69 thm 400 cruise rpm question after rebuild

[Brad H.]

Guys,
I have a numbers matching 69 350/300 attached to a TH400. I just had the transmission rebuilt by a very reputable transmission company and I am experiencing a difference in performance. I did ask the shop owner if he could firm the shifts up just a little. At a constant 60mph cruise the engine must produce about 2-300 rpm more than before the rebuild. The increase in rpm is greater at higher speed. The shifts did firm up a bit, but I don't care for the higher cruise rpm. I have read that the modulator can be adjusted to change the shift points some, but would any modulator adjustment bring the cruise rpm down?
I hope some of the Automatic experts out there may have some ideas.
Thanks for your help,
Brad

[John H.]

Re: 69 thm 400 cruise rpm question after rebuild

Guys,
I have a numbers matching 69 350/300 attached to a TH400. I just had the transmission rebuilt by a very reputable transmission company and I am experiencing a difference in performance. I did ask the shop owner if he could firm the shifts up just a little. At a constant 60mph cruise the engine must produce about 2-300 rpm more than before the rebuild. The increase in rpm is greater at higher speed. The shifts did firm up a bit, but I don't care for the higher cruise rpm. I have read that the modulator can be adjusted to change the shift points some, but would any modulator adjustment bring the cruise rpm down?
I hope some of the Automatic experts out there may have some ideas.
Thanks for your help,
Brad

Brad -

I'm not an automatic guy, but I know enough about them to know that the modulator or a "shift kit" won't affect cruise rpm at any given speed; that can only result from slippage or a looser torque converter. Did the trans guy modify the converter or install a "high-stall" converter?

[Brad H.]

Re: 69 thm 400 cruise rpm question after rebuild

John,
It supposedly the same torque converter. I will check on that tomorrow.
thanks

[Brad H.]

Re: 69 thm 400 cruise rpm question after rebuild

John,
I did briefly describe this problem with the shop owner. He told me the modulator could be adjusted to help this condition. I posted this because I didn't believe this to be the case, but was not enough of an expert to know for sure.

[William C.]

Re: 69 thm 400 cruise rpm question after rebuild

If you are in top gear, slippage internal to the trans itself should not be an issue if properly rebuilt, that leaves two options, the incorrect drive gear for the speedo was installed (have you verified indicated speed with milepost/stopwatch @ 60MPH?) or the trans/converter has excessive slippage.

[Brad H.]

Re: 69 thm 400 cruise rpm question after rebuild

Bill,
I am comparing apples to apples. Prior to the transmission rebuild with the same speedometer the engine rpm was about 300 rpm slower at 60mph than it does now with the same same transmission rebuilt. Is there any validity to the statement that the "modulator" can be adjusted. I suppose that the modulator/ shift kit could have pushed the top gear shift point up such that the trans has not actually shifted into top gear. I'm trying to get some information before I go back to the trans shop.
Thanks very much

[William C.]

Re: 69 thm 400 cruise rpm question after rebuild

OK, we are talking about a turbo 400 trans here, which has only three speeds in the trans, all of which are a fixed gear multiplication when the clutches are applied. The modulator affects the pressure of the application, but once they are fully applied should provide no measurable slippage, thus the speeds you had prior to the rebuild at a given RPM shoul equal the before rebuild speeds exactly with respect to the trans itself. The torque converter is essentially a torque-multiplying fluid clutch which has an internal slippage by design, but that slippage should be minimal at a speed above the "lockup" point Say 2% for an approximation. You look to be seeing numbers in the 10% range which is WAY too much, and should be generating noticible heat buildup in the trans. NOT good! I'd be looking for a MUCH better answer from the trans rebuilder, starting with did he replalce the original Convertor, and if so does he still have it and is there a satisfactory explaination as to why? Do you have a temp gauge on the trans? I'm assuming no changes to the rear gear... or the speedo driven gear.

[John D.]

Re: 69 thm 400 cruise rpm question after rebuild

It may be a good idea to check the trans fluid. If there's slippage you may see clutch material and notice a burning smell...

[Brad H.]

Re: 69 thm 400 cruise rpm question after rebuild

Thanks guys
Brad

[William C.]

Re: 69 thm 400 cruise rpm question after rebuild

One other thought, if the trans had a complete rebuild, did they put an incorrect speedo gear in it? Is your speedo vs tach still correlating the same as before at a given speed?

[Brad H.]

Re: 69 thm 400 cruise rpm question after rebuild

Good idea Bill, I'll check on that as well.
Thanks a million!
Brad