1970 Mercedes 250 Automatic Transmission Question

[gnarlington1970]

Hello all, I am a little stuck in diagnosing my transmission flaring issue and I thought I could run it past all of you and hopefully get some suggestions of where to look next. I just bought the car off the original owner. It looks 100% like new and has been very well maintained. That said, The previous owner said that about 15 years ago the automatic transmission has flaring quite a bit between 2nd and 3rd. Being that we live in a small town without a Mercedes dealer, the local trans shops were not able to diagnose it. A couple shops thought it might be the transmission modulator. Anyway, the car has been doing this for 15 years and the trans fluid looks like red cool aid and has no burnt smell at all. Here are the things I have read about to check and have performed... 1. Trans modulator holds good vacuum testing with banjo fitting disconnected at intake manifold (plus it does not leak trans fluid when disconnected at trans) 2.With engine running, there is very good vacuum at the disconnected banjo fitting at the trans.
3. When driving the car with the vacuum banjo bolt at trans disconnected there is no difference in shifting.
This is the part that I don't understand. Can the trans modulator still be bad even if the diaphragm holds vacuum? The fact that it makes no difference if it is connected or not leads me to believe that it is the modulator. Does anyone agree? As far as I can tell, there is no adjustment I can make that will adjust the amount of flaring between gears (Like the diesel version). Any suggestions would be greatly appreciated. Thanks a lot for your time.
-Spencer

[Stretch]

Welcome to the forum Spencer

Have you got a W114 250 with a 722.104 transmission? I've been looking through the Russian EPC over on EverythingBenz - Mercedes-Benz Forum and Web Search Using Google trying to work out what you have.

I guess you've got a solid rod connection to the throttle linkage as well as the vacuum dash pot. Can you confirm this?

Do you know when the transmission fluid and filter were last changed?

[bcbens@embarqma]

I don't know if this will help or not. Either way it is good info to have. I copied it off the web a couple of years ago from a source that I no longer remember.

The later automatic used in your car is the K4C 025 and was used in this model from May 1969 through June of 1972.

It is easily identified by it's four bolt transmission pan as you observed.

Shift points on this version are regulated by rpms, accelerator position and engine vacuum.

There is a metal vacuum line which runs from the intake manifold to the vacuum modulator on the right side lower part of the transmission.

In addition a linkage rod runs from the engine to the transmission.

This linkage rod originates under the intake manifold and is attached to the accelerator linkage lever.

It runs back to the transmission where influences modulator pressure and shift points depending on accelerator position. During engine repairs this linkage or the engine accelerator linkage can be maladjusted causing shift problems in the transmission.

It is critical to have the engine linkages set properly before performing linkage adjustments on the transmission.

There is a procedure for setting the engine accelerator linkages.

Stu Ritter wrote an excellent article in the Nov.-Dec. 2003 issue of " The Star Magazine" on the mechanical injection systems. He covered engine linkage adjustment and setting along with much more. Very worthwhile reading.

I also have the factory set up procedure for the linkages.

So if you do not have any references here I can post this information also.

Anyway, after all the engine accelerator linkages are set to specs, the transmission should shift as follows:

With the shift lever in " D" with low accelerator pedal input the transmission should shift from 1st to 2nd at around 5-6 mph, then shift from 2nd to third at around 17 to 18 mph, then shift from 3rd to 4th at around 24 mph.

At full accelerator pedal but (not kick down), with the transmission in "D", the shift from 1st to 2nd should occur at around 22 mph, the shift from 2nd to 3rd should be around 32 mph, and the 3rd to 4th shift should occur around 56 mph.

Depending on the rear axle ratio these figures may vary slightly.

Changing the shift points can be done after setting the engine linkages to specs.

Simply shorten the the mechanical linkage going from the engine (below the intake manifold) to the transmission simply by screwing the threaded ball joint inward or outward.

Lengthening the rod will lower all rpm shift points.

Shortening the rod will increase rpms of all shift points.

Again I stress set the engine linkages first since this will often cure shift problems and make the engine run better!
Some other possible problems not due to linkage adjustments, may be a bad modulator valve.

These screw off the side of the transmission and can be replaced.

There is also an internal adjustment here but a bit complex since the transmission modulator pressure should be monitored with a gauge during adjustments.

A bad modulator may have a ruptured diaphragm and trans fluid will show up in the intake manifold and on #6 spark plug.

In this case a new factory set modulator valve is installed.

Internal problems are always a possibility but not likely if your shifts are all too high or all too low.

This information is not relevant to early automatics used in these cars (K4A 025) with the 16 bolt transmission oil pans.

These transmissions have different mechanical inputs for the shift points.

Keep us up to date and good luck.

[Frank Reiner]

Spencer:

Your description does indeed sound like low modulator pressure.
With the vacuum line disconnected from the modulator, and with the aid of a small mirror and flashlight, peer into the open end of the modulator. An adjustment screw lurks within (usually a hex socket). Clockwise will raise the modulated pressure. If there is no response to adjustment, the trans tail housing will have to be removed to determine the problem with the modulating valve which is contained in the tail housing. If there is a response, then the throttle pressure rod will require readjustment to restore the proper shift points. (See preceding post above)