Need help diagnosing a transmission problem

[psaboic]

Hi folks.

First let me say I spent about 2 hours reading posts of similar problems and I am now more confused than ever. Info overload I guess.

Anyway, yesterday, out of the blue, my 83 300D with 250,350 miles suddenly started downshifting from 2nd to 1st with a harsh "clunk". The upshifts are still ok, but seem a tad bit firmer than before. All other downshifts are fine, just the 2-1 downshift is harder and making a noticeable "clunk"

So far I have checked the transmission fluid level when warm and it is fine, nice an pink colored, and not burnt smelling. I last changed it about 10,000 miles/12 months ago along with the filter. I also tried turning the whole climate control system off with the dash switch, but it still downshifts hard.

After doing some reading, it sounds like I have one or more of these issues.

1. Vacuum leak somewhere
2. Bad B2 piston
3. Bad Vacuum Modulator on the Transmission

I need some advice on where to start troubleshooting this. I think the best place to start is a possible vacuum leak. That said, again after reading I am totally confused on how to isolate the various systems to check for vacuum leaks (i.e.. which lines to disconnect and where to hook up my MityVac)

If anybody can tell me where to start or have a decent step-by-step I would appreciate it.

I should also say, I have no key shut off issues, and the power brakes work just fine so......

Thanks in advance,

Glenn

[mespe]

One method I found for troubelshooting vacuum leaks is called the smoke test. blow smoke into the vacuum system and look to see where it comes out. But you got to realize that there are check valves used so you might need several attempts.

The no shut-off requires alot of vacuum loss.

How fast are you going when you get the clunk?

[psaboic]

I have no easy method or way to try a smoke test as I live out in the sticks so to speak, and the nearest shop that could possibly do a smoke test is over a hundred miles away. However, I am usually going about 15 mph or less when it "clunks" from 2nd to 1st.

[tangofox007]

Originally Posted by psaboic

After doing some reading, it sounds like I have one or more of these issues.

1. ......
2. Bad B2 piston
3. ......

What theory would link the B2 piston to a hard 2-1 downshift?

[kerry]

It's a vacuum leak. Push the far right button in the climate control and see if the problem goes away. If it does, the leak is in one of the climate control pods. If it doesn't