C5 Facelift 4HP20 holding 2nd gear! Fixed

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MGmike
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Joined: 29 Aug 2020, 18:29
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C5 Facelift 4HP20 holding 2nd gear! Fixed

Post by MGmike »

Hi All,
problem number two that has me stumped and all suggestions welcome.

Symptoms:
on moderate acceleration the gearbox fails to change from 2nd to 3rd when the steering is turned left (exiting a roundabout/bend in road) or the straight ahead position. It holds second until the steering is turned to the right about and 1/8 of a turn or I manually change up.

As I have an almost complete spares car of a similar vintage, I've replaced the gearbox ECU and the Comm unit (under steering wheel) without success. The Comm unit steering angle has been successfully calibrated according to the computer!

Where do I go next to find the source of the problem? Is it something to do with the ESP inhibiting a gear change whilst cornering but why in the straight ahead position? Could it be the ABS unit?

If anyone has a wiring diagram of the gearbox controls or a functional description of how it works I would very grateful.

TIA
Michael
Hell Razor5543
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Re: C5 Facelift 4HP20 holding 2nd gear! Any suggestions?

Post by Hell Razor5543 »

This may seem a silly question, but are the wheels/tyres the right size for the car? I remember a Forum Member who had an issue with their XM, and it was because the wheels/tyres were incorrect for that car. Once they were replaced with ones of the correct dimensions the XM was able to read the wheel rotations without confusion.
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Sloppysod
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Re: C5 Facelift 4HP20 holding 2nd gear! Any suggestions?

Post by Sloppysod »

I suffered the same on my X7.
tyre circumference is probably the answer, run a tape measure around the tyre to confirm the are the same each axle.
Here is my post from this topic (Last post)

viewtopic.php?t=52892&hilit=Tyres

".........., it was the rolling diameter of the tyres, I put a tape measure around the circumference of the rear tyres and had about 15mm difference. I changed the tyres and never had the problem again. I was lucky my spare was the same as one of those I measured so I could test before spending alot of money.
I think it was down to the car "thinking" it was loosing traction due to the difference in rotational speed, and therefore would not aggrevate the issue by changing gear.
Good luck.
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MGmike
Posts: 146
Joined: 29 Aug 2020, 18:29
x 39

Re: C5 Facelift 4HP20 holding 2nd gear! Any suggestions?

Post by MGmike »

Thanks for the pointers Gents.
The wheels are the standard for the car but TBH I've not checked the tyres. I've had the car a couple of years but have done very few miles and it's fitted with whatever it came to me with.
I was planning on doing a live data run to check the ABS sensors are reporting the correct wheel speeds. Fingers crossed it is something like this and easily fixed!
MGmike
Posts: 146
Joined: 29 Aug 2020, 18:29
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Re: C5 Facelift 4HP20 holding 2nd gear! Fixed

Post by MGmike »

Well Guys I must say... you are outstanding :clap: :clap:
I haven't done the actual measurements but a few days of extensive testing have confirmed this is now fixed.
The OSR tyre was down to the markers and ready for replacement, all the others are mid life around 4mm. Replaced the one tyre with one from my spares and hay presto all good.

Thanks again and I just wish I'd asked earlier which would have saved me countless hours thinking of things that could cause it #-o

Mods, could you update the header to fixed please?