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  #1  
Old 12-20-2011, 08:21 PM
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300SD bizarre shifting immediately after changing engine mounts

I recently changed my engine mounts (83 300SD) and now my transmission shifts suddenly into 2nd at around 20mph. It then shifts into 3rd, within +5mph, almost immediately, but I do notice that it does engage into 2nd gear momentarily.

Is it possible maybe by moving the engine that the bowden cable was adjusted, and that be affecting the shifting? Is the bowden cable responsible for shifting timing, or is that a vacuum component?

I've been noticing some flaring from 1st to 2nd, but I suspect the B2 band brake for that, and I am curious if my recent engine work (lifting the engine up and down 3-6 inches several times) may have caused it - or if there are bigger issues.

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  #2  
Old 12-20-2011, 08:25 PM
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And if it IS the bowden cable, how do I adjust it?
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Old 12-20-2011, 08:34 PM
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I have a 84 300D which did the exact same thing. Rolling at 800RPM in 4th gear at 20MPH really doesnt give alot of oomph to start moving...On my car, there's a plastic white screw thing that effectively shortens or lengthens a cable connected to the rest of the throttle linkage contraption on the valve cover.

More length = higher shifting
Less length = lower shifting

It's on the passenger side (on my car). The cable tells the transmission where the throttle is, and from what i've read is involved in comparing throttle position pressure to the internal transmission governor.

I have mine set to shift at 2000-2300RPM, which seems a bit high, but I also seem to be rolling at 3000RPM at 70MPH anyway...so it seems to be a reasonable adjustment
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Old 12-20-2011, 09:26 PM
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When you say 'more length', do you mean that the cable is looser where the rubber sheathing is, or that the white screw has more threading exposed? I tried both ways (short screw, long screw) and there seemed to be some difference, but it almost seemed like the screw was going to come out (really long) before it gets to nearly any reasonable shifting. Almost all the way out had it shift into 4th at 40mph. I distinctly remember it should be at 60mph.

Having the screw really far out still had substantial slack in the cable. I suspect that it slide out the other end slightly (it's just way too much at the engine end), though I'm not sure if I can tighten the transmission end.
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  #5  
Old 12-20-2011, 10:39 PM
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After fiddling with the bowden cable, I have made the observation that the tranny shifting is not just a function of RPMs but also throttle.

I'm having a frustrating time getting it perfect (rather, what I remember as normal).... The more I turn the white screw out (toward the back), it delays shifting, but it depends on the throttle.

When I accelerate slowly (less throttle and less pull on the cable), the gears shift at a much lower RPM than if I accelerate quickly (more throttle and more pull on cable).

Problem is that in order to get it to shift at a reasonable RPM when I accelerate quickly (higher throttle/pull on cable), I have to sacrifice having it shift quickly, at a low speed when accelerating slower (less throttle/pull on cable). Best I can get it at a low RPM (tach is broken, so I can't know for sure) is to shift to 2nd at 15mph and then 3rd at 25mph, then 4th at 45mph. Getting it to shift decently at a normal acceleration causes it to wait WAYYYY too long when I actually 'floor' it.

And now after all this, it shifts hard. Not sure if I messed up a vacuum line (didn't seem so), or what. =(

Not sure if there's something else the issue.
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  #6  
Old 12-21-2011, 11:07 PM
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Transmission shifting

The bowden cable was never adjusted from it's regular place for years - I even noticed a black stain on the white nut that marked where it's been. The cable definitely feels loose, but I'm not so sure that adjusting the white nut out 1/4 inch fixes the issue completely.

When I adjust the bowden's white nut, 2-3 shifting is timed decently, but then 3-4 shifting is too late. When I try to make 3-4 shifting ok, 2-3 shifting is too soon, ie at 20mph when at a reasonable throttle. Isn't there a factory spec shift point for certain MPH?

I was noticing some flaring from 1-2, is it possible there are some valve/piston issues? With the bowden cable adjusted, it shifts hard (I think I have to tweak the modulator after changing the bowden so much), but no more flaring.

I just suspect changing the bowden cable (away from it's normal position) is a bad idea because it might indicate other underlying problems that need attention. Is it possible that something loosened up with the bowden cable on the transmission end, ie inside the pan (to explain the sudden loose end on my throttle end)?

I know the modulator eases shifting smoother, but I'm not sure if that has an effect on timing too, when timing has become abnormal.

Last edited by tomas_maly; 12-21-2011 at 11:22 PM.
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  #7  
Old 12-21-2011, 11:16 PM
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Location: Miami, FL
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I suspect you have a vac line unplugged or damaged/cracked somewhere as a result of the motor mount work. My SD did this when I unplugged a vacuum line that leads to the modulator. It did almost exactly as you're describing, it would rapidly go through the gears and end up in 4th very quickly. Before adjusting the bowden cable I would start by retracing all the lines under the hood and under the car, anywhere one could be unplugged, to see if that is indeed the problem. This behavior after some fairly involved work occuring so suddenly leads me to believe you will find the solution to your problem through this avenue.
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  #8  
Old 12-21-2011, 11:26 PM
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Sounds plausible. I had a pain trying to get the transmission to sit properly on the new tranny mount. So I lifted up the transmission separately from the engine (just 2 inches in the rear), and could've knocked something loose. Will check it out.
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  #9  
Old 12-22-2011, 12:43 AM
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Vacuum modulator checked ok. Pulled black wire from Y-connector near rear top of injection pump, and heard a strong suction noise, so it was holding the vacuum fine. Also directly hooked hand vacuum pump to line and it stayed steady.

Unless there are other lines to check (I only saw one vacuum line go to the transmission), I think that just leaves the bowden cable.

I got my tach working (cardboard inside the round plug to improve contact), and at the original bowden cable setting, it went 1-2 at 1800rpm/10mph, 2-3 at 2000rpm/15mph, then 3-4 at 2000rpm/20mph. But it took longer at the same throttle to reach 2000rpm once in 3rd. Seems odd it would reach 2000rpm quickly in 2nd, but maybe that's normal.

I adjusted the bowden cable and now (at a gentle throttle) I have it at 1-2 at 2000rpm / 15mph, 2-3 @ 2000rpm / 35mph, 3-4 @ 2500rpm / 50mph. Does that seem reasonable?

Is it normal for 2nd gear to gain RPM's quicker than 3rd gear? It still shifts at the same point, just that it reaches it sooner, it seems. I could be worrying about nothing at this point, and the hard shifting could be throwing me a bit off.

My understanding is that the bowden cable can get stretched over time and that it may require periodic adjustment.


Last edited by tomas_maly; 12-22-2011 at 02:02 AM.
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