Automatic Transmission harsh shifting.

[ecurie]

The automatic transmission ( a GM TH400) on my 1979 Rolls Royce Silver Shadow had developped some problems.
It was leaking oil and the shift from 1st to 2nd was a bit harsh, unless I released the throttle a bit.

I rebuild the transmission, which went well.
All the leaks have dissapeared (apart from a small one, where the alloy sump has been overtightened and which will be repaired) and when cold the shifts between gears are unnoticeable.
However, after some time, with the engine and transmission warmed up, the shift from 1st to 2nd becomes harsh again, unless I give it full throttle.
The shift between 2nd and 3rd is always perfectly smooth, whatever engine/transmission temperature is.

Every suggestion is welcome.

[kevin]

The Borg Warner 35 box in my Singer is very sensitive to the fluid level being spot on, if it drops slightly low then it becomes 'grabby'.
Have you tried flushing the system with fresh fluid a few times? on mine, the torque converter fluid cannot be drained so relies on being flushed through.

Kevin

[Flatlander]

The automatic transmission ( a GM TH400) on my 1979 Rolls Royce Silver Shadow had developped some problems.
It was leaking oil and the shift from 1st to 2nd was a bit harsh, unless I released the throttle a bit.

I rebuild the transmission, which went well.
All the leaks have dissapeared (apart from a small one, where the alloy sump has been overtightened and which will be repaired) and when cold the shifts between gears are unnoticeable.
However, after some time, with the engine and transmission warmed up, the shift from 1st to 2nd becomes harsh again, unless I give it full throttle.
The shift between 2nd and 3rd is always perfectly smooth, whatever engine/transmission temperature is.

Every suggestion is welcome.

You wrote that you rebuilt the transmission yourself. Did you by any chance use an aftermarket kit from the US? List the parts that you used.

[alabbasi]

Sounds like a vacuum issue. Is the vacuum hose to the vacuum modulator secured properly?

[ecurie]

Thanks everyone for the help.

Kevin,
I checked the fluid level on several occasions and it was always spot on.
During the rebuild I tried to drain the torque converter as good as I could but you just can't really empty it. So it might be an idea to flush the whole system. Certainly worth a try.

The rebuild kit came from Flying Spares, the leading specialist for Rolls Royce and Bentley spares, so I guess the quality of the kit is good.

I already checked the vacuum modulator as I too felt this might be the likely culprit. The vacuum hose is wel attached and there is no transmission fluid on the inside.
I feel that the car is upshifting quite late and kickdown comes too soon even when I adjusted the vacuum modulator. I don't know if this could be related.

Benoit

[kevin]

The torque converter on my box cannot be drained either so I just changed the fluid (7 times I seem to remember) until it eventually ran red (colour of the clean fluid) after a run. Once clean, I changed it again for good luck.

Kev

[alabbasi]

The vacuum modulator can be adjust with a screw to tighten and soften shifts. I also think that there is a bowden cable that connects to a governer on the back of the transmission that can also adjust shifting. At least how I remember on my 73 GMC pickup truck.

[Mitsuru]

If all else has been checked you said you rebuilt the full gearbox, you did the valve-body as well?

What about the oil grade used I know that Chrysler Automatics can be extremely sensitive to the grade of oil. Put TF3 in and it doesn't like it as it was designed for tf+4 the 41te/a604 transmission.

if not that then I'm just thinking on the springs on those valves, as I replaced mine when I had the valve body out of mine.

[Flatlander]

First off, the TH400 torque converter can be drained. The plug is sometimes hard to find, but its a little square thing 50 or 60mm away from the centre shaft.

Assuming the rebuild kit was a genuine RR one, then nothing in it should be the cause of the problem. If it had been something like a TransGo kit, or other US aftermarket one, the problem would have been in one of two small holes in the valve body. The yanks quite often enlarge these to make the shifts harder. One covers the one to two shift, the other two to three.

Assuming the oil is the correct type, and at the correct level, its simply going to be a case of incorrect adjustment. If you have a manual for the car, check the specs for the gearchanges, then compare them against reality. Then use the manual to adjust everything to factory specs.

As someone stated above, all US made automatics are sensitive to the type of ATF used.

Incidentally, the TH400 is originally a GM transmission. Of the period (here I go upsetting people with an opinion!) it was without a doubt the worst of the "Big 3" automatics. Quite why RR chose it is beyond me. Its nowhere near in the same class as Chryslers A727, or even Fords C6 (which was based on the A727!).

[alabbasi]

Incidentally, the TH400 is originally a GM transmission. Of the period (here I go upsetting people with an opinion!) it was without a doubt the worst of the "Big 3" automatics. Quite why RR chose it is beyond me. Its nowhere near in the same class as Chryslers A727, or even Fords C6 (which was based on the A727!).

That's fighting talk right there. The TH350 and TH400 are fantastic transmissions , also used in the Jaguar V12. I've used a GMC or Chevy as a tow vehicle for many years, and any miles. Usually pulling a full size trailer with an S class Mercedes on top at 80mph including hills. Never worried about the transmissions.

RR and Jaguar used it becase competition was Mercedes and the TH400 was a better transmission then the 722.0.