The legendary 722.5 Gearbox on a W124!!!!!

[Benzsc1]

Right this is my first chance to post this in a forum where I am hoping I can get some answers.

My C124 300-24 has the dreaded (When they go wrong) 722.5 5 speed auto box the original box failed two years ago so I sourced a replacement 722.5 box out of an early W210 300 Diesel. Apart from having to swap the end casing on the gearbox as mounting points between the W210 and W124 differ that it the boxes are identical (Not sure if MB beefed up the internals for the torquey diesel engine or not??). When this 5speed box works well its a pleasure to drive I think.......... but WHEN it works is the big statement!

The problem!

Ever since I bought the car in 2002 the gearbox gave me signs of an underlying issue by and large at first the gearbox would perform normally and smoothly occasionally I would notice it would miss 4th gear completely and change from 3rd in to fifth. This gradually got worse to the point 4th gear rarely engaged however the car still drove smoothly and TBH I was not fully aware of the issue as the car was still so drivable.

Anyway it failed two year ago by way of the clutches and blets wearing out causing major slippage.

So with the new box in its like a new car only its still occasionally missing 4th gear so basically the same issue. I drove the W210 the box came out of and that drove very well and did not miss a gear at all?????

I understand there is an electro hydraulic valve that control 5th gear I understand is prone to causing the fault specifically with this missing 4th gear syndrome.

My concern is with another box that was working fine the issue should have gone away or am I missing something?

Any help would be appreciatted here.

Thanks Simon.

 

[mattc]

read through this thread, particularly post 7

 

[Benzsc1]

Thank Matt,

Had a read unless I am missing something this is to do with adjusting the gearbox so it can be fine tuned to shift smoothly???...... Although I can see your logic in that as I have swapped the tramsmission out already and still the same fault is evident albeit not in such a ferocious fashion as it was before (I blame the previous box being worn out from the strain of this fault) adjustment may be the key reason. I am not sure though what adjustment on a 722.5 box can be made to aid 4th gear engaging and not just skipping straight to fifth.

Thanks again.

 

[ray_hennig]

4th gear engages for half a second then into 5th ...

Mine started doing this after the engine/gearbox mounts were replaced. Maybe a vacuum tube got trapped or damaged?

I also have the impression that the 'box just isn't listening to inputs like depressing the accelerator to prompt a change-down. Again, I suspect vacuum issues.

My next move is going to be checking the vacuum circuit to the 'box and replacing ALL rubber joints and any dodgy looking tubes to ensure there are no leaks.

Don't know when I'll get around to it but I will report back with result.

Also looking forward to hearing how you get on.

RayH

 

[Benzsc1]

Cheers Ray I will keep you posted (Scuse the pun!! )

Before I venture into this area I have to get her started but Before I joined this forum there was very little or specialised knowledge about these gearboxes so its great to hear other members have the same cars with similar issues.

Although lets face it would be great if we all never had to experience these issues as for me the best part of owning a Mercedes is driving it!! And thats coming from someone who cannot leave cars especially engines alone I am currently rebuilding an old MK1 VW LT35 turbo diesel engine with uprated power for reason I have no clue off but anyway it is fun isnt it and hopefully I will end up a quick LT35 Turbo Diesel truck (Probably the only one in the country cause evryone else is sensible and scraps old LT's

 

[ray_hennig]

Ho Benzsc1

These 5 speed boxes aren't that rare. The later 124s (in Europe, at least) had them and the SL sports (129?) too, I think.

For me, it's never been a problem, aside from this recent 4th/5th issue.

I live deep in the Dordogne countryside so W124 specialists are thin on the ground. I have a German friend who's been importing MBs into France for years - now things have changed, the traffic's the reverse, classic MBs, rust-free being shipped back to Germany!

Any way, I'll try to keep you and others posted on this. I can't believe it's that complicated. Having said that, it is worth consulting some of the gurus. Some of the chaps on this forum know their onions (nothing about MBs but everything about onions) so we await their comment.

I know that many have responded already to this issue so please don't think I'm spamming the group.

Best to all.

RayH

PS: My auto box was refilled (+ filter change) 20K Kms ago. The local guy used Renault-branded auto-box fluid. My attitude was that these MB auto boxes are pretty bloody agricultural and bomb-proof. Are we MB guys really that precious?

 

[Benzsc1]

Hi Ray not so much that these boxes are rare they are just a bit of an odd ball similar to the 300-24 engine really. I know they were offerred as an option to all 24 valve Mercs between 1989 and 1992 not sure if they were standard on 129 SL's following which the electronically control gearboxes made an appearance as did the HFM injected M104's 320 engines dubbed as the real M104 engines by some as a pose to our engines that are essentially an M103 300 engine with the 24v head bolted on.........

TBH with the Renualt oil I don't think it makes a difference who produces the oil its the grade thats important for engines obviously the better brands do instill more confidence in some however I am sure the Renualt branded ATF will be just as good as Merc branded stuff after all ATF is ATF IMO......