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  #1  
Old 04-29-2011, 12:51 PM
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'81 300SD new tranny goes clunk

Just put a used tranny in the 81 300SD. It shifts reasonably well and shift points are right so I think I have no problem with the Bowden cable or the new modulator. However I get a bad clunk at every downshift.

I have read all the many posts on clunk and guess it must be a vac problem but I have tested with the mityvac and am not sure what is supposed to leak and what is not.

Breakdown - starting with the rubber Y connection coming from the top of the VCV. The line going to the tranny holds 25psi; the short 2" line from the Y going back to the VCV holds 9psi; so no leak there. Modulator is not leaking. Not sure why the VCV holds only 9psi but the old tranny shifted fine before I ran it out of fluid so I think the VCV is OK.

The line going to the EGR black box does not hold vac (it is now plugged with a golf tee as seen in the picture). The line going to the very large brake booster(?) line holds no vac. Since my brakes work fine, I assume this is designed with no vaccum. The EGR line is now out of the equation.

Summary for top rubber Y - vac at tranny line and vac coming from VCV. The other lines have no vac and I assume they are designed that way.

Now for the bottom rubber Y - no vac. One line just vents to the cabin so must be designed for no vac. The other line goes to EGR black box and has no vac (it is now plugged with a white golf tee too). This EGR line is also out of the equation.

Summary - guess the bottom Y is not designed to hold vaccum.

So does anyone think I have a vac leak in a spot not designed to leak? If not, then why the clunk at downshift?

Appreciate your thoughts. See picture and white golf tees.

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'81 300SD new tranny goes clunk-dscf0658.jpg  
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  #2  
Old 04-29-2011, 01:07 PM
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As long as the line to the modulator holds vacuum, you are fine.


Bowden cable (TV / pressure cable) should be slack until the rack begins to move. No tension in cable until RPMs increase with pedal travel.

once that is set, Tee into the modulator vac line and drive around with a vac gauge; take notes on vac levels. (highest and lowest at partial throttle and full throttle)
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Old 04-29-2011, 01:17 PM
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Please post back what worked or not.

I am having a similar problem after disconnecting Vacuum Lines, Fuel Injection Pump and Oil Filter Housing.

Within the next hour I will be checking the Vacuum Connections compared to the pic in the manual. (However, I tagged my Vac Lines before I removed them so I do not sure how I could have connected them wrong.)

Will check with Vac Gauge also.
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Old 04-29-2011, 01:55 PM
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I'll second that on the request for feedback on what helps this.

Is your problem on all downshifts, or just when it drops into second when you approach a stop?

I had this problem with my '82 and was told at the time it was worn clutch packs. I sold it to a friend, so I want to update him if there's a fix lurking.
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  #5  
Old 04-29-2011, 02:42 PM
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Quote:
Originally Posted by Diesel911 View Post
Please post back what worked or not.

I am having a similar problem after disconnecting Vacuum Lines, Fuel Injection Pump and Oil Filter Housing.

Within the next hour I will be checking the Vacuum Connections compared to the pic in the manual. (However, I tagged my Vac Lines before I removed them so I do not sure how I could have connected them wrong.)

Will check with Vac Gauge also.
In my case the Trans was down shifting OK before I started to do the other work I did; so, for me not the Clutch Pac.
I also found I had reconnected the Vac Lines I removed OK.

I disconnected the line to the Transmission from the Y connector a tested with a Mighty Vac type Tools and had zero Vacuum.
Sure enough I found that the other end was disconnected from the Modulator. I guess it came loose during my struggle with the Oil Filter Housing or other Vacu Lines.

After replacing the short length of Vac Hose that hooks to the Plastic Tubing and hooking it up to the Modulator the Trans is back to normal.

However, after doing the above when I connect the Might Vac I can pump it up to around 22 hgs of Vac but it bleeds back to zero in about 2 seconds.
Is My Modulator leaking Vacuum?
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  #6  
Old 04-29-2011, 03:08 PM
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Well, I am glad the texting while driving police did not see me. I drove around and here is the story.

Starts and idles at 9 inches. When you touch the gas it goes down immediately to about zero-2 inches (Mityvac has a box that says zero which appears to cover everything from zero to four inches). At full throttle it is about zero. While driving/coasting it registers about 9 again. It upshifts smoothly in the zero psi box. But the downshift is at 9inches again and is a clunk.

