Gearbox behaviour/TC lockup

[Jay_g]

Afternoon all,

Quick question regarding how our box 'behaves'. Something that was bought tommy attention my a friend whilst on the way home.

So cruising along in any gear, put your foot down and the revs jump about 300-400 revs? Not necessarily slip, the revs jump up , stay there and you waft along.

I've always assumed this was just the joys of having a TC, and until it "locked on" then you would always have that "slack/play"?

Question number 1) when does out TC lock up? I've always assumed it was 3-4K?

I was cruising along at 40mph in D, and just blipped the throttle, revs done the same as above and went from 2k-3k (I've got a video which a friend recorded, you get the gist.. ignore the whistling!) so is this normal?
https://vimeo.com/194049260

Now comes my second question, and I'm 99% I know the answer.

The other day, I overtook someone in 5th. About 75% throttle, revs shot up abot 2k, nothing happened, sharp bang/judder and we took off. Battery had been dead, and car not really driven for the last month so thought that may of been the end of it (yeah, you grow naive to these cars..)

Then while on the way home, the same happened. So thought I would try and have a play see if I can pinpoint it.

https://vimeo.com/194050314

Again, video isn't great but you get the point.

Just to try and explain what's happening. Car is in full manual at this point. 5th gear,
I put my foot down. Pretty quickly the revs jump up to 4K (same as issue above). For some reason the box then jumps down to 4th, and you can hear the thud it makes as it does it. Don't know why it jumps down to 4th?

Car is MRC mapped and I'm fairly sure that removes the kickdown (apart from the switch, and I was no where near that), it defo doesn't downchange in any other gear

It only does this in fifth, and if you load it up in the first half of rev range! I can do a launch 1st-5th and it's fine the whole way up the box. All other gears are fine.box was serviced last October, about 8k ago. So im pretty sure 5th is done, and the box needs a refurb, but quite a strange gear to go?
.

[bam_bam]

First of all, don't overtake from low speed in 5th, pull it down a couple of cogs on the paddles. Consider how temperamental these slushies are and apply some mechanical sympathy.

When you request some boost under certain parameters, the TC will open, helps turbos to spool, protects drivetrain and probably help with refinement. So that rise in revs is the TC opening, all good.

The slip and bang in fifth isn't good, it could be a brain fart, I never drove in "D" unless I was stuck in traffic, I'd recommend to doing this to, the TCU was always flaky with part throttle.

[Mɐʇʇ]

In addition to Bam's comments - particularly about the TC disengaging to pick up revs and get it on boost + having some mechanical sympathy, I'd recommend 2 more things.
1. Do a service on the gearbox - new oil and filter.
2. Leave it a couple of weeks.

What I learnt recently is the the TCU measures the amount of time it takes to change gear (which is affected by the condition of the oil) and adjusts the engagements in the box to be optimal. I'd noticed on a few cars when I've done the auto box oil that the gear change is quite harsh for a day or 2 after - and was educated the other day that it takes a little while for the brain to learn the viscosity & lubricity of the new oil.

Finally - the revs increasing. If you're cruising along, in say, 4th @ 2000 rpm and you gently & smoothly put your foot down - do the revs jump up - ie is the TC locking correctly? Not locking correctly is the first indicator that it's time to start saving up.

[HPsauce]

Same gearbox as the D2 S8 and similar behaviour with regards to revs rising. How many miles on it and have you ever had the fluid/filter changed?
Another current discussion among S8 owners is wear on the F125 switch. I, along with several others, have had it replaced recently and it can make a hell of a difference.

That said, serious problems in the 4th/5th range I've NEVER seen reported before.

As for using "D" all the time, why not, the TCU is supposed to respond appropriately to whatever demands you place on the transmission.

[jockthedog]

5th gear clunk is not good. The clutch pack is slipping, but the sound of it, likely due to too-low pressure in the hyraulics.

Change the oil and filter, and see how it goes, but you may need the valve body reco'd too (500 quid will get you that).

Worst case is worst case - engine out, box off, 5k+ bill. Avoid that if possible!

[bam_bam]

As for using "D" all the time, why not, the TCU is supposed to respond appropriately to whatever demands you place on the transmission.

I found "D" stood for dim-witted, after my stage 3 it just wasn't relevant, so many times I'd given it half or three quarter throttle only for it to stick to a long gear and just sit there, kinda dangerous. The manoeuvre didn't require WOT and a three gear kick down, just half throttle and one less cog, "D" could never muster that. Besides, I knew what I wanted the car to do next so why not just select the gears myself. If it was just running in traffic, I left it in dimwit, anything else, tiptro. Dimwit mode might make more sense in an S8.

[HPsauce]

Dimwit mode might make more sense in an S8.

Depends which one your gearbox is coded for. The default is a bit tedious but that's easily changed with VCDS to make it much more "driveable".

[bam_bam]

Yeah, it had a revised MRC TCU map with higher rev shift points too. The fact remained, "more drivable" still wasn't drivable enough. Tiptro for me, thanks.

[Jay_g]

Thanks peeps!!

To be honest, I never load the box up like that, which is probably why I never found it before yesterday. Before the map I would of never been able to, as the box would of dropped a gear or 2.

The oil was changed about 8k miles ago, last September timeish. Is it worth getting it done again?

I'll be honest, in my mind I'm pretty much prepared for it to be trailered off to Unit20 for recon box. For now, I'll enjoy the car and see if gets worse, and avoid loading it up in the lower revs

Shop, I only ever drive it in manual TBH, it's a little bit more "fun"!

Having a bit of a play with it, i noticed it does similar in 4th, but without the bang.

In fact boot it at about 3k, the revs rise cleanly, then all of a sudden the revs will jump up to 4.5k(about 1k sudden increase), with no real motion change and will keep pulling hard. When you let off, the revs will drop by about 1k revs back down. Almost like it's slipping a little, but still pulling? I've noticed it does the exact same thing in 5th at the same revs, except 5th you get the bang.

I'm gonna see how it goes, and maybe look into getting the valve body done as that could be a cheap fix!!

[CHRISFLEE]

I came on this forum with exactly the same issue and your post was the first one I found..!

My Wife's RS6 does a similar thing. Seems as though from 3,000rom the revs just jump if you push the pedal hard but there is no further accelaration, a bit like wheel spin in ice. At a junction the car has before now lit up all gear lights on the dash when in drive and will just rev without movement. It requires an of and on again to reset. That mechanical clunk, or a kick up the backside has been present since we bought the car in April under heavy acce;aration.

I have had the diagnostics checked and it threw up 'incorrect gear ratios'. I have been told it will only get worse and it is a gearbox rebuild or replacement. Have been quoted £3,000.

Sorry I couldn't bring good news...

[jockthedog]

Thanks peeps!!


In fact boot it at about 3k, the revs rise cleanly, then all of a sudden the revs will jump up to 4.5k(about 1k sudden increase), with no real motion change and will keep pulling hard.

It sounds like it is dropping a gear - 1k higher revs but still pulling hard? Wouldn't happen if it was slipping.

[Jay_g]

Which is what I thought, but the gear indicator was still showing 4th! (When it does it in 5th it shows that it drops down the 4th most of the time!!)

[jockthedog]

Does the f125 switch relay the current gear?
That might be the small issue.

car looks like it's working, kicking down but just not relaying to dis?

[Jay_g]

It really is a strange one. I was accelerating hard in 4th yesterday, changed up to fifth, which was fine, soon as I let off the accelerator the revs dropped about 1k (as before), so I'm fairly certain there is a slip. It's just not that vicious, but some reason doesn't affect the pull lol!