Tach/Cruise Issue

[TFucili]

Lately I have noticed that with the cruise set, if I get to a downgrade where the cruise backs off of the throttle, the tach will bounce around about a 1000 RPM range of what is actual. As soon as it picks up a bit of throttle the problem goes away. I have only seen it happen with the cruise set. Any ideas? Truck is an '05 auto.

 

[rbattelle]

The engine isn't actually swinging RPM around, right?

I believe the ECM feeds the tachometer, so that might imply an ECM problem. But why only on the decel? That's weird. Maybe there's a loose wire that gets looser when decelerating, then tightens up again on acceleration? But if that were the case, I'd expect the same behavior even without the cruise on.

I think your best bet at this point is to just wait it out and see if it gets worse.

Ryan

 

[TFucili]

The engine isn't actually swinging RPM around, right?



I believe the ECM feeds the tachometer, so that might imply an ECM problem. But why only on the decel? That's weird. Maybe there's a loose wire that gets looser when decelerating, then tightens up again on acceleration? But if that were the case, I'd expect the same behavior even without the cruise on.



I think your best bet at this point is to just wait it out and see if it gets worse.



Ryan

Yes, wait and see has been my approach, but weird behavior like that makes me think I have a bigger problem brewing. Plus, my local dealer was Obamacized, so if I need a dealer, it is a hassle now.

 

[gordon.rhodes]

I had my tach (instrument cluster) go bad at about 60,000 miles. Now I am facing the same problem at 109,000 miles. When the engine is at idle, the engine RPM should be around 750. When I am at idle my tach will read anywhere from 500 to 750 RPM. Sometimes the reading will drop even lower. I have used a hand tach and verify that the engine RPM is 750. Dodge has offered to replace the instrument cluster as they did before, but this time they also agreed to take a small cash donation of 600 dollars. Has anyone else had this sort of problem before? The ECM does control most of the engine functions. But the RPM is picked up by a MPU if I am not mistaken. I do not want to spend 600 dollars for a new instrument cluster if I really do not need one.



Gordon

 

[Missouri Mule]

I believe your cam position sensor reads rpm unless you still have a crank position sensor on the engine then that will send a signal to the ECM.

 

[JFought]

its offical. they replaced my cluster under the extended warranty> 71K now everything is reading correct... .