transmission shifting hard - PeachParts Mercedes-Benz Forum

danwashuk · #1 10-06-2007, 06:37 PM

This is my first post to the boards. I recently purchased a 1974 450 SL and I have learned quite a bit reading the posts over the past few days. I thought I had found the solution to my hard shifting problem but the response I read does not seem to apply to my transmission. I suspected a vacume problem and I have located the vacume line at the intake manifold, followed it down to the connection point at the transmission, but it looks intact. It's a metal line all the way to the connection at the transmission. I looked for an adjustment of the modulator valve, but I don't see one. I also tried to see if I could buy a modulator valve, but The parts supply houses I have checked so far on the internet don't seem to have one in stock. Am I on the right track here??? Any help would be appreciated.........Thanks........Dan

danwashuk · #2 10-07-2007, 07:08 PM

Today I attempted to remove the metal vacume line. I was unsucessful!!! The way it wraps around the bell housing and transmission made it impossible to get out. I had hoped to inspect it for damage. I would like to test it with a vacume source but I don't know how to attach lines to the peculiar connections at the ends of the tubung. Of course I still don't know if this vacume source is at the root of my hard shifting transmission.

danwashuk · #3 10-08-2007, 10:47 AM

I am thinking about cutting the metal line at both ends, keeping the connectors and disposing of the line in between. I can replace the line in between with rubber vacume hose. This way, at least, I can be assurred that the line has no damage in the areas I can't see.

Also I plan to go through my entire vacume system replacing as much of it as I can. The hard plastic lines seem ok but most of the rubber lines look pretty old and tired. It could be that I have leaks causing insufficient vacume at the transmission.

Of course I could be completely wrong about what is going on with my transmission. I had hoped to get some help from the experienced folks that I have found on this site, but so far no responses to my post.

sgaar · #4 10-08-2007, 11:08 AM

It is not that we are not here (not that I am that experienced, but anyway...) and I would love to give you some pointers but I truly don't know.

The plan to replace the line with a rubber hose seems on sound paper but I am not sure how it will play out in the real world.

Do you have the repair manual for your car? There might be some hints as how to remove the metal line without damaging it. Also, have you checked the fluid levels and the color? You need to bring the motor up to operating temp and then check the level. The fluid should be light red if it is OK. Maybe a ATF change will solve some of the problem. That is properly what I would do before I have another go at the vacuum line.

MB claims that the tranny and rear diff are closed systems but common sense dictates that the fluids should be changed at least every 100K miles. If you change the tranny fluid you should also change the filter.

JFawcett · #5 10-08-2007, 11:22 AM

I wouldn't go cutting on the transmission lines.......Mercedes made that line metal for a reason.
Can you provide some additional details on when the transmission is shifting hard? Is it when the car is cold, does it shift hard in all gears or just 2nd to 3rd? Hard acceleration?
Does it get better or worse ever or is it common problem?
Could be a weak pump on either end of transmission......
Fluid could be low.....or old.....
Any evidence of leak and/or road damage?

danwashuk · #6 10-08-2007, 03:36 PM

Thank for the great replies. I am very happy to hear from both sgarr and JFawcett.

Yes, I have two repair manuals, the CD from Mercerdes and the Chilton paper manual. Neither manual provides any information on the transmission, other than how to change the fluid. I found no mention of the vacume line. The fluid looks ok and smells ok to me, but I like the suggestion of changing it. I just learned recently that I can do this myself, thanks to Mercedes providing a method to drain the torque convertor.

OK I won't cut the metal line, but I wish I could figure out how to test it's integrity.

The hard shift is most pronounced going from first to second, It will bark the tires if too much gas is given. It shifts hard from second to third also but not as pronounced. That may be because by then I am not accelerating as much since I am pretty much up to the speed limit. It doesn't matter whether it is hot or cold, and even when accelerating very slowly it shifts hard.

I see no evidence of leaks or road dammage.

Thanks again for the replies.

I think my next step will be to replace the fluid, and change the filter. Also I have some vacume lines that although they don't appear to be leaking, look pretty deteriorated, so I will replace those also.

450slcya · #7 10-08-2007, 06:16 PM

It is my understanding that to adjust the vacuum modulator it must first be removed.

The threaded portion of the modulator that screws to the transmission has a screw adjustment in the inside of the fitting, as it were.

I cannot tell you in which direction to turn the adjuster. Some trial and error on your part might be in order.

Dave.

danwashuk · #8 10-09-2007, 10:02 AM

Hey Dave..... thanks a bunch for that information!!!!!!!!!!

I thought about removing the modulator, but was unsure about what sort of trouble I might cause by doing so. Do you think transmission fluid will come pouring out when I remove it????? Seems likely to me.

