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Old 06-11-2016, 04:07 PM
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Join Date: Mar 2015
Location: Houston, TX
Posts: 1
W123 300D Transmission Issues

I've got a beater 1984 300D that I haven't driven regularly in a while. When cold, it shifts just fine all the way up to 4th, although maybe some of the upshifts are a little harsh. Once it gets up to operating temp, the shift points seem to rise unreasonably high, to the point where it never gets out of 2nd when driving around town. It will shift into 3rd if you get it going fast enough (like 5000 RPM), but I haven't driven it on the highway yet to see if it will go into 4th at all.

The fluid level is fine, the fluid looks good (red and non-aerated), and it doesn't smell burnt. Adding a bottle of Lucas Transmission Fix (which has given me good results in other marginally-shifting vehicles) made no noticeable difference. The transmission does have a slow leak, so it's possible it was run low on fluid at some point, but I don't recall it having this behavior immediately before I parked it for a year.

I'm hoping the interior guts are still good and there's just an adjustment that needs to made somewhere (vacuum modulator, bowden cable, etc.). Unfortunately, everything I'm reading about this transmission is pretty complicated and I'm not sure where to start. My current plan is to make sure I still have healthy vacuum to the modulator, but if that's not the problem I'm not sure where to go from there.

Any suggestions?

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Old 06-12-2016, 03:35 PM
vstech's Avatar
Join Date: Mar 2006
Location: Mount Holly, NC
Posts: 26,389
My first suggestion would be to pull up the drivers floormat and look under the accelerator for the kick down switch and make sure it's free, or even unplug it.

If that doesn't fix it, open the hood and make sure the Bowden cable is attached to the linkage, and moving freely.

Next I'd drain the pan and torque converter of fluid, and refill with cheap Walmart brand fluid, get the fluid level correct at operating temp on a level surface.

This way you know what is in there.

Next get a bottle of trans-X fluid solvent and drive for a week or so, then drain the fluid again and drop the pan and change the filter and gasket. Refill with high quality dextronIII fluid.

If no improvements are found after this... Consult a transmission rebuilder.

John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!
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