Replacing both engine mounts and transmission mount on the 1984 300DT. Should I loosen BOTH engine mounts, jack up the engine then change out the transmission mount FIRST?

I did my transmission mount after the engine mounts by a couple of months. I don't think it really matters which order you do things in.

Put a thick board under the tranny pan and jack it up just enough to support it. Remove the transmission crossmember and the mount. LOL at how flattened the old one is compared to the new one. Install the new mount, then jack up the transmission enough that you can reinstall the crossmember and mounting bolts. Check tightness, remove jack and board. Have beverage of choice.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

I usually just undo the transmission mount and jack up the tail. If you undo the engine mounts at the same time, that may be too many degrees of freedom. Do the engine mounts once the transmission is secured. When you lift, make sure you mind the transmission cooler lines, fan shroud and the radiator hoses. Safest to remove all that stuff, but it can be done with everything in place if you're careful not to lift too high.

You really should (but not absolutely necessary) loosen the large clamping nut on the drive shaft. It allows a bit more movement of the front guibo and reduces the amount of twist/stress the guibo receives.

I disagree with Diseasel300's advice. This is a TWO beverage job!

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“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now

I am unfamiliar with the location of the large clamping nut on the driver shaft, where do I find it?

If you have engine shocks, release one end before you life the engine.

Sixto
98 E320s sedan and wagon

Anything different about doing say the transmission mount on a manual 1981 240D?? Other than the witchcraft needed to figure out which part number the manual transmission mount really is of course....


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Nope. Check for clearance on the cooling fan blades as the OM61X series are notorious for collapsing the LH side mounts due to diesel fuel and power steering fluid leaks.

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“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now

nice tutorial with photos....

https://www.ifixit.com/Guide/Mercedes+W123+Motor+Mount+Replacement/20760

Don't loosen the engine mounts. That is difficult and unneeded. Support the transmission on a jack w/ wide board under the tranny pan and newspapers to distribute the load. Lift it just 1/8" to relieve force on the tranny mount. As I recall, you must unbolt the short "cross-member" get the tranny mount out since it sits down in a U channel.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans

Don't forget to disconnect the throttle linkage BEFORE you lift the engine...and also disconnect one end on each of the the engine shocks as advised by Sixto.

Place cardboard bewteen the radiator and fan to protect the radiator fins from damage by the lower fan blades tilting forward as you lift the engine.

Here is a direct link to the free W123 FSM hosted by MBUSA.

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022

As you can see in the photo, I removed the throttle linkage from the connection on the firewall. I believe the linkage on the left side of the firewall pivot connects to the gas pedal in the car. Do I need to disconnect it?

Attached Thumbnails

 

No, other than to lubricate the linkage with a little ATF...since it is more accessible.

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022

Do the mounts one at a time. I suppose you could loosen the mount bolts a little (a very little), but don't disconnect more than one mount at a time. Too much unnecessary movement will make it hard to line back up. That drive train is Heavy.

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1985 300TD 1981 Scirocco 1.6D conv 1986 Golf 1.6D 2003 Golf TDI

I didn't have to drop the crossmember to do the tranny mount in my 84 300td. I just took the bolts out, jacked the trans up and changed it out.
Just obviously don't jack it up more than you absolutely have to.

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1984 300TD -- summer daily driver
Many others that aren't Mercedes...