722.120 transmission question - PeachParts Mercedes-Benz Forum

turbozeke418 · #1 03-20-2019, 11:22 AM

WHY is the only transmission that works with a 1978-80 300SD a 722.120? If I used a 722.3xx transmission, it won't work? It's the same OM617 engine as the rest of them. You would think all of the transmissions would be interchangeable like the engines are. FRUSTRATING!!!

Frank Reiner · #2 03-20-2019, 03:07 PM

Quote:

Originally Posted by turbozeke418

WHY is the only transmission that works with a 1978-80 300SD a 722.120? If I used a 722.3xx transmission, it won't work? It's the same OM617 engine as the rest of them. You would think all of the transmissions would be interchangeable like the engines are. FRUSTRATING!!!

A 722.3 would "work" with the 617. To install it in the W116 chassis the linkages and cable from a 617/722.3 car (W123 or W126) will be required, as well as a mount crossmember, and possibly a front driveline of a different length.
Your problem seems to be isolated to W116s located in Monmouth County.

Assault · #3 03-20-2019, 09:35 PM

What are you trans symptoms?

722.120
No expert by any means Different gear ratios might be part of it. Even though a w116 450SEL and 300SD share a 3.07 ratio transmission their gear ratios are different.

Also Transmissions after 1980 no longer had mechanical speedometers, all electronic.

Also if your trans is shifting poorly check the trans fluid level level should be at the max line mark when vehicle is at operating temp, check vacuum pressure to the modulator (can be done from engine bay, if leaks check modulator directly) then check b1 piston.

I sent you a PM in regards to a 78-80 300SD transmissions. Always good to a keep a spare or two...

turbozeke418 · #4 03-21-2019, 02:49 AM

Quote:

Originally Posted by Assault

What are you trans symptoms?

Also if your trans is shifting poorly check the trans fluid level level should be at the max line mark when vehicle is at operating temp, check vacuum pressure to the modulator (can be done from engine bay, if leaks check modulator directly) then check b1 piston.

I double checked the fluid level and it was higher than the top of the line. I removed enough to get it just to the top of the top line and it drove even worse. It also started flaring into each gear. I put the same amount in that I drained out and it shifts nice again.
I disconnected the solenoid so it starts of in 2nd and shifts through all gear with out flaring. I'll just drive it like this for now until I find a replacement.

MB140300SD · #5 03-21-2019, 07:42 PM

This is not correct. The 81 123 Wagon had an electronic speedo, but the rest of the 123 line had mechanical 722.3 trans. The 124 diesels for sure had 722.3 and 722.4 with mechanical speedo cable also. Not sure about the gas models but they probably did also.

Quote:

Originally Posted by Assault

What are you trans symptoms?

722.120
No expert by any means Different gear ratios might be part of it. Even though a w116 450SEL and 300SD share a 3.07 ratio transmission their gear ratios are different.

Also Transmissions after 1980 no longer had mechanical speedometers, all electronic.

Also if your trans is shifting poorly check the trans fluid level level should be at the max line mark when vehicle is at operating temp, check vacuum pressure to the modulator (can be done from engine bay, if leaks check modulator directly) then check b1 piston.

I sent you a PM in regards to a 78-80 300SD transmissions. Always good to a keep a spare or two...

Assault · #6 03-22-2019, 04:23 PM

Okay, That makes sense My first w116 1980 had the transmission replaced with one from a 300D I had assumed it was a pre-80. Unfortunately, ( I assume) due to different gear ratios the end of the transmission destroyed itself and refused to shift into any other gear but 1 and R. I had replaced the speedometer gear in the tail end of the housing, because the one that was on there was completely chewed up, once replaced and transmissions died after a few thousand miles, pulled the gear out again and it was toast.

Stick with a 722.120 Transmission in a 78-80 300SD, learn from my headache.