MB Gearbox: Odd change-up

[ray_hennig]

1991 300CE-24 5-Speed Auto

Just driven from Dordogne to UK and back. Auto box now behaving oddly.

Box changes from 2nd to 3rd as usual, excellent acceleration, everything.

BUT, it now seems to change properly into 4th and then, after about one second, change into 5th. In fact, it stays in 4th for so short a time that one has to concentrate to know that it's made the change.

It does this even if the lever is in the "up-to-4th-only" position (left of normal drive). "E" and "S" positions now seem to make little or no difference. So, basically, 4th gear is no longer available.

Fuel consumption may be up now - 750 KM before reserve, rather than 800+ KM, although that's just an impression and also we were pretty heavily laden.

I wonder what's happenned?

Otherwise, a good trip with much improved night vision due to new windscreen.

RayH

 

[Dieselman]

The gearbox on your car is hydraulic with electronic selection for 5th gear, similar to an overdrive.
It sounds like whatever makes the selection for 5th is doing so irrespective of speed or load so as soon as it sees 4th engaged it jumps to 5th straight away.
There is an ECU for this selection but it needs the inputs checking before condemning.

 

[ray_hennig]

Yes, that sounds logical. There's a switch at the gearbox for 'neutral safety' and, doubtless, the 4/5th manual select (I don't think they're separate switches, are they?).

Maybe there's a problem with that switch?

It's always a question of 2 steps forward and 1 back.

Oh well.

RayH

 

[Dieselman]

Try disconnecting the ECU or solenoid and see what happens. This should bring 4th back into play at the expense of 5th, which might be useful for local journeys.

 

[punjabi]

It is strange why the otherwise normal behaviour of auto tranny's become erratic after long journey's , my 300te developed a somewhat similar tendency but with a delayed changeover from 1st to 2nd, for 2 days it is fine then it starts again - just waiting to change the oil over the hols!

 

[Ade B]

Have you checked the ATF, how many miles since the last change and how many miles on the car - not sure if this will help much but it may help other peeps with their diagnosis..

The 'electric' 5th gear appears to have a reputation for being troublesome at high mileage, but from what I've read its reverse that normally plays up first.

Mine has so far behaved itsself.

Hope you get to the bottom of it.


Ade

 

[ray_hennig]

Gearbox: Other symptoms

Been on some local trips and other symptoms are:

1. Change-down at, say, less than 40 mph is very hesitant - when normally if would change quickly down to, I think, fourth, it now hesitates for some seconds and selects 3rd.

2. Fourth is very obviously 'there' but it's always engaged only for a split second before 5th. Bizarre.

3. Climbing a hill, coming out of a bend, hesitation again but then smartly into 3rd, pulling strongly until shifted to 5th.

I'm tempted to go looking for vacuum problems and also the linkage between accelerator and 'box. I've adjusted this in the past with excellent 'trial and error' results.

The comment above about unplugging the 'box ECU is worth following up.

I'll report further but am currently under pressure with yuletide issues - doubtless fogged, post-Christmas brain will need to be re-booted before pursuing this one.

Also, rythmic while (back left, I think) making me wonder if we're in for another rear bearing but the rythm is exactly the same as that under braking (new ATE pads/discs all round!).

It never ends, but what can one expect at 340 K KMS?

Thanks for comments so far and the best to all for Xmas 2008!!

RayH