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  #31  
Old 26th January 2018, 09:06 PM
jimh jimh is offline
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Yep, limp as soon as it starts.
Maybe even during cranking, before it starts - it's hard to tell.
Maybe I'll try filming it and watching back to be sure.

I could try preventing it from starting somehow to see if it goes into limp when just cranking and not starting I suppose.
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  #32  
Old 30th January 2018, 09:30 AM
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YorkshireBloke YorkshireBloke is offline
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Hi,

Interesting thread, did you make any more progress?

Robert
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  #33  
Old 30th January 2018, 09:41 AM
HPsauce HPsauce is offline
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All rather odd. Have you tried starting in Neutral rather than Park?
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  #34  
Old 30th January 2018, 01:48 PM
MikkiJayne MikkiJayne is offline
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The only thing I can think of that would cause limp as soon as the engine starts is the input speed sensor giving an implausible reading. Or, the TCU thinks it's been started in gear if the cable is incorrectly adjusted and just tweaking the switch out of P. The input speed sensor going from 0 to idle rpm would certainly be implausible if the TCU thought it was in gear.

Need those codes

Also worth checking VCDS to see what gear the TCU thinks is selected before and during fault conditions.

Note to self: TCU's don't think. Don't anthropomorphise machines - they don't like it.

Last edited by MikkiJayne; 30th January 2018 at 01:50 PM.
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  #35  
Old 30th January 2018, 06:07 PM
jimh jimh is offline
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Yes, made some progress over the weekend. I've been holding off posting results as I wanted a few cold starts under my belt to know it's ok.

I started the car Sat morning with the laptop connected. As sods law dictates, the error did not occur that time. I checked for codes anyway, and there were none stored.

I got a 100Amp battery drop tester, connected it up to the battery and it read 12.0V. I then started the car. The initial Volt drop when cranking was down to 8V.
After the car ran for a minute, I shut it off and the Volt meter was then sitting at about 11.6V. I did the 100A drop test which dropped down to 9.5V.
I took the battery inside and charged it.
At full charge, battery was reading 12.6V. 100A drop test dropped that to 10V, recovering to 12.0V.

I wasn't happy with that, so I went and bought a new battery.
New battery drop test went down to 11V and recovered back up to 12.6V.
New battery in the car dropped to minimum of 9.8V when cranking.

I had not noticed any other battery related type problems and this car is my daily driver (about 30miles a day commute).
But the new battery does somehow "feel" stronger when cranking and the gearbox error has not happened since (only been a few days, but there was frost this morning).

Assuming the problem does not return, I think that initial drop to 8V when cranking was too low to keep the TCU alive / happy (it has emotions now).
That makes sense to me with no codes stored. If it was the TCU dropping out due to momentary low Voltage, it might not have a chance to store codes.
Like expecting a Windows log to log a power cut I suppose.
This would mean, any error such as this, it's worth checking battery Voltage when cranking. General internet consensus seems to be that trouble occurs with car electrical systems if Voltage ever drops below 9V (seems to be the magic number).

If anyone is really interested, I video'd myself doing the battery tests (mostly as I wanted to capture the momentary Volt drop easier, but then I got carried away and put it on Youtube in case I wanted to refer to it...

https://www.youtube.com/watch?v=VG-2...ature=youtu.be
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  #36  
Old 14th February 2018, 09:59 AM
oldnick oldnick is offline
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as baldrick might say ...I have a cunning idea
for the last couple of winters , like so many , have had the dreaded limp mode in cold weather on an intermittent basis when engaging reverse so that the passenger door mirror wouldn't dip
this winter , despite if being so cold that the drivers window was frozen up yesterday , have had no problems at all

but now the mirror doesn't dip at all
could it be that it is a fault in the mirror mechanism rather than the F125 ?
or just a coincidence ?
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  #37  
Old 21st October 2018, 08:43 AM
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Green Goblin Green Goblin is offline
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Just reading this, I had the same thing with a low battery. When I was using my work car, the S8 sat for a couple of weeks. The battery on the S8 went pretty low - just on the cusp of being able to start the car (which it did, just barely) but threw the error on the dash as described above and gearbox in limp mode. All the lights for the gearbox lit up on the dash and it was stuck in one gear.

After charging the battery and restarting, it was fine. Never had it since then and that was over a year ago. I drive the S8 more now anyway which obviously keeps the battery in better condition.

So in conclusion, anyone with a limp mode gearbox, just make sure the battery is fully charged before moving on to all the other processes of elimination (F125 switch, gearbox oil, etc. etc.). It does seem though that this happens only when the battery is ridiculously low - as in just barely able to crank the engine but succeed in doing so and then starting (if that makes sense lol).

Iíve bought a CTEK MXS 7.0 to keep the battery charged/topped up for any prolonged stationary periods. Itís a superb charger btw.
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  #38  
Old 24th October 2018, 08:46 PM
Dakes55 Dakes55 is offline
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Default Limp Mode

I had exactly the same problem as you, all my symptoms were the same, I would say im 99% sure its your F125 multifunction switch on your gearbox, the brass tracks inside the switch go bad, what I did to confirm that it was the switch was to do a test with VCDS, and this showed that when the gear lever was moved from P to R there was no electrical connection, thus causing the car to bring up all the PRNDS lights on the dash and go into limp mode, I bought a genuine ZF F125 switch for £350, and it cured the problem, the car now drives great, I would say that its a bit of a pain to do the job, if you need further advice on this just PM me and I will give you the procedure for testing, with the values you should be getting

Last edited by Dakes55; 24th October 2018 at 08:56 PM.
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