Mercedes auto gearbox query

outfox · 11-07-2019 10:52am #1

I've a 1970 250ce W114. It has the 4 speed auto box, although it's really only a 3 speed as it starts off in second gear. The car goes into top gear at about 28-30 MPH (not km/h). That seems very low. Top speed for the car is around 110 MPH I think, which would make you think that the revs would be up around 8000 RPM at that speed, if it's in top gear by 30 MPH.
Does anyone else find this with the Merc auto boxes from that era? I would have thought that they should not hit top gear until at least 40 MPH.

hi5 · 11-07-2019 6:25pm

Common enough for that era as not many motorways then and heavy bodies with under powered engines.
Many Mercedes 4 speed auto boxes right up to the 90's were not overdrive boxes, 4th came out at 1:1.
I had a 1991 300d w124 like that, the diff was around 2.5 so that kept the revs down.

outfox · 11-07-2019 9:01pm

hi5 wrote: »

Common enough for that era as not many motorways then and heavy bodies with under powered engines.
Many Mercedes 4 speed auto boxes right up to the 90's were not overdrive boxes, 4th came out at 1:1.
I had a 1991 300d w124 like that, the diff was around 2.5 so that kept the revs down.

There's a lovely 300d for sale at the moment on DoneDeal. Caught my eye. It's amazing what a set of monoblocks does for a Merc.
With your 300d, can you remember what speed it was doing before it hit top gear?

Type 17 · 12-07-2019 7:53am

I have a W116 with a 3.5 and 3-speed auto and it hits top at around 30mph too (higher if engine load/throttle is higher).
Sounds normal enough - manufacturers want the car to hit top gear for fuel economy by the time they hit the urban speed limit (50km/h).
Lots of older cars revved high at motorway speeds - eg: 4-speed Mk1 Golfs cruised at 3,750rpm at 70mph.

Seweryn · 12-07-2019 8:57am

outfox wrote: »

There's a lovely 300d for sale at the moment on DoneDeal. Caught my eye. It's amazing what a set of monoblocks does for a Merc.
With your 300d, can you remember what speed it was doing before it hit top gear?

All M-B transmissions (or should I say the car setup) made up to 1990's are like that. Top gear is engaged at 40 - 45 km/h if you don't push it hard.
One of these things I am not a big fan of in their gearing and have a slight preference towards manual boxes for that reason.
Regarding rev/min at top speed, you can calculate it easily knowing the wheel size, top gear ratio and differential ratio. The reading of engine speed at low rpm will be higher than calculated due to slip in torque converter.

outfox · 12-07-2019 1:40pm

Thanks guys. I was beginning to wonder if there was a problem with the gearbox.
It's just had a big service (€2500), and it's running sweet now.
Next thing is to sort out new carpets for the interior, and to get the wood replaced or shined up on the dash.

Neilw · 13-07-2019 11:53am

You have vacuum problems going to the gearbox if it's starting in second and going into top gear so soon. Those auto boxes are changed by vacuum and if there's a problems or weak vacuum it messes up the shifts.

Went through it on a 300gd with a 4 speed auto.

Type 17 · 13-07-2019 12:54pm

Some 4-speed MB autos (250 & 280, and maybe others) Start off in second by design. They only drop to first if you floor it off the line, or are going slow on a steep hill under heavy load, eg: full car on a narrow mountain road.

911s · 13-07-2019 5:12pm

Most start off in 2nd, G Wagen starts off in first due to extra weight. As far as I know the vacuum to gearbox is just to ‘modulate’ the shifts, make them less harsh and smooth the change

Neilw · 13-07-2019 6:14pm

911s wrote: »

Most start off in 2nd, G Wagen starts off in first due to extra weight. As far as I know the vacuum to gearbox is just to ‘modulate’ the shifts, make them less harsh and smooth the change

Vacuum also controls how soon or late the box changes, mine was changing into 4th a 30mph because of vacuum leaks.
I bought a vacuum test kit and replaced all the leaking hoses and it was fine after that, the engine would rev out now problem and change when it's supposed to.

outfox · 14-07-2019 12:34pm

I wasn't aware of the vacuum link to the gearbox. Must suss that out with the mechanic.

Tommyboy40 · 14-07-2019 12:59pm

They have a w/s selector by the shift. In winter mode it starts in second to minimise wheel spin. Early so's did the same

Mercedes auto gearbox query

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