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Old 10-26-2001, 08:15 PM
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Join Date: Jun 2001
Location: Denver, Colorado
Posts: 127
78' 300D transmission: too smooth to be true!!!

hey wassup every1:
my mechanic was tryin to fix my transmission about 3 months ago to have it shift close to the shift points found on the spedometer (the yellow dots between the kph and mph)....anyway after he had done several hours of tweeking the shift points, the car shifted terribly, it always shifted way too low in respects to rpms....and as a result it got terrible acceleration. I keep telling him to check for vacumm leaks and or anything that has to do with the vacumm element that might affect transmission shifts...He keeps saying that the transmision only operates off of pressure, and that there are no vacumm lines that go into the modulator or governor. He claims that instead of the transmission, there might be something wrong with the control pressure linkage system, the linkage system thats from the acceration pedal all the way down to the transmission. He says it might be too short or too long.

in the mean seems that i've been treating the transmission too rough... I'm always shifting the car manually using L,S,D shifts on the transmission and shifting it near those yellow dots on the spedometer. now for some reason when ever i shift to the next gear (from first to second gear), its way too smooth and there is no noticable jerks at all. It feels like a new honda civic transmision...way too smooth. I've been driving it like this for close to 3 months long do you guys think before it see the end of its light on this transmission? your responses are appreciated.


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Old 10-26-2001, 09:23 PM
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Join Date: Aug 2001
Location: Evansville, Indiana
Posts: 8,150

Those dots on the speedo are maximun rpm (redline) indicators. Going any faster will overspeed the engine (usually causing a catastrophic failure, like a broken rod). The only way to get the tranny to shift that late is to use the shift lever -- the transmission on it's own shifts earlier. You have to floor the accelerator to get this car to go -- it starts in second gear otherwise. Heavy foot is highly advised for acceleration.

I don't know if the '78 has a vacuum modulator -- you also have a control cable from the accelerator and an electric kickdown switch on the floor under the gas pedal. If the kickdown switch isn't working, the wire is broken, or the solenoid bad (of fuse blown), the tranny upshifts fairly early. W123 chassis has a vacuum modulator, W115 do not (and I think a 78 is a W123).

If you cannot feel the shifts, change the fluid pronto -- I fear that the transmission is toast from overheating and/or clutch failure. These do shift rather smoothly for the most part, but if you don't get at least a sensation of shifting at full throttle with manual shifts, the clutches are going. Check for metal shavings in the pan when you change the tanny filter -- metal shavings of any kind and more that a teaspoon or two of black powder indicates serious clutch wear.

A 78 300D isn't ever going to be a road rocket -- it will keep up if you push it hard, but you are going to lose the stoplight drags every time, so give up and take it easier -- the car will last longer that way!


1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!
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