W124 5speed auto

[abogas]

Hi, newbie here! I have a 1996 cab that I've any number of little issues to sort out but the most worrying is the gearbox issue. I've trawled through lot of threads on it but none seem to help! The problem is its missing out 4th gear, going 3rd to fifth instead. I've made a blink tester but when I test pin 13 all it does is light the led at about half power and I get nothing else. Hope a genius is to hand!

Thanks in anticipation

 

[bolide]

Are you sure it's going from 3rd to 5th? Fixing 5-speed control systems is hard work because there isn't much in the way of diagnostic kit. There's a PDF that describes the action of the ECU that controls the 4-5-4 switch - get a copy of that and check that all the signals are present

Nick Froome

 

[abogas]

I'm fairly sure as it changes up through 2nd, 3rd then a big jump upto 5th? But does about 3000rpm @70mph. Also clicking it back and fore across the gate has no effect. Don't suppose you have the PDF do you, tried searching till my fingers bled!

 

[jaymanek]

You are certain that the car is a 5 speed? And if you are counting gears then you know it starts in 2nd?

 

[abogas]

Hi, yes I'm sure it's a 5 speed as the the fours don't have the side to side plastic gate on them do they? I've looked at a few four speeders and the shifter gate is different.

 

[abogas]

Oh and yes they only start in first If you floor it!

 

[Dieselman]

4 speeders have a habit of missing top gear when worn, it sounds as though yours is doing this then selecting 5th, which is an add on gear.

 

[bolide]

The difference between 4th and 5th at 70 mph is about 700 rpm. What revs is your car doing at 60 in 3rd & 5th? Or 70 if it'll do 70 in 3rd

You may find a subscription to WIS & EPC useful. Google it

Nick Froome

 

[WDB124066]

5th gear module - Benzworld.org - Mercedes-Benz Discussion Forum

5 speed auto problem - Benzworld.org - Mercedes-Benz Discussion Forum

 

[ray_hennig]

Hi, yes I'm sure it's a 5 speed as the the fours don't have the side to side plastic gate on them do they? I've looked at a few four speeders and the shifter gate is different.

Over the years I've had many 4th/5th issues and clearing fault codes was the answer. However, mine is the 300-24 with KE Jetronic and the MAS box. I don't know how your more recent car handles this.

But, and this is worth doing, even if it isn't the root cause of the issue. Change the 4th/5th microswitch. I got one for about £25 shipped to France and it took me about 30 mins to install.

The important thing is that the 4th/5th problem was coming back more frequently. Each time I cleared the codes, all was well for a while. With the new microswitch, all seems back to normal.

Good luck.

RayH

 

[abogas]

I might well try that, thinking ill have to work my way through the system testing what I can but the fact I'm not getting sense when I test pin 13 makes me worry it could be an ecu etc problem which could be expensive!!

 

[ray_hennig]

I might well try that, thinking ill have to work my way through the system testing what I can but the fact I'm not getting sense when I test pin 13 makes me worry it could be an ecu etc problem which could be expensive!!

My assumptions following replacement of the 4th/5th switch are that the intermittent contacts were sending confusing signals to whatever intelligence is in the system. The system then went into this sort of "safe mode" with low up-changes and only a passing flirtation with 4th.

In your shoes, things are a little different but, for the price and ease of fitting of the switch, I would go for it.

Do keep us posted.

R

PS: Have you studied your system? What controls auto-box changes? It is a few years later than mine so I don't know.

My policy is always, "Start with the cheap stuff. Don't get all morbid and assume that ECUs are dead until proven otherwise".

 

[grober]

722 automatic transmissions

These are both excellent links to diagrams and explanations of how these earlier gearboxes work and include references to shift controls and trouble shooting tips.

http://www.w124performance.com/docs/mb/other/tranny_722_repair.pdf

and

http://www.w124performance.com/docs/mb/other/tranny_722_Mitchell.pdf


Mods could this be made a sticky?

 

[Niks]

70mph sits at 2750rpm on my 124 (5-speed), 80mph is at 3000rpm.

 

[ray_hennig]

70mph sits at 2750rpm on my 124 (5-speed), 80mph is at 3000rpm.

Sounds about right to me.

Are you suggesting that this is a problem?

More comment needed.

R

 

[abogas]

Cheers for the advice, soon as I get a minute I'll dig into the switch and see what's happening with it.........just the dodgy code/ non code that's got me worried!! I'll post soon as I've tested the switch.