2004 Century Erratic shifing after warm up

[MWelge]

I have 2004 Century. I had just did a head gasket and front seal replacement. Ever since, I have had an issue after the car goes about 5 miles or so my speedometer bounces between 10-40 mph and then the transmission starts shifting up and down. I have replaced the vehicle speed sensor by the passenger side cv axle. I have also tried a used instrument cluster. Any help would be appreciated.

 

[MelsRegal]

I have 2004 Century. I had just did a head gasket and front seal replacement. Ever since, I have had an issue after the car goes about 5 miles or so my speedometer bounces between 10-40 mph and then the transmission starts shifting up and down. I have replaced the vehicle speed sensor by the passenger side cv axle. I have also tried a used instrument cluster. Any help would be appreciated.

Check the wire harness going to the sensor.

 

[RD_Atlanta]

Definitely agree with MelsRegal, Almost all of the shifting tables in the PCM program relies on an accurate MPH. It's imperative that you get the MPH signal stable to prevent damaging the 4T65e transmission.

 

[02LTD2.0]

Check throttle position sensor for smooth operation throughout its travel. It's just a rheostat so you can check the resistance with a volt meter.

 

[MWelge]

Check the wire harness going to the sensor.

Does that signal wire go to the transmission first or the cluster and what connector is it so I can check continuity?
Thanks again for your help

 

[RD_Atlanta]

Does that signal wire go to the transmission first or the cluster and what connector is it so I can check continuity?
Thanks again for your he

The vss (vehicle speed sensor) originates in the 4t65e transmission to the P04 PCM. The fact that the mph became erratic & the transmission shifting also became erratic when you performed the recent repairs implies the core cause is likely in the vss signal from the transmission to the PCM. This is a pulse signal that gets converted to a rate (mph) in the PCM, so it's a little difficult to check. As was previously suggested, examine the wire harness and connectors from the transmission to include all ground points in & around the area you were working to include possible pinch points in the harness. One thing is certain, the PCM is seeing an erratic vss. It's certainly possible the vss sensor has developed a problem at the same time you did your recent work but I would first exhaust potential vss connection issues between the 4t65e and the PCM to include any nearby grounding points.

As for the throttle position sensor, it is true that the transmission shift points are effected almost exclusively by throttle position & vehicle speed. But since the speedometer is also behaving erratic suggest to me the vss is the likely reference causing the issue. It would be more convenient if it were the throttle position because as previously well stated, it's the easier of the two references to check.

In summary, I think your down to three strong suspects: the 4t65e internal vss sensor, the harness/connectors (in between) / grounds or the PCM. I think that I would first closely examine the harness, connectors & grounds in and around where you were working.

 

[MWelge]

Thanks for your help. I remember as I was putting the car back together that the power steering hose was giving some issues on the connector of the speed sensor. I did not think it was that bad. After your suggestions and that remembering that, I replaced the connector and am not having any issues after a long test drive. Thanks again

 

[RD_Atlanta]

Perhaps consider doing what I do, visit the site once a day to view the new posts, one's knowledge will certainly increase over time. There's lots of knowledgeable people sharing. We even have a knowledgeable Martian on the site sharing knowledge.