1983 300SD No Downshift While Moving - PeachParts Mercedes-Benz Forum

EzBusiness · #1 02-17-2012, 02:31 AM

Hello Everyone,

I really Need your help. I recently purchased a 1983 300SD that has a particular issue I can't figure out. It takes off fine in 1st gear up shifts perfect to 2nd, 3rd & 4th. It is just a little slow to engage every time I put in gear but once in gear it up shifts great..
The problem are:

1) if I try to downshift by pushing the kickdown switch the tranny goes in what seems to be neutral or between gears. The only way to get it into gear is to manually shift to third while letting off the accelerator and then it will engage again.

2) Every time I slow down and let off the pedal the trans is suppose to downshift from 4th to 3rd, when i re-accelerate it just disengages the gear and goes to neutral until i come to a complete stop and RPM's are 650 or I let off the pedal and manually downshift. Also when I put it in gear from park to Rev or 1st if the RPM's are more than 650 it will not go into gear

I have checked the Vacuum going to the Modulator at Idle it is at 18, I also checked the line going to the modulator for leaks and there is no leaks. I also changed the filter and fluid. I don't know what to check next.

I would appreciate any suggestions.

EzBusiness
Phoenix, Az
1987 190D 250k plus
1983 300SD 250k plus

gatorblue92 · #2 02-17-2012, 06:21 AM

Are you certain the kickdown switch is functioning correctly?

EzBusiness · #3 02-17-2012, 12:33 PM

Hello Gatorblue92

I think so because when I hit the pedeal to the floor while in 4th it wants to downshift but it goes into neutral or between gears just accelerates with no response. How ever if im in 2nd and I hit the pedal it does shift down to 1st. That is why I think the switch is good.

Thanks,

EzBusiness · #4 04-22-2013, 05:00 PM

I have been trying now for a year to figure this out on and off. The car will downshift and up-shift properly from 1-2, 2-3- 3-2 and 2-1. I am able to go up and down gears 1 thru 3 without any problems, i only have the problem when I get into forth and I need to downshift back to 3rd or lower. If i push the pedal all the way down the kick-down switch does activate but it seems to go into neutral, if i move the shifter manually from 4th to 3rd it goes into neutral also.

The conclusion is the kick-down switch is working but for some reason the gear isn't engaging. The only way I get a gear to engage is to come to a complete stop and manually put it into low gear and then the gears work fine going up.

I hope someone has had experience with something like this to guide me in the right direction.

EzBusiness

abrankhalid · #5 04-22-2013, 07:03 PM

The kickdown switch under the gas pedal is attached to a magnetic solenoid at the transmission. I do not know how to check it but its one part I thought was sticking on my 87 190d. I ruled it out due to proper downshifts after a fluid and filter change. I would recommend removing the solenoid from the transmission ( you don't have to take the transmission out. Its right by the vacuum modulator on the rear right of the transmission at the very bottom ) and inspecting if it actuates properly when the kickdown switch is depressed .
I might be wrong but that is the only logical reason i can think of.

Since its related only to the kick down to 3rd from 4th, it might be a worn out valve in the valve body that is leaking out the pressure. Some one with more knowledge of the internal workings of the transmission might be able to tell you if there is a specific part that governs only the 4th to 3rd downshift.

Stretch · #6 04-23-2013, 04:01 AM

Ideally you need to measure the transmission with hydraulic pressure gauges - can you do that? Have you got the technology?

####

But if you want guess work =>

It could be a valve body related problem.

If you look in this thread =>

http://www.peachparts.com/shopforum/diesel-discussion/323259-722-303-bits-photo-shoot-3.html

I've labeled each of the positions of the valves so people can work out what to take apart. You need to check the valves 11 and 12 which unfortunately are difficult to reach with the valve body in the transmission / car.

As the K2 clutch is engaged / disengaged for 4th you also need to look at valves 49, 51 and 59...

If it isn't a valve body problem then you need to investigate the governor (although that shouldn't be a problem if the other gears shift as they should <= THIS IS REALLY IMPORTANT! If this isn't the case then you are chasing your tail)