300SD transmission downshift clunk

[MartinFT]

I have an 84 300SD that the previous owner did a bunch of work on, including some changes to the vacuum system. The car shifts wonderfully, but when downshifting into 2nd (or maybe it's 1st) there is a single "clunk".

Here is what he says he did:

Engine -

- cleaned and painted valve cover, installed new valve cover gasket
- camshaft valve adjustment
- installed various SS hardware on different engine components
- cleaned up turbocharger, installed new turbocharger gaskets
- installed new K&N air filter on turbocharger
- removed EGR vacuum lines and EGR solenoids from vacuum system
- installed new glow plugs
- cleaned up fuel injector fuel line routing
- tested VCV (vacuum control valve) and leak checked vacuum system lines, checked vacuum system at various points to make sure vacuum in/hg is set to proper specifications
- bled air from fuel system
- recharged air conditioning system using R134 freon
- fabricated and installed custom crank case vent system
- lubricated throttle linkage system
- new filters installed - oil, power steering, fuel

Transmission -

- installed new VCV, adjusted base hydraulic pressure to 42psi
- installed new (upgraded version) B2 piston
- adjusted bowden cable
- changed transmission fluid and installed new filter
- installed new fluid pan gasket

I read that the 300SD uses vacuum on the downshift only, and I suspect something he did is causing the clunk. I replaced the guide rod bushings as they were a wreck and thought that could be where the clunk was coming from, but it didn't help.

Any advice, anyone?

[leathermang]

Check your u joints...

[mannys9130]

Also check the flex disks, the trans and motor mounts, and the differential back lash.

[MartinFT]

What to look for in the u joints... just play? There is a little play in the driveshaft when turning. The noise "clunk" only happens on the one final downshift when braking.

[leathermang]

LOL... " JUST " play ? Yes, they are not supposed to have any play...

[tyl604]

To make sure vac is not the problem, you unhook the vac line from the VAV and connect a Mityvac to the line running to the vacuum modulator. The upshift will be fine because it does not need vac. After you reach fourth gear, pump up the vac with the Mityvac and then come to a stop. The downshifts should happen with no clunk. If you still have a downshift clunk, it is not because of vac; it is something else.

[sixto]

Do you mean downshifting when accelerating or downshifting as you slow to a stop? If it's as you slow to a stop, check the control pressure (Bowden) cable tension.

Sixto
83 300SD

Hello MartinFT,

I believe what you hear/feel is the downshift from 3 to 2, probably just as you come to a complete stop when using the brakes? These gearboxes end up in second when you are sitting at a stop and do a quick downshift into first as soon as you touch the accelerator. I'm not sure if the 84 version of the 722 gearbox is the same as my 81 but mine does this also. It isn't a heavy clunk almost as if there is play in the driveline. This I ruled out as the car only has 125K km (~77K mi.) so I doubt if this is the issue.

I found that by increasing the play in the Bowden Cable to ~3mm and turning down the idle speed to about 550 RPM the clunk was reduced significantly.

I haven't been able to track down the problem but I plan to tackle it late next year by changing a spring pack in the gearbox. I'm moving back to Canada in May of 2016 so I have all modifications on hold until after the move and the home construction.

I have purchased a Superior Shift Kit for the 722 and it includes springs to increase or decrease the firmness of the shift.

Superior Transmission - Superior K722A-D

Hope it helps.