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1983 300D Turbo
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120 Posts
Discussion Starter · #1 ·
I've been searching hi & low for some diy'er diagnostics to fix hard shitting.
by hard shifting, she seems to be taking longer to up shift and she clunks in when she does.
Here's what I know:
- the vaccum lines are routed according to a diagram i found on this site.
- the vacum line into the vaccum control valve (on top of the injector pump) reads 20 at idle.
- the vaccum reads 0 coming out of the VCV
- the line down to the modulator (green plastic unit on the side of the trans) holds vaccum.
- there is no apreciable difference when I bypass the VCV ( I know mabe that was a silly test but I've isolated problems in the past this way :eek:) )
-i've recently changed the vaccum valve levers (on top of the valve cover).

Ive found one page that seems to have a lot of info but I don't get it. transmission

can any one point me where to go next?

BTW i've been using the gauge on my vacuum pump for readings, I assume that's as good as a separate gauge! am I right?
 

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1981 300CD, 1973 280C
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448 Posts
I think the VCV is leaking. Should be 20 in and close to 20 out, then as you advance the throttle midway the vacuum should drop approx half way to 10 or so and at wide open throttle virtually no vacuum signal. Because the vac pump provides a steady vacuum regardless of throttle opening the system is designed to "leak" vacuum or dump it as the throttle is advanced. That way the modulator sees a varible vacuum as driven much like a gas engine would do. Dont assume the vac valve on the valve cover holds vacuum simply because you replaced the levers. I've had brand new valves leak coming across the parts counter! Did you read the New Page 1 article on vacuum tests? Its a sub title under Mercedes maintenence tips.
 

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1983 300D Turbo
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120 Posts
Discussion Starter · #4 ·
thanks for the input nut.
I'm cool with testing the valves once I know for sure the VCV is or isn't the culprit.
I was at the wreckers & dug up 2 VCV's & tested them by putting vacuum to them; neither of them have any vacuum coming out. And, this doesn't change with increased throttle.
So, according to my read of your reply, the VCV is the "leaker". Is it a crazy coincidence that the two I found at the wreckers behave the same way?
DO I go ahead & buy a new one?
is there another test to be sure it is shot?
 

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1983 300D Turbo
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120 Posts
Discussion Starter · #6 ·
I read this one: Mercedes Vacuum Troubleshooting

now here's a total twist.
I noticed that the cable that's attached to the throttle linkage was restricting its full travel.
when I disconnected it I achived fantastically smooth shifts but way too soon.
I'm prettry sure I've found the problem!!!
the adjustment at the trottle linkage is inn all the way.
Is there a length adjust,ment at the other end of the cable. I don't have access to a hoist right now & want to plan my next step.
 

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1981 300CD, 1973 280C
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448 Posts
My only experience was with my 81 which did not use the bowden cable. Generally the only adjustment is at the point where it pivots. I'm told the retainer for that cable tended to get brittle and break which would not enable a proper adjustment.
 
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