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Old 04-19-2000, 07:05 PM
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The trans on my '84 300SD with 95,000 on it 'snatches' or 'jerks' when it shifts, especially noticable at slow acceleration. I think I read here that there is an adjustment for this. How is it done? Thanks!

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Old 04-19-2000, 09:10 PM
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Join Date: Mar 1999
Location: Suwanee, GA, USA
Posts: 4,712
Any time someone asks about a transmission adjustment, I will tell them to go to a shop. You have to check and make sure that the basics are intact before adjustment is justified.

Donnie Drummonds
1992 500E (very soon I hope
1981 280GE SWB
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Old 04-19-2000, 10:01 PM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
This is a really complicated adjustment because there are about four different things to be adjusted and they all effect one another.

The basic concept is to have the modulator receive vacuum that would resemble the vacuum of a gas motor (the diesel has no throttle and thus no manifold vacuum). This process starts using the vacuum created by the vacuum pump for the brakes. The vacuum flows through a precise oriface (the first hurdle - it gets plugged due to its small size) toward the modulator. A branch goes to the proportioning valve at the rear of the injection pump (white plastic). This is where the size of the supply oriface is critical as this valve is a precise leak designed to take 12-15in. vac at idle to zero in. vacuum at full throttle. If the oriface is too small the leak has too much effect (vacuum drops too fast). If the oriface is too large the leak will have reduced effect the 15in signal at idle will stay high or only drop partially as the throttle goes full.

Now is where it gets complicated. The proportioning valve is adjustable. The application rate is adjustable by lengthening/shortening the control rod. Some have an internal adjustment for range, ex: 10 to zero versus 15 to zero.

Next, actually first should be the basic modulator pressure at zero vacuum with hi idle or above, engine rpm. This should be set by gauge for anyone not very experienced. We set all rebuilds to proper pressures otherwise we usually go by feel.

To throw a wrench into the works, proper coordination of the vacuum being applied often has to do with when the trans shifts which is affected be the control pressure cable adjustment.

To get the best shift may require this versus that judgements of all these positions creating a virtually infinite number of possibilities.

And beside all that you must have good power to get proper shifting. Boost at the aneroid absolutely should be verified before a bunch of compensatory adjustments are attempted.

Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician
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Old 04-26-2000, 09:39 AM
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Thanks for your informative reply! And I will eventually have a tech take a look, as it still is not perfect. But as in the normal course of things when I buy a car, I changed the trans fluid and filter (looked like it had never been done in 95k) and the shifting improved greatly! Just reaffirms your comment about need to get the basics right first.

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