72 280se Auto Trans 1st gear slipping when warm - Mercedes-Benz Forum

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post #1 of 7 (permalink) Old 08-10-2016, 10:06 AM Thread Starter
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72 280se Auto Trans 1st gear slipping when warm

Hey guys. My 108 project is back on after it took home in my new garage. I've gone through and cleaned/replaced injectors that had poor spray patterns and now have a happy running/idling 108.

I've also changed the Trans oil/filter with Fuchs Titan ATF 3353 that was recommended from Pelican Parts. The oil fluid drained was nice and pink, and the pan showed little signs of friction material (good!)

One issue that I haven't been able to tackle is the trans slips after the engine has been heated up in 1st gear. Once it gets going, there are no issues. Could this be a vacuum modulator issue? Worn frictions (again fluid is pink and pretty)?

Any help is appreciated.

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post #2 of 7 (permalink) Old 08-10-2016, 01:47 PM
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The torque converter (of the 3 Speed trans in a 4.5 r107) takes lot of fluid and might take a while to get the torque converter filled. Have you been continuing to check the fluid level on level ground with the car running?


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post #3 of 7 (permalink) Old 08-10-2016, 01:51 PM
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These transmissions are normally suppose to start in 2nd gear. if you put the indicator in 2, then it will start in 1st gear. So first make sure what gear it is starting in, then let us know what gear is slipping.

If your shifts are very smooth, then try tightening the tiny screw on the vacuum modulator 1/2 a turn. It is on the side of the trans and kept from turning with a 10mm nut. This should make your shifts harder. These trans should have a crisp shift.
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post #4 of 7 (permalink) Old 08-10-2016, 02:03 PM Thread Starter
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For reference this is a 72 280se 2.8 MFI engine with a K4C transmission (4 bolt pan)

shifts on the car are firm when its not slipping which makes me think its a vacuum related issue.
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post #5 of 7 (permalink) Old 08-11-2016, 05:01 PM Thread Starter
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Hey guys. Thanks for providing some more feedback. Here's what I did today:

1. Got the car up to running temp. shifts delayed but firm when cold
2. Drove the car in both D and S (snow). Could not reproduce any type of slippage for 5 min of city driving
3. Last start from 0 the trans slipped in D, I shifted to S and it shifted but continued to slip. revved the throttle slightly to get the trans to grab and it finally went as expected however if you step on the throttle. it slips again (line pressure??)
4. Started from 0 again, could not reproduce.

The issue is intermittent, appears to both both first and 2nd gear when it occurs. What controls line pressure on these Trans? Should I hook up a gauge and test?

Thanks again
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post #6 of 7 (permalink) Old 08-12-2016, 09:49 AM
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I copied this off the internet a couple of years ago. Don't know who wrote it, but it's very accurate as to what you need to do.

My '71 250C shifts perfectly from 1st to 2nd to 3rd to 4th. Just as described below.

Based on my experiences, FIRST make sure you have NO vacuum leaks. Make sure the metal tube vacuum line coming off of the rear of your intake manifold is connected securely at the manifold and to the vacuum modulator. SECOND ensure that your carb linkages are adjusted correctly. THIRD keep adjusting the linkage that connects your carb linkage to the transmission until you get the shifts where you want them. With that linkage disconnected or missing your car will most likely start off in second and shift into third and fourth quite rapidly.
I hope this helps you. I struggled with mine off and on for months before getting it to where I was happy with it. Good luck to you.

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 mis adjusted 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 kickdown), with the transmission in "D", the shift from 1st to 2nd should occur at around 22mph, the shift from 2nd to 3rd should be around 32mph, and the 3rd to 4th shift should occur around 56mph.

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 relavant to early automatics used in these cars (K4A 025) with the 16 bolt transmission oil pans.

These tranmissions have different mechanical inputs for the shift points.

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post #7 of 7 (permalink) Old 08-13-2016, 10:45 AM Thread Starter
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Date registered: Nov 2014
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Hello. I have a couple questions:

1. Is there any access panel to the top of the transmission tunnel? I've read about this with the SL guys but haven't seen anything.
2. Would it make any sense to connect a trans pressure gauge? If so, where is the access port?
3. What type of VAC should I expect at idle? 18in mg?

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