78 123 Transmission in 77 114 300D, Hard Shift, High RPM's
 

I recently put a junkyard transmission from a 1978 300D 123 into my 1977 300D W115, Everything is put back together but it does not like to shift into 1-2 or 2-3 until the engine RPM's get so high that it sounds like the engine is going to blow. I've read that the 77 did not have vacuum but I am not aware if the 78 used a vacuum system on the transmission. Also I was not able to find a connection on the 78 transmission for the kickdown linkage.
any help would be appreciated.

It seems like I installed a vacuum regulated transmission, my old transmission was manually controlled with the kickdown lever. Is there anyway to remedy this? Could I rig up a vacuum line to the transmission? would it help it shift normally?

Im pretty sure that W123 tranny you put in will be just fine because its also controled by linkage.

It seems to be shifting a bit better the more I drive it, although still pretty hard and at high RPM,s, could be the trans had dry seals and belts. It actually shifted into 4th at 55mph today. There might be hope yet!

I figure you have little to loose trying some internal transmission treatment product. I suspect they contain a petroleum agent that swells the seals. Have to drive 1500 miles or so sometimes to tell if it totally worked. Dried internal seals from sitting around unused are not that uncommon.

Tried adjusting the control pressure cable / lever? Sounds to me like the transmission thinks you're giving it more go pedal then you are.

You're sure the kickdown solenoid isn't stuck on, right?

-Jason

I checked the kickdown swich and it is not stuck. The transmission that I pulled out had a connection for the lever that is connected to the throttle linkage, whereas the transmission I installed from a 1978 300d has no connection for the linkage rod, but it has a red plastic cap on the right rear of the transmission that looks like a fitting for a vacuum linehttp://www.peachparts.com/shopforum/...cons/icon5.gif. I'll try to post some picts of the installed transmission, so as to clarify what I am refering to.

Weren't 1977 MB's 123's and not 115's? My 1977 123 300d had a transmission that was controlled by a series of rods that went back to the right side of the transmission from the throttle linkage down thru the intake manifold area. Not sure if the 78's had that same rod linkage or not but my impression is that there has to be some mechanical connection similar to the Bowden cable which controls the shift points. Vacuum controlled the harshness/softness of shifts on later 123's but the shift points were controlled by the mechanical bowden cable.

It sounds to me like you're gonna have to get the accelerator/vacuum linkages from a car with this setup so you can send shifting vacuum to the tranny modulator. One problem here is how the accelerator linkage connects to the firewall.

me bad! the 114 is 1976 not 1977, 76 was the change over year from 114 to 123 model

I'd put a MityVac on the modulator and start pulling some vacuum as I'm driving and see how it effects the shifting. If it solves the problem, you'll have to figure out whether you can just put a steady vacuum on that modulator or if you're going to have to get a VCV to vary the vacuum.

I know this sounds stupid, but where is the modulator ?:confused:?, there is no vacuum line connected to the transmission and nowhere on the transmission to connect the manual linkage.

On the earlier trannies the modulator is on the passenger side of tranny.