Tiptronic Shift Problem (996)

[Shmockiebaby]

Good evening,

I have a 1999 C2 with tiptronic. Dealer serviced, has had it's required fluid service done. 120K miles, runs great Except for one recent hiccup.....

It will not shift up to 5th gear about 50% of the time. When it does this, if I move the lever to manual and use the tiptronic switch on the wheel (either one), if shifts up just fine. Put it back in auto, and and it's back to normal. It may do it the next time it downshifts, or it may not do it again for a hour.

No codes, no lights. It must be electronic, because it easily shifts with the tip switches just fine. And it will go back to normal operation for a while, and then act up again.

It downshifts normally when slowing down, accelerating, etc. It holds a gear in corners or under acceleration, like it should. Upshifts into all other gears normally, under both hard and light acceleration - has never hung up in any other gear, except the 4-5 shift.

Had the battery disconnected for a day during an oil filler tube replacement. At first I thought it was re-learning, so I let it hang in 5th for several miles on very light throttle at 3500 rpm, but it wouldn't upshift. It did seem to start at about this time, or shortly after.

I have reviewed the shop manual several times, and I cannot seem to figure out what would cause this without causing problems on other gears (kick down switch, TPS, shift position switch, etc).

Any ideas? HAs anyone seen this before? Thanks in advance for any suggestions.

[Shmockiebaby]

I should have been more clear - upshift only. It will downshift out of 5th appropriately.

[Shmockiebaby]

I have noticed that no one replies to any Tiptronic issues on any of the Porsche tech forums. Apparently no one has the knowledge. Neither does my dealer. They have suggested I go to an independent. That shop has said they cannot read the ECU with their tool.

So I have decided to become an expert in all things Tiptronic. I will post what I have learned (and am continuing to learn) as I diagnose and repair this issue.

I am a ASE Certified Master Diagnostic tech. But I do not practice my skills anymore. I am now in a different area of the auto and heavy truck industry. But I do have the background to know where to start.

After the independent shop said they could not read the ECU, I decided to buy Durametric "enthusiast" version cable and software. If you are wondering, it works great on a MS Surface Pro II via USB. It DOES NOT give you actual values for the Tip - although it give actual, live values for the engine. Very useful.

It tells me that I had P0740 code, for converter lock up. This confirms what the dealer told me. But they told me two possible resolutions - replace converter, or replace ECU ("now go away, please!").

I studied the manual carefully. What signals the Tip ECU to downshift? What tells it that it is OK to upshift? What tells it to hold a lower gear in a corner? What will prevent lock up of the converter?

The dealer had checked the fluid at temp, and it was full, so I ruled that out.

Throttle pedal switch would prevent any upshift til redline. Ruled this out. Throttle position switch (on throttle body) could stick - replaced it, no change. There have been report of intermittent brake light switch issues with these cars. OK, let's try that.

Brake light switch completely cured the upshift to 5th gear under moderate acceleration in a straight line!! Viola! And inexpensive. But it would still hang in a lower gear after a hard acceleration or downshift, especially in a turn.

I did notice that it may upshift after a left turn, but not after a right turn. Hmmm. How does the Tip know that you are in a turn, and that it should hold a shift? Wheel speed sensors - the same ones that work with the ABS ECU. Filed that away to investigate.

And then, while cutting the grass, I noticed that my right rear tire looked low. I checked it - about `18 PSI. Very low. I inflated it to proper level, cleared code, and test drove.

Yup. Problem solved. Wheel speed difference told the Tip ECU that it was in a corner, and it would hold a downshift.

I remembered from my days working with Toyota's that their earliest low pressure system worked the same way - no sensors in the wheels, just a look at wheel speed, to see if a tire was low.

So, for thru price of an LOF at the dealer, I own the Durametric, and I fixed the problem in the trans. Easily.

I did all of my brakes, plugs, modifications, other stuff, and just left certain work to the dealer. I think I will be doing it all myself now, since they basically told me to pound sand and didn't bother to do a basic diagnosis.

I hope this info helps someone else! Take care.

[zelrik911]

This is EXACTLY why I love this Forum!!!

I dont have a Tiptronic, but I may buy one in the future if my SC gets too hard to handle (I am 67 yo). So these tips are going into my memory bank before I start to forget everything.

Dealers these days seem to have a very narrow experience/skill base - is this because they have sacked all the older & wiser techs and rely on software? Or have these experienced guys gone off to do their own thing as Independents (or retirees)?

[otto_kretschmer]

you have an intermittent problem, aka a ghost

I like mechanical watches, carburetors, pushrods and hydraulic lifters

too bad Werner Heisenberg isn't with us anymore

not knowing anything of the system, I would look for a problem in the wiring harness

any crimped wires, anything that looks like its gotten hot and cooked or burned

looked for loose connectors and clean the grounds

I wish I could give you something more specific but I've tried to stay away from cars with overly complicated electrical systems

[Shmockiebaby]

Just about everyone, at just about every manufacturer, have quit servicing auto trans altogether. They just install a new or reman unit. There are very few who have disassembled a rebuilt one.

The experienced guys have retired. In my opinion, they did not even rely on the software. They printed the diagnostic tree out of the manual, and attached it to my RO. They did not even bother to think it thru.

Having been inside many ATMs (and rebuilt several) myself in the past, I do not find it as intimidating. But I can understand why a tech who has never seen the innards of one would be confused. But really, this diagnosis was all external to the trans. Just working through the clues.

They always (supposedly) check tire pressure, tread depth, and brake pad wear - I have yet to go back and re-read the sheet they attached to the RO two weeks ago.....

[Shmockiebaby]

Not intermittent, and not a ghost. I've dealt with those. This was easily repeated, easily duplicated, and when chased down, easily diagnosed and repaired.

Intermittent and NPF ("no problem found") are understandable frustrations for the tech, even the most skilled tech. But the dealer just did not try - they sent me packing.

Re-read the post - I have diagnosed and repaired the problem. But thanks for the info. Wiring checks out - sensors and solenoids in electronically controlled trans is at pretty low voltages, so burned wires are a thing of the past. Fortunately, I was able to sort it out before I had to pull out the multi-meter and start checking voltages and values on the sensors.

Thanks for your input.

[Misamai]

i can't shift up anymore in manual from steering wheel.
down shift fine on steering wheel.

up and down shift fine in auto.