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At fwy speeds does crankshaft continue to spin at same speed when auto trans shifted to neutral?

Greetings Car Talk people! I am trying to isolate a vibration that has developed when I reach 40-45 mph and dies out after 50-55. New carrier bearing seems to have helped, but not entirely. The torque converter was recently changed as it was cracked and leaking tranny fluid. The vibration existed before and after the swap, so the torque converter is likely not the source. The torque converter locks up on this model at 40 mph, but I’m thinking it must not be locked when cruising in neutral above 40mph, else the engine would continue at high rpm rather than slow to idle speed, so any torque spinning the driveshaft when cruising at highway speed in neutral would have to come from the differential. This is where my knowledge is deficient. Can anyone shed some light on this for me? At 45mph, say, does the driveshaft continue to spin at the same speed when the tranny is shifted to neutral and the vehicle maintains the 45 mph, say like on a slight downslope, or does it slow appreciatively? Knowing which behavior takes place would allow me to rule the driveshaft in or out as a source of the vibration, as the the vibration continues when shifting into neutral at 45mph. As in the past, looking forward to the benefit of the collective wisdom of the Car Talk community.

Jack
87 Dakota V6 auto trans

The driveshaft will rotate at the speed the differential input shaft rotates.

So yes, if you shift the transmission into neutral, and the driveshaft is the source of the vibration, the vibration can still occur.

Tester

When you shift into neutral the tranny disconnects the engine from the driveshaft internally. The inertia of the vehicle does not pull the engine along. The engine winds down to (in an '87) idle speed if allowed to.

If the vibration continues without change when at speed and shifted into neutral, it’s coming from either the driveshaft, a U-joint, the differential, or the tranny output shaft.
OR a wheel or tire. Have you tried moving the wheels around to see if moving the rears to the front changes the vibration? It’s free and you might get lucky.

Really? Free? Where? I relocated 200 miles from where I bought the tires, so going there is not an option. Hmm… about three months ago I changed the front pads and switched the tires front to rear while I was at it. Can seem to remember if the vibration was happening before that. Will any tire shop do this for free?

In your driveway.
You mean to tell me that you can’t rotate your own wheels?
If you can’t even do this, you might as well just take the truck to a shop and let them diagnose and repair it.

You never mentioned that you had already switched the wheels front to rear. Had you done so, I would not have suggested it.

1 Like

Haha… oh, you mean THAT kind if free.

When you replaced the carrier bearing, did you re-assemble the driveshaft correctly, are the u-joints properly lined up?

@Jack_Dak87 Is this the same vehicle that you could not find someone to balance the drive shaft ?

You refer to both what the driveshaft and the crankshaft do when you shift to neutral and presumably let up on the gas and start coasting at 45 mph. The crankshaft will continue to rotate, but not at the cruising rpm of maybe 2000 rpm. Instead it will drop to the engine idle rpm of around 800 rpm. The driveshaft will continue to turn at the same rpm it was before you shifted into neutral. It will gradually slow down as the vehicle speed slows during the coast. If the vibration continues the same as before when you shift to neutral and start to coast, you can pretty reliably eliminate the engine/crankshaft as the cause. But that wouldn’t necessarily mean the problem was the driveshaft. Could be that, or the differential, an axle shaft, or wheel/tire problems. It’s pretty common when diy’ers work on their driveshaft to forget before removing it to place witness marks on the shaft and flanges it attaches to so they can install it in the same configuration. That can result in a vibration. Has the driveshaft ever been removed and re-installed?