So - idle at 9inches
drives at 0-5inches based upon throttle - more throttle = less vacuum
upshifts at 0-2inches smoothly with normal throttle
downshifts at 9inches with a clunk whether this is coming to a full stop or just downshifting from third to second.

Seems to me that it is bleeding down perfectly in response to throttle. Should I try turning the modulator CCW about one click? Two clicks? Quarter turn?

Diesel - in response to your question I think your modulator is leaking. Mine pumps to 25inches and holds it; no leak at all. At least for about a minute or so until I released the pressure. Indy tells me he installed a new vac modulator when he installed the used tranny.

Last edited by tyl604; 05-10-2011 at 02:10 PM.
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  #7  
Old 04-29-2011, 03:41 PM
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That vacuum is too low. Put it into the standard range (10" - 15 ")

Start from 15" and work your way down to 10" if you don't find suitable shifting.

If none of that works, then mess with the modulator.


Also, might I recommend upgrading the system with a vacuum amplifier?

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Old 04-29-2011, 03:44 PM
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JT - do not remember how to increase the vac from reading all those posts. What do I mess with?
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Old 04-29-2011, 03:48 PM
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First make sure the lever on the VCV closes to 2mm of the stop at full throttle.

Then, open the side of the VCV (a little domed cap) and very gently turn the nut inside. Please read a little before doing this. I think people adjust this improperly and mess up their valves.

I recommended the vac amp because its easy to adjust and smooths out the vacuum signal.
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Old 04-29-2011, 03:50 PM
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Steve is your man!

http://continentalimports.com/ser_ic20242.html
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Old 04-29-2011, 09:29 PM
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JT - your post says move the lever so it comes to within 2mm of the stop at full throttle. Brotherton says 0.5mm unless I am reading it wrong. That is a very big difference. Who is right? Right now I have a 1/4 inch gap which is huge.

Am I measuring correctly? Pull the throttle linkage all the way and watch the lever on the vcv move. The gap with the throttle pulled all the way back is 1/4 inch!!

Once I get the gap right, do I move the nut under the dome CW or CCW?
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Old 04-29-2011, 10:22 PM
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Doh.

Always trust guys like Steve before me. .5mm is correct (it is also .020 inches = confusion) 1/4 " is quite big. Remove that before adjusting VCV.

CW = more vacuum / higher initial vacuum signal to trans. Adjust in small increments and test drive each time.
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Old 04-29-2011, 10:47 PM
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Quote:
Originally Posted by tyl604 View Post
Well, I am glad the texting while driving police did not see me. I drove around and here is the story.

Starts and idles at 9psi. When you touch the gas it goes down immediately to about zero-2psi (Mityvac has a box that says zero which appears to cover everything from zero to four psi). At full throttle it is about zero. While driving/coasting it registers about 9 again. It upshifts smoothly in the zero psi box. But the downshift is at 9psi again and is a clunk.

So - idle at 9psi
drives at 0-5psi based upon throttle - more throttle = less psi
upshifts at 0-2psi smoothly with normal throttle
downshifts at 9psi with a clunk whether this is coming to a full stop or just downshifting from third to second.

Seems to me that it is bleeding down perfectly in response to throttle. Should I try turning the modulator CCW about one click? Two clicks? Quarter turn?

Diesel - in response to your question I think your modulator is leaking. Mine pumps to 25psi and holds it; no leak at all. At least for about a minute or so until I released the pressure. Indy tells me he installed a new vac modulator when he installed the used tranny.
Thanks.

What I found odd is that with the Vac Hose disconnected at the Modulator I had the hard down shift but with the forward gear shifts there was not much of a change when the Vacu line was connected?
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  #14  
Old 04-29-2011, 10:52 PM
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Quote:
Originally Posted by tyl604 View Post
JT - do not remember how to increase the vac from reading all those posts. What do I mess with?
Article on adjusting the Vac.
http://www.continentalimports.com/ser_ic20242.html
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  #15  
Old 04-30-2011, 10:40 AM
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JT - got it. Had intended to do it this AM but wife is dragging me to Stone Mountain. Maybe this afternoon.

Thanks for the feedback.

There must something different about the way MB designed the downshift. In most instances you will be off the throttle when it needs to downshift so the vac should have climbed to max; that would seem to give you a harsh downshift except that MB must have designed something to handle it. On the upshift you generally are giving it throttle and the vac is going away so a smooth upshift. But on the downshift the opposite is true. So how did they design it to downshift?

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