transmission fault re appeared after 150mile

[ilunn]

Last week I had a repair on an 722.8 transmission ECU out of a 2007 A180, The reassembly went well and the fault code reset without any problems, The car then drove fine and has done around 150 mile (100 of being on the motorway). Yesterday the same fault code (P0793 intermediate speed sensor) has reappeared again with the car going back into limp mode. Have anyone come across this before and if so any ideas what I should look for or what else could cause this code to come on?
The code is the only code being generated and is being read by a general purpose code reader (Autolink AL419) with a second P.C based code reader (Scantool) to back up the code. I

 

[toolman1954]

Hi,
This would be the part in question??
https://www.alibaba.com/product-detail/722-8-Transmission-Control-Unit-Transmission_60279910258.html
I see also some discussion here on the subject.
B200 CVT playing up. Help? - Page 15 - Mercedes-Benz Forum
I suppose the first question to consider, is what kind of repair did you have done to the ECU?.... Dry joint repair ?.... Component failure? ... How much does a new ECU cost ? Dare say it will not be cheap. My thinking is why repair when a new part may have been a better option. No one ever seems to try to repair 722.6 electroplates as it is just not cost effective. Hope you get it sorted soon.

Steve

 

[MickyP64]

It used to be the case that you could only buy the ECU together with the valve body on the W169, but these days you can buy the ECU separately for much less than the combined cost of both. I’ve had mine changed twice now (the second time was due to mis-diagnosis by the indie I took it to…Grrrr). First time round cost about £1600 for ECU/Valve Body (thankfully covered by dealership warranty), the second time (though un-necessary) it cost me about £400 for ECU on its own.

When I had the work done first time round the mechanic told me that the new ECUs had been through ‘product enhancement’, meaning (I guess) that MB had improved the design of Speed Sensor arrangement - I was told that the problem shouldn’t reoccur with the new 'improved' units.

I’m not sure what current prices are for a repair (I think somewhere in the region of £350 the last time I looked) – it makes no sense to me to have the unit repaired these days when you can buy a new unit for around the same price – I wish more people were aware of this!

Additionally, when I had the CVT re-built on our car (there’s a thread on here somewhere detailing this) the transmission specialist I took it to had used a well-known repair service a few times in the past and didn’t rate repairing as an option, siting it as unreliable.

 

[ilunn]

Yes tryed to get ecu on its own and there is a mercedes part number on the system but they are no longer available I'm told by the dealer that mercedes were selling refurbished units but they are also not available and the only way to get them is with complete valve assembly the price was around £800 + Vat then the car would need taking to them to pair the unit with rest of the ecu's on car with additional cost being incurred for there time to do this, and trailer hire to transport car to them. The cost of repair £230 with lifetime warranty. I'm hoping to have chat with them today to see what next. I'm wanting to check all other possibilities such as wear in mechanicals causing sensor to move to far away to read reliably or anything else any one can suggest.

 

[MickyP64]

Oh, I see, doesn’t sound good does it!...Is it worth ringing around a few indies for a second opinion?

I don’t know what the deal is with MB, maybe it’s a case of whatever they have one on the shelf at the time. When I had the speed sensor issue I could only get the Valve Body and ECU together with the option of refurb or new, when it came to having the work done I was told that the refurb unit on the self at Milton Keynes had already sold and that there was only a new one left!...So I had no choice and it seems that they don’t carry a large stock of them.

When the CVT on our car failed due to mechanical issues a couple of years later (https://www.mbclub.co.uk/forums/engine/201789-cvt-rebuild.html) the indie I took it to mistakenly diagnosed the ECU as the problem (I was sceptical that this was the issue at the time and mentioned it to them, but bowed to their apparent superior knowledge and expertise)…Anyhow that aside they were apparently able to source a new ECU on its own on that occasion, and I was told by them that MB now sold them separate – that was only about two years ago and we’ve had the car about five years. Maybe MB have drawn a line under the CVT version of the A & B Class cars since they don’t build them anymore, and therefore they no longer manufacture the part?…Just a hunch, it would be nice to know for sure.

I think the transmission specialist that I took our car to also told me that the tolerances vary between each CVT unit, which might explain why a new ECU needs to be uniquely coded to the car it’s fitted to.

When my car was un-drivable I had it towed by the RAC from my house to the transmission specialist and it was way cheaper than any other tow companies I got quotes from. There is another way around this (not sure if I should mention it but I will). If you are a member with full membership and you happen to break down perhaps a mile or so from your house they will give you the option tow it free of charge back to your house or to a garage of your choice. Ever since I decided to take the car out of main dealership servicing and lost the Mobilolife cover I’ve been an RAC member and it’s paid off more than once.

Did you send the ECU off to be diagnosed and repaired by the way? I thought that particular service offered a lifetime warranty on their repair?...Even if my Trans specialist didn’t rate them.

