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Sudden TACC max speed drops

tridge

Member
Mar 15, 2021
12
1
Birmingham, UK
Just got back from a trip down south (M5, M40, M25, M23) used TACC most of the way there and back, some short stints of autosteer but not much. On the way down all was fine, but coming back up the M40, once I got past High Wycombe the TACC started suddenly dropping its max speed from 70mph to 50 or 55mph everytime I went past a junction turn off, pretty consistently at the hard shoulder apex of the junction. Did it both in inside lane and middle lane (don't remember it doing it in outside lane). Must have done it half a dozen times at least. Has anyone else experienced this?

I found some old threads from the US but nothing recent or in the UK.

Only seemed to do it on M40 northbound, no other motorways I travelled on today. It was getting dark/had gone dark (but it was dark on the M5 bit too and I didn't have the issue). May be related that I've noticed my GPS seems to be off too? Only noticed past few days, I'm either on the very left edge of whatever road or in some cases I've been 20m off to the left of the road driving through houses - it's been bad enough at times that the routing has been constantly changing to try and find me. Car (2021 LR) was in SC for a new rear windscreen just over a week ago (heater elements damaged from factory) - could they have damaged the GPS and affected the TACC? Don't know where the GPS is located...

Sorry for the rambling post but I've had a long drive to ponder all contributing factors and possibilities!
 

Alex987854

Member
Aug 30, 2019
183
152
UK
The advice for most software related problems is try reboot - remove any usb devices then foot on brake pedal, hold down both buttons. There are also suggestions that allowing the car to sleep can fix some things. Leave it overnight with sentry turned off.

If neither of these works its a SC fix. They may be able to do it remotely.
 

VanillaAir_UK

Supporting Member
Jun 17, 2019
7,141
4,665
Surrey, UK
Just got back from a trip down south (M5, M40, M25, M23) used TACC most of the way there and back, some short stints of autosteer but not much. On the way down all was fine, but coming back up the M40, once I got past High Wycombe the TACC started suddenly dropping its max speed from 70mph to 50 or 55mph everytime I went past a junction turn off, pretty consistently at the hard shoulder apex of the junction. Did it both in inside lane and middle lane (don't remember it doing it in outside lane). Must have done it half a dozen times at least. Has anyone else experienced this?

Yes, although apart from one, it occurred in same section but going towards London. In the end I think it was put down to map and/or GPS error as on at least one of the occasions the map had the car starting to go up the offramp even though physically the car remained on the motorway. That was on TACC. Other direction I was 'driving' so on NoA and it suddenly wanted to start taking me off. I initially put it down to user error and a mix up in the stalks, but not so sure after the London bound experiences.
 

browellm

Member
Oct 4, 2019
363
302
Notts
I've had a similar experience recently where the A52 bridges over the A1, intersecting it close to right angles. The system seems to get caught out by criss-crossing roads quite badly. Didn't someone mention there are some shocking sections of the A14 that causes TACC to wig out completely?
 

tridge

Member
Mar 15, 2021
12
1
Birmingham, UK
I've had a similar experience recently where the A52 bridges over the A1, intersecting it close to right angles. The system seems to get caught out by criss-crossing roads quite badly. Didn't someone mention there are some shocking sections of the A14 that causes TACC to wig out completely?
This would make sense, except in my case the recognised speed limit never changed from 70mph?
 

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