W124 help needed

[MB-tex]

Hi All

I have a series of problems that I want to resolve. Car has been checked over by my mechanic who knows these cars and by an auto gearbox garage who also have experience of this model. They have both not been able to resolve the issue (partly due to my budget). I am having the car looked at on Tue by some sort of Merc specialist guru type that my mechanic uses so hopefully this will help too. But I thought I'd hit on the wealth of knowledge on this forum as well to try to get my car back to her normal lovely condition...

1996 w124 E280

Car is regularly serviced and maintained by A J Robertson in Hove. Currently running but with two issues as below.

Problems as they have presented themselves:

Approx 2 years ago the cruise control suddenly stopped working. Car otherwise ran perfectly with no other issues. Did not get fixed.

About 4/5 months ago I noticed the car’s idle speed became irregular when waiting at traffic lights etc…Very faint at first but now more pronounced.

During a recent long journey on the continent (Toulouse and back) this became a problem when the engine would almost cut out at times when slowing down for a roundabout and there would be a power lag when trying to set off again. This has become less of a problem now I use the car less.

During the trip on the continent the car began to stop changing up into 5th gear. At first it would be occasionally and it has now stopped using 5th gear entirely. The other gears seem to change correctly. The kickdown switch still works but changes from 4th to 3rd at highway speeds! The engine is on 3000rpm at 60ish mph. In 5th it was at 2500rpm at 65mph. Switching the gearlever between 5th and 4th has no effect (gearbox stays in 4th). When on highway I now stay at 55mph (2500rpm).
The car has no other issues I am aware of.

Have done copious research and know it is throttle body/solenoid/4th5th switch/deteriorating loom etc... but mechanics have not come up with anything definitive yet so was hoping the above symptoms might give a precise thing to follow up. Throttle body new from merc is £2000!

Thanks

 

[kth286]

my initial reaction is the gearbox change to 5th gear.

there is a diagnostic plug in engine bay bulkhead.

pin 13 is the electronic diagnostic for the 5th gear change.

You need someone with the code reader (a simple device) and the list of relevant codes and the understanding of their meaning.

someone old school is the ideal.

I guess you know that the 4 speeds change mechanically but that the 5th gear changes electronically.

 

[bolide]

Cruise is done by the TB and the cruise ECU which is separate to the engine ECU so look at those, and the TB loom, for the fault

Your symptoms of the car bogging down out of roundabouts is identical to a car I had. The fault was found & fixed in under an hour - the speed sensor signal to the ECU was missing, so the engine / ECU didn't know what was going on when getting back on the throttle when exiting the roundabout. Fault was a bad connection behind the battery

From memory the speed control signal is fed via one pin of the OVP relay socket

I sorted the cruise fault, Alan who used to work at Rivervale sorted the 5-speed problem. AJ and Alan are old buddies

Nick Froome

 

[WDB124066]

Brake light switch for the CC if all else fails...

 

[neilrr]

Is A J Robertson in Hove a MB specialist?

 

[MB-tex]

I am told A J Robertson's specialism is BMWs but he is a Mercedes expert too.

 

[MB-tex]

Thanks for the advice folks. I am taking the car to see the merc guru this eve and will pass on the info posted. I will report back when I have resolved the issues.

 

[MB-tex]

Who'd have thunk it?

Merc guru says...air mass sensor!

I will get a new one fitted and let you know if it clears up the faults...

Can you all keep your fingers crossed for me please.

Thanks

 

[WDB124066]

It will not clear up ALL your faults on its own.

 

[ray_hennig]

Have done copious research and know it is throttle body/solenoid/4th5th switch/deteriorating loom etc... but mechanics have not come up with anything definitive yet so was hoping the above symptoms might give a precise thing to follow up. Throttle body new from merc is £2000!

Thanks

I'm not saying this will solve your problem but I do advise changing the 4th/5th switch. It's cheap, easy and, in my case, solved a problem where the electronics (such as they are) got confused and set the gearbox into a sort of 'safe' mode with no 4th.

Different car but the 'box may be the same.

Bonne chance.

RayH

 

[MB-tex]

Yes, I was a bit surprised by this fix-all solution and asked if it would also cure the 5th gear issue. He explained that it could well do so. I can only (at this stage) bow to the knowledge of this guy who is undoubtedly an expert. I think he spent the best part of two hours looking it over. I will keep going till I get it fixed.

The car did briefly use 5th a couple of days ago and I realised how much I miss it. I have been setting out early and crawling along at 55! The car does feel sportier though with power available instantly in 4th gear.

Thanks again for the advice.

 

[bing]

It will not clear up ALL your faults on its own.

+1 . Did the guru plug it in and check for fault codes ?

 

[MB-tex]

"Did the guru plug it in and check for fault codes ?"

I believe so. I dropped the car off and was not there while they worked on it. I got the impression that they did a lot of investigations. I hope they did. This car is normally so reliable I have become unused to this sort of automotive conundrum.

But maybe they just drank tea and debated the Scottish independence vote?

 

[MB-tex]

Solved

Hi All

One new air mass meter and regeneration valve later and...surging tickover cured and fifth gear back...

Good news.

 

[WDB124066]

Good news.