This forum is worth it's weight in gold..................

danwashuk · #9 10-22-2007, 11:01 AM

Last weekend I replaces all the black rubber vacuume lines I could find. This weekend I changes the transmission oil and filter. It took me about an hour to find the drain plug on the torque convertor. Bump the ignition then crawl under the car and look for the plug. I marked the torque convertor with a felt pen through the inspection hole each time so that I could see where I had been. Eventually I found it and got it drained. I am happy to report that the transmission is shifting much better now!!! It still shifts a little hard but I am no longer burning rubber when it shifts to second gear. Also, my vacuum system seems to be working better now. Door locks more positive, and the car seems to hold vacuume longer when the car is turned off.

I may try fooling around with the modulater at some point in the future, but for now I want to drive it!!!!!

230/8 · #10 10-22-2007, 11:20 AM

dan:

You are on the right track...keep searching out the vacuum lines and replace all of the soft rubber bits. Careful with the plastic tubing, it gets brittle and can crack when changing the rubber hoses. There are also some one-way valves that can fail so once you have all the lines secure you should look at them to cure any remaining leak-down issues. Have you changed out the big line to the MAP sensor? This is on the DS fender well panel and goes to the back of the engine. Check its condition, and then secure it with a small clamp. Do the same for the larger auxiliary air hoses to and from the idle air valve. These are just push on connections. I have seen early factory photos that show these were clamped on early models...probably some bean-counter wanted to save a buck. Once all the vacuum lines are tight the engine will probably run better, too.

230/8

danwashuk · #11 10-22-2007, 02:09 PM

Thanks 230/8 for your reply. It there a source for that plastic tubing???

I looked at the one way valves, I found that I could not suck air through them with my mouth and when I turned them around the other way I could suck air through them but it sounded like I was playing a comb with tissue paper. Like the diaphragm inside was oscillating. Is that normal??? I have two of them on my car and both exibited this feature.

I have not changed out the big line to the MAP sensor, in fact, I don't know what a MAP sensor is, but I will do my homework and find out, as well as check the line for leaks and a clamp.

I am aware of the big lines to the idle air valve, I didn't see any leaks there but they are old and probably need replacement.

Thanks again for your help................Dan

230/8 · #12 10-22-2007, 04:27 PM

Check with Phil for the tubing, or the dealer, or the Classic Center. Valves seem OK, MAP sensor is about the size of a can of corn, down below the power brake booster, aluminum with a large electrical plug in one end. Should be a black plastic cap on the other end. Rubber hose connects along side the electric plug.

230/8

danwashuk · #13 10-23-2007, 07:14 PM

I replaced the hose from the MAP sensor to the intake manifold, and put hose clamps on both ends. Also doing my homework on the MAP (manifold absolute pressure) sensor.

Transmission is still shifting hard, not as bad as it was before I changed the oil and filter but still too hard.

I am still persuing the vacuume lines looking for leaks.......

aam · #14 10-23-2007, 09:57 PM

Quote:

Originally Posted by danwashuk

I replaced the hose from the MAP sensor to the intake manifold, and put hose clamps on both ends. Also doing my homework on the MAP (manifold absolute pressure) sensor.

Transmission is still shifting hard, not as bad as it was before I changed the oil and filter but still too hard.

I am still persuing the vacuume lines looking for leaks.......

Go to autozone get a vacuum pump it is free.
With this tool you can ck for leaks on the lines, you may have to make some adapters to fit.
I am late on info. about draining the converder, but any ways here it goes.
You turn the engine over, from the crank shaft pulley.
There is a big bolt dip in the center , with a 1/2 drive and a 27mm socket.
Turn the same direction the engine turns, till you see the converter plug.
Easy if you know.
Not with the key.

Robert Squires · #15 10-27-2007, 05:57 PM

* Danwashuk, have you determined whether your steel vacuum line to the modulating valve is blocked? Is there vacuum at the modulating valve end with the engine running?
* The modulating pressure is adjusted with the modulating valve installed, and a fluid pressure gauge connected to the modulating pressure port (right next to the valve). Take off the rubber cap on the modulating valve to expose the adjustment key (old/new styles vary somewhat).
* The final pressure reading is taken while driving the car, usually at 30mph in D. You do a basic adjustment in the shop and then take it out on the road for a final check. This is why most trans pressure gauges are attached to 6-8' hoses so that you can hold the gauge in the car (hose through the window) while you are road testing.
* If you replace the modulating valve, you will need to adjust the mod pressure because the valve doesn't come set from the factory (and because one valve applies to several transmissions, each with a different mod pressure).