Jolt, when shifting from Park to Reverse (TH 400)

[Ruedi K.]

Hello together,

I am experiencing a problem with the turbo hydramatic 400 automatic transmission in my car. (1970 LS5 Corvette).

Whenever I shift from P to reverse or from reverse to 3 I notice a jolt. It's like a punch and you can really feel it.

Could this be the modulator of the transmission? However, the car shifts nicely when I drive it. This is only when I do not move and put the tranny in reverse. Idle speed is at 550 RPM.

Before I order any parts I would like to ask you for advice, what could cause this problem? Transmission had been rebuilt some years ago. Maybe somebody has had the same issue?

Any help is really appreciated.

Thank you

Best regards from Switzerland

Ruedi

[Mark E.]

Re: Jolt, when shifting from Park to Reverse (TH 400)

I'm not an automatic tranny expert, but a "jolt" when shifting into reverse or drive might also be caused by too much drive train lash. So check for worn u-joints, differential, yoke during your trouble shooting.

Regarding the tranny, a good service manual should have a symptom/trouble shooting guide that might provide insight.

Are there many TH400 shops in Switzerland?

[Mark E.]

Re: Jolt, when shifting from Park to Reverse (TH 400)

You might check the motor mounts also.

[Paul D.]

Re: Jolt, when shifting from Park to Reverse (TH 400)

Most likely source of this type complaint is the pinion mount cushions. Check them for looseness or deterioration. Chip.

[Mark E.]

Re: Jolt, when shifting from Park to Reverse (TH 400)

Good catch. Yes, check the front differential mount also. This and other drive train lash possibilities mentioned can cause a clunking.

But from your description I'm not sure if you're hearing the drive train clunking or if the transmission is just shifting into gear too harshly. Knowing the difference will help you trouble shoot this.

[Ray G.]

Re: Jolt, when shifting from Park to Reverse (TH 400)

Hello;
Idle speed correct ?
Ray

[Patrick H.]

Re: Jolt, when shifting from Park to Reverse (TH 400)

I assume you've check the fluid level as well? I've seen this with low fluid.
Just a thought in addition to what has already been said.

[Steven B.]

Re: Jolt, when shifting from Park to Reverse (TH 400)

How does the shift into reverse or drive compare to the 1-2 and 2-3 up-shifts? A shift kit can make some of these shifts seem harsh along with a torque converter that has a low rpm stall speed. In actuality, a firm shift is more desirable than a soft shift. Soft shifts are OK for the family car where you don't want to give everyone whiplash on an up-shift, but for a performance car, a firm shift is more desirable. Smooth shifting is generally accomplished by easing clutch engagement, accomplished by allowing some initial clutch slippage when the pistons apply. The compromise is that frictional heat is generated that ultimately breaks down internal components sooner. If your "jolt" includes the car wanting to move while the brake is applied, it sounds like firm engagement, and if you are experiencing something else (especially if your up-shifts are not firm), the problem very well could be some of the components mentioned earlier such as the mounts or some slop in the drive train. Others may have additional thoughts.

Steve

[Russ S.]

Re: Jolt, when shifting from Park to Reverse (TH 400)

When I purchased my 73 new with a turbo 400 it shifted from park to reverse or from neutral to drive with a jolt and still does. Unless your talking about a severe jolt I would say it is probably normal.

[Ruedi K.]

Re: Jolt, when shifting from Park to Reverse (TH 400)

Hello together,

Thanks for all the replies. There are no shops specifically for the TH 400 transmission here in Switzerland. I will tackle this next winter and will check the points you mentioned. It drives well, the jolt is not that hard. Checked fluid level and was as it should be. Motor mounts are new. Shifts quite softely when driving.

Now looking forward to the next meet. Had enough rain so far, here in Switzerland

Thank you

Have a good week

Rudy