 

[KeithJG]

Not the same model as myself but being gearbox including electro hydraulic problems if replaced will need a dealer or indy with ONLINE STAR.

The reason is because this is a top security part and has to be matched with the owner and log book of the car in Germany.

MB did do this service of TCU separate but no longer do so. Probably they realised they could earn more money the new way.

Had to have mine done but you do get a 24 month guarantee.

 

[MickyP64]

Just another thought – when I had the Valve Body and ECU changed, the old unit went back to MB to be refurbed and then I assume resold…I thought this was a bit cheeky of MB since they had £1600 off me to fit a new unit and then they get to re-sell my old one again refurbed for £800! So if there haven’t been any recent dodgy units swopped out MB wouldn’t have any to refurbish and then sell on.

 

[KeithJG]

That is true what you say they are refurbished but i will say the sensor and valves are all replaced as Olly let me take some photos and you can see the difference between an old speed sensor and a new one!

Will say again at least you get 24 months warranty on the parts so they daren`t use old ones.

On the subject of not having any to replace they do make sure that this is a very common problem before they offered the service??

 

[ilunn]

Hi Guys
The company that tested and did the repair on the ecu offer a lifetime warranty so I'm in talks with them but could do with finding something (software/diagnostic unit) to be able to read live data (the one I have does not do live data for transmission). Its looking like strip down again for unit to be retested.
Mickyp64 ref "CVT on our car failed due to mechanical issues a couple" can you highlight any more on "mechanical issues" just in case its mechanical on mine.
Thanks.

 

[MickyP64]

Only what's in my post here: https://www.mbclub.co.uk/forums/engine/201789-cvt-rebuild.html Bearing/Shaft wear on the primary pulley arrangement was the mechanical failure I had – new parts and a re-build of the CVT (with two year warranty) cured the problem, though it cost about £2400 for this service. ECU was not at fault on this occasion.

 

[ilunn]

Only what's in my post here: https://www.mbclub.co.uk/forums/engine/201789-cvt-rebuild.html Bearing/Shaft wear on the primary pulley arrangement was the mechanical failure I had – new parts and a re-build of the CVT (with two year warranty) cured the problem, though it cost about £2400 for this service. ECU was not at fault on this occasion.

I'm wondering if mechanical wear might be my current problem! Ive just done a 50 mile trip tonight up the A1 with know problems at at all!!.
What ever happened to bullet proof Merc's like the old 190e from the late 80's seems there not the top manufacture they used to be, they used to do something like a 25 year rust warranty if I remember correct.

 

[MickyP64]

The problem is that it’s so difficult to diagnose what the issue is even with diagnostics - the question is, is it mechanical or electrical? You really need STAR diagnostics rather than generic OBD too.

The ECU/speed sensor seems to be by far the common issue with the MB CVT, the problem is that it’s intermittent – the car will drive perfectly well for some of the time and then the fault will re-appear and can be cleared by turning off/on the ignition. My car was drivable several times when I had this problem and after a re-start and I was able to drive it across London to the garage to have it diagnosed and repaired, I was also able to drive for several hundred miles on the motorway before the fault would re-appear. Your problem seems to be consistent with speed sensor issues - I suspect you need to have a chat with the guys that did the repair or (if you can) source a new or refurbished ECU, though there comes a point sadly when these cars are probably beyond economical repair.

I’ve not yet heard of anyone else having the mechanical failure that our car had (though the transmission guy said he'd seen a few). Once the CVT on our car had failed mechanically it was un-drivable as soon as it happened and obviously the fault couldn't be cleared by ignition off/on. There were no prior symptoms or indications that this was going to happen (nothing intermittent like ECU problems). The transmission felt like it was slipping in gear rather than going into limp mode. I have wondered if it’s anything to do with the fact that our car is an A200T and the power is just a little too much for the transmission or that maybe the car gets driven a little harder due to its nature - it surprises people sometimes, they don’t expect to get burned by an A Class! (though I don’t drive like a nutter, I’m not a petrol head).

I don’t think you’re the first person to question the build quality and reliability of modern Mercs, don’t get me started on rust or customer service. I too bought a Merc because I thought that they had a bullet proof reputation - I’ve now spent way more on our car keeping it maintained than the car is actually worth, but I guess cars are just a money pit anyway. Before I bought our present car I had an old VW Golf that I paid £600 for and it was probably the most reliable car I ever had. But strangely myself and the Mrs quite like our A Class, we must be mad!

Hope you’re able to get yourself sorted without too much expense.

 

[ilunn]

OK update. back into limp mode again but this time different codes being generated but as pending so no codes stored to erase! codes now are P0717 and P0718, I need to speak to repair company again.

 

[ilunn]

spoke to repair company so last attempt before spares/repair add go's out. I'm going to whip the ecu out again 3rd time (i'm getting quick at it now) and they are going to have another go at repair. Just need to keep my fingers crossed.