rexpontius

Dear all,

first of all I will give a short introduction:
My name is Michel rexpontius and I Just bought my first Porsche.
I also have three Alfa Romeos (156 GTA, 164 Q4 an 164 QV).
In the past years I have gained a lot of expertise on the Alfa v6 24v engines, but always wanted to have a Porsche 928.

So yesterday I finally did it and bougt me a Black 1986 S 32v imported from the USA with 94k Miles on the clock.

It needs some TLC but I am quite confident I will be able to tackle most problems.

Besides many common problems (electric windows not working, seat adjustments not working, etc) there is a larger problem with the auto gearbox.

It will start in 2nd gear and when it shifts to 3rd it is very brusk, with a massive clunk through the driveline.

There are no strange noises whatsoever.

Before running of to get a 2nd hand transmission I was wondering if there are some things like oil / oilfilter / etc I should check, which can cause this kind of behaviour?


All help is very much appreciated

Richter12x2

Sounds like the vacuum line going to your transmission is disconnected.




(ignore the green circle, the numbers and letters should match the first picture)

If you remove the airbox where the airfilter lives (worm clamp underneath, and a nut on either side holding it down once the intake tubes are off) you should find the vacuum manifold near the air inlet.

rexpontius

Would it be such a simple problem???

Does this vacuum line go from inlet manifold of the engine to the gearbox?

I have litte experience with auto gearboxes, could you briefly explain why it would need a vacuum?

Thanks

JHowell37

+1 on the vacuum line.

You'll want to check the vacuum line. Pay close attention at the front where it comes down from the back of the engine. It connects to a steel line that runs along the length of the torque tube. Then at the rear, there's a rubber vacuum hose that connects at the back of the steel tube and goes to the vacuum modulator. The front vacuum hose is hard plastic that can collapse over time with exposure to heat. If it does this, it will seal itself so it won't be readily apparent that you have a vacuum issue because the engine won't run any different.

You should definitely change the transmission fluid and filter.

Richter12x2

This is one of the only auto gearboxes I've seen that's this way, but it uses vacuum pressure and a modulator on the transmission to soften up the shift engagement. There's a neat little adjuster on the side of the transmission, but unless you've had work done on it, I wouldn't expect it to be out of adjustment.

Congrats on the new 928, by the way!

My '85 had the opposite problem, it would run up on every shift before engaging, unless I was at WOT. (You know how fast you're going when it shifts 2-3 at WOT? It almost never happens on normal driving!) I was looking for a new transmission too when SeanR said "Try your valve" I gave it a couple of turns and now it shifts almost like my Steptronic 540i. The previous owner had the transmission serviced by someone who didn't really understand 928s though, or didn't finish adjusting it when I picked it up from their shop, anyway.

Anyways, if you search for Harsh Shifts, you'll find that problem covered in detail. Check for a disconnected or leaking vacuum line first. Even if you replace all the vacuum lines (not a bad idea for a 24 year old car!) you're looking at maybe $10 in parts. I tested all mine with a Mityvac and found 2 bad connections, too, but now I KNOW my vacuum is perfect.

Also, if you have a vacuum leak because of a disconnected line, it's going to affect your air/mix and fuel pressure as well, so there's horsepower on the table if it's running loose.

Richter12x2

Oh, I also edited and slid the diagrams in for you. I had them from when I was asking another question before I did my whole vacuum line refit. You're looking for E, to go back to your transmission.

Also, once it's running well, Porken's chips with a 1987 fuel pressure regulator are the best money you will spend for an '85/'86 car!

rexpontius

Thanks for all input

I will check it first thing this weekend, along with oil + filter.

Is there any specific procedure for the gearbox oil change?

I used the product advisor at www.kroon-oil.com where they recomment ATF ATF Dexron II-D 7,3 liters.

Would the vacuum problem also explain why the car takes off in 2nd gear?

The engine runs great, but taking off in 2nd gear makes it feel like a small engined car :-)

Richter12x2

It's because of all the horsepower that it wants to start in 2nd. According to Roger (a lot of guys around here call the Auto transmissions "Rogerboxes" because of him.) If you shift the gear selector down to 2, then shift to 3, and back to 2, it will kickdown to 1, then up to 2 and shift normally as you move the stick from then on. If you leave it in drive as we're all taught to do, it will start in 2.

Landseer

Remember, the 86 should always start in first.
Needs its bowden cable tightened to keep it from instantly going into second.
That adjustment occurs at the cable connections all the way forward at the radiator.

G Man

Hi and welcome. I was having the clunk you describe with the trans in my 84'. My vac lines going to the modulator were all good. I used a vac testor on the modulator and found it would not hold vacume. A new modulator solved the issue for me. There is a procedure for adjusting the modulator pressure but it takes some special gauges. I just adjusted mine by the "seat of the pants" feel. Not sure about 86' modles but on my 84' I found it necessary to drop the back part of the exhaust system in order to remove the heat shield and gain access to the modulator. I reinstalled the exhaust minus the heat shield so I could make adjustments to the modulator until I was happy with the shifting. Once I was satisfied with how the trans shifted I dropped the exhaust and reinstalled the heat shield followed by the exhaust. Sounds labor intensive but I can get the rear section of the exhaust out in about 10 min once the car is in the air. As far as starting in second gear, that is the the norm for mine. Not sure about the 86'. Plenty of info on this site about that topic. And just in case you don't know, do not remove the cap from the trans resivour with the engine not running. You will have a lake of ATF on the floor if you do. The level is checked with the engine running. Good luck!

rexpontius

Wow, that is incredibly cool :-)

Have driven the car not more than 10 kilometers, yet so have lots to discover about it.

Found many postings regarding auto. transmission when searching this forum, but could not find a posting explaining the correct procedure for changing trans fluid + filter.

Could you point me in the right direction?

Will certainly post some pict when I start working on the car this weekend.

G Man

This may help some.

http://www.nichols.nu/tip275.htm

Landseer

Jim, how did you set the bowden cable?

When you are driving at 65 mph or so in 4th, and you hit the gas hard, will it downshift at about 1/2 to 2/3 throttle, then do a second downshift if the pedal hits the kickdown switch on the floor?

Tampa 928s

My 85 Euro had a rubber line connected to a metal vacuum line running down the firewall hooked to a 2 inch rubber vacuum line that was hooked to the rear metal vacuum line. I found the hose above the passenger side heat shield rotted. That took care of the hard shifting, look for it or you may have just a rubber hose connected to the solid line only, use a miti-vac to test it.

G Man

Landseer. I know I will get a downshift as you describe, Not sure about it dropping into second. I'm pretty sure it does. I have the cable adjusted just to the point of no slack in the line.