Transmission problem

[jmead]

Alright...got my car running smoothly, now it turns out I've got a transmission problem.

It turns out the vacuum line that connects to the manifold was loose, and leaking vacuum, causing the transmission to lurch at certain speeds depending on the throttle position.

I re-attached the hose securely, and the car is much smoother, but it will still ocasionally lurch around during a shift. This is mainly at high speeds (45+). What more can be done to fix this? Is it possible the hose itself is leaking? What other parts could cause this to happen? I've read something about a "vacuum modulator" in the tranny. What does it do?

I've got a 76 280z, auto tranny, stock. Thanks for any input

[kmack]

How many miles are on the car/transmission?

The lurching could be worn friction plates that need to be replaced. I would also suggest to at least do a full fluid flush and filter replacement.

[jmead]

The engine and transmission both have about 160k, no rebuilds or replacements that I know of.

So you think it could be pressure plates? I know for a fact that the lurching is related to the vacuum signal the tranny is getting. Are the pressure plates "activated" by this signal? (I'm sorry, I don't know very much about the internal workings of an auto tranny)

How difficult is this replacement?

[kmack]

I would try to have the tranny flushed first. It should run about $100, but it's cheaper than rebuilding if it will fix the problem. A good flush will include flushing the torque converter. Basically, they will need to run fresh fluid through one of the cooling lines (at the radiator, and while the engine is running) until fresh fluid comes out the other cooling line. This will clean almost all the crap and build-up out of the tranny, which may help.

The vacuum signal could be just exaggerating the problem. When the pressure and friction plates wear, the tranny will slip while engaged. The lurching sounds like the beginnings of slipage.

As far as replacement, most times you can re-use the pressure plates, just scuff them up with sandpaper to get the old glaze off. The friction plates will need to be replaced with new. The cost of parts for rebuilding an auto tranny is usually not more than $200-$300 including friction plates, bearings, seals, and filter. It's the labor involved that jacks the price up.

If you're careful and pay attention to how it all comes apart, you can rebuild it yourself. It's like taking apart and putting back together a puzzle. Be careful when removing the "valve body" from the bottom side of the tranny. There are small ball bearings used as check valves that go in certain places. If they fall out and you put them in the wrong place, the tranny will not shift right if it works at all. Try to find a manual first.

Or if you are not comfortable doing the rebuild yourself, pull the tranny out of the car and take it to a transmission shop. That will save you a few hundred $$'s there.