Slipping transmission '79 300SD

Greetings Everyone, Need some input on a transmission problem. My '79 300SD began slipping yesterday, and flairing badly between the 3-4 shift. Change was quite sudden, as in between one intersection and the next. It has been shifting fine up until now. It will now hold RPM only at light throttle, and will slip badly under load in gears 2, 3 & 4 and flairs on all shifts under throttle. Fluid & filter were changed less than 1K ago, and pan showed no debris or metal then. Today I tested the modulator, and it does not hold vacuum, but is getting signal from the pump. As a test I unhooked it and plugged the lines and test drove. No perceptable difference. Will a bad modulator cause this severe of slippage? I am thinking that 0 vac to the modulator should increase pressures and harden the shifts, but maybe I'm thinking wrong. Now I'm looking for more ideas. Anyone know of other components or systems I can check and/or replace while in the car, or is this one just toast and is it time to rebuild? Any thoughts are helpful. Thanks.

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Body and Interior Specific Aid

Chassis Model Number

W116

Recheck the last thing you did before the trouble started.

This is a common troubleshooting procedure. Not to offend because your post did not say who did the fluid and filter change, but since it was done soon before you started having problems - I would start back there. Your symptoms sound very familiar to what happens when the fluid is low or pump pressure is down.

Have you checked the fluid level today with engine hot and running and transmission in park. Are you certain the correct fluid was placed in the tranmission. (Believe it or not I had a car come in with the same symptoms and someone had put engine oil in the trans!). Was the correct filter replaced and installed properly. (I came across another bad transmission where the filter was sitting in the bottom of the pan). Did the torque converter get drained and was the plug installed and tighened.

You are correct in your assumptions about vacuum. When I have a transmission that is shifting soft or flaring I often remove vacuum to the modulator for a quick test. If the tranmission is ok your shifts should get hard and jerky immediately.

If all those things mentioned above check out ok, then I suspect your pump or the pump drive mechanism is failing. There could be other internal issues as well. If the fluid smells burnt you have some slipping clutch paks. Hope that is not the case, but if it is start looking for a replacement. The only transmissions that will work in your car are the 1978 to 1980 300SD and the 1978 to 1980 280SE. Kent

rechecked work, and found more problems

Kent, Thanks for the help. No offenses taken in any way, as human error is often the basis for problems. I did have 2 years of formal training, though it was 20+ years ago and mainly on domestic stuff. Just lots of day to day experience since then, more on heavy trucks than cars, so I'm still learning the little differences in German engineering. For some background, this is a recent purchase and had been obviously neglected for awhile. I had only driven it enough for an initial shakedown before starting work. I did the trans service myself. Checked the fluid level when working on it yesterday and it was full when hot in park. Fluid was Dexron III and filter matched the one removed. I did not drain the converter during service. Dropped the pan again this morning and filter was in place. Then the bad news. Fresh metal filings in the pan and particles that look like clutch material. Fluid wasn't burned yet, but we only drove it a few miles after it started to slip. Also discovered some other problems that others might find interesting. After doing some more line tracing I discovered the the vacuum switches on the valve cover were completely bypassed, with lines that were plugged internally or that lead in a loop. The modulator had been getting little or no vacuum before my initial testing when I discovered a kinked line. So either whoever worked on it in the past was the worlds worst mechanic or did some ingenious jerry rigging to get it to work for a while longer. I know finding exact diagrams for these cars can be tough, but it is worth the time and money to have one! So, it looks like it's time for a replacement. Any recomends about good trans builders that don't charge an arm and a leg would be great. Thanks for all you do Kent!

update on slipping trans

Just an update on this problem. Kent, you called it right. The pump had failed. Cause was a spun stator support bushing. Luckily I found a local indy shop that did a good job for a resonable price. And thanks to Kent & Kaia, we found all the parts we needed. Moral of the story: you never know when a part may break, but always do your preventive maintenance. It couldn't prevent this problem, but I found other issues working on the car that are now fixed and will improve its reliability. Watch for the little things. At least this failed at home instead of on a trip!