggested as soon as I can. House full of family today can't get to garage.

 

Have 1.5v on both sides of the plug same with engine turning over and 950 ohms for the sensor.

Going to get a new sensor. We had the same starting problems on trucks with CKP sensor just before it would not start at all.

 

that peppi.. smart man!

 

Sync is short for the word synchronization. Synchronization is keeping "step" between the physical position of the flywheel and where the electronics or ECM THINKS where the flywheel is.
If you were to look at a late model crankshaft, there are notches or slots in the flywheel around the circumference of one of the wheels. Along the way there is a "missing" tooth or large gap between the notches. The crank sensor sees those notches as ferrous (magnetic) metal density.
The Large gap indicates TDC for the number 1 cylinder and the other notches are position markers for the flywheel as it turns. The firmware in the ECM knows what the count should be from when it sees a #1 sync position (large gap) to when that large gap comes back around.

Each time it sees the large gap,, it is "sync-ed" to the number one piston. If the count of the smaller steps changes each time around,, the flywheel actually looses "sync" with where the ECM "thinks" it is.

The ECM must ALWAYS know exactly what position the flywheel is in order to fire the injectors and spark at the proper time. Remember that the large gap is for the #1 cylinder. The spark is fired for the #2 cylinder by counting the "close notches" in the flywheel. If it does not sync to the large gap, the number 2 cylinder will fire at the wrong time also since it is counted from the #1 starting point.

If this does not help, let me know.

 

Jackie999 thanks, now I understand how CKP sensor works and from all the testing that I was able to do think the sensor is the problem. As soon as I have time will get another one from dealer.

 

Jackie999 really know his stuff!

Glad to have you here.

 

does anyone know ohm value of 98 ultra crankshaft sensor//