P38 - strange transmission? behaviour

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kooky_guy

kooky_guy

Well-Known Member
Posts
2,410
Location
Sandhurst, Berkshire
Hi guys.

Drove up to cumbria today and it was doing some odd things on the motorway.

It's a bit hard to explain but basically I'd be cruising at 70 quite happily and then all of a sudden it would be as though it kicked down a gear without the throttle being increased so it would suddenly slow down. Then it would change back up again and carry on as normal.

It did this about half a dozen times on the journey.

All else seems fine. Anyone got any ideas or suggestions?

I've got to go on to scotland tomorrow after a laning session so I'm a bit nervous now.

I'll check the trans fluid in the morning but I was wondering if anyone had come across anything similar?

Guy.
 
is it actually kicking down? do the revs shoot up? or is it just a sudden deceleration and then coming back to normal? to me it sounds like fuel starvation.

hey, you've got an even 1000 posts! congratulations! :p
 
Before I start I'll apologise for the hijack.
Mine is doing something similar changing from 3rd to 4th, it changes then immediately goes back to 3rd, i release the accelerator and it goes into 4th no probs and stays there. Its a bit jerky lol and wondered if changing the oil and cleaning the strainer/filter might fix it.
 
kooky_guy said:
Hi guys.

Drove up to cumbria today and it was doing some odd things on the motorway.

It's a bit hard to explain but basically I'd be cruising at 70 quite happily and then all of a sudden it would be as though it kicked down a gear without the throttle being increased so it would suddenly slow down. Then it would change back up again and carry on as normal.

It did this about half a dozen times on the journey.

All else seems fine. Anyone got any ideas or suggestions?

I've got to go on to scotland tomorrow after a laning session so I'm a bit nervous now.

I'll check the trans fluid in the morning but I was wondering if anyone had come across anything similar?

Guy.
Click to expand...

Check and adjust throttle cable to Rave then check and adjust kickdown cable to RAVE, you will get this if it is to tight.
 
Hi guys.

I didn't think the p38 had a kickdown cable..?

Well it's getting more and more frequent. Came up with gearbox fault while laning this morning and went into limp mode. Seems to reset itself if I stop and restart the engine.

It's doing it at all speeds and all loads now although it got us to scotland ok.

Would the gearbox fault message have logged a fault code?

Checked the atf this morning and it seemed fine.

Not sure what to do now. :( Auto boxes are a bit of a mystery to me!

Guy
 
have to agree kooky guy the TCU controls various combinations of solenoid valve 1 and solenoid valve 2 .solenoid 1 energised in D2 & D3 solenoid 2 energised in D1 & D2
there are various other inputs throttle angle, engine speed, engine torque, gear position ,mode selection hi/low signal and output shaft speed
 
Think the battery is ok. It was new a couple of months ago and it's starting fine.

I did notice that the cable to the cruise control bellows was a bit tight so I've backed that off a bit (doesn't work anyway) and I'll see if that makes any difference.

Guy.
 
kooky_guy said:
Hi guys.

I didn't think the p38 had a kickdown cable..?

Well it's getting more and more frequent. Came up with gearbox fault while laning this morning and went into limp mode. Seems to reset itself if I stop and restart the engine.

It's doing it at all speeds and all loads now although it got us to scotland ok.

Would the gearbox fault message have logged a fault code?

Checked the atf this morning and it seemed fine.

Not sure what to do now. :( Auto boxes are a bit of a mystery to me!

Guy
Click to expand...

If you have a 1998 4.6 it does have akickdown cable, check it before you start looking for anything else. If the setting is correct then you can start looking elswhere.
 
wammers said:
If you have a 1998 4.6 it does have akickdown cable, check it before you start looking for anything else. If the setting is correct then you can start looking elswhere.
Click to expand...

Hmm. Well I don't know where it is if it has got one. Doesn't seem to be anything around the throttle linkage where I would expect it to be.

Unfortunately I'm on holiday so no access to rave and very small selection of tools.

Guy
 
kooky_guy said:
Hmm. Well I don't know where it is if it has got one. Doesn't seem to be anything around the throttle linkage where I would expect it to be.

Unfortunately I'm on holiday so no access to rave and very small selection of tools.

Guy
Click to expand...

Ok my mistake signals are taken from throttle position sensor for kickdown. No kickdown cable as such. Would take a look at sensor positioning. Or maybe a bad connection to it. Maybe adjusting throttle cable to give a little slack or cruise cable to tight.
 
wammers said:
Ok my mistake signals are taken from throttle position sensor for kickdown. No kickdown cable as such. Would take a look at sensor positioning. Or maybe a bad connection to it. Maybe adjusting throttle cable to give a little slack or cruise cable to tight.
Click to expand...

Cheers for looking that up Wammers.

Blast. I was getting excited about the kickdown cable idea as it seemed to match the symtoms.

I've slackened off the cruise control cable as it was definitely too tight and will see how that goes today.

Cheers for all the suggestions guys.

Guy
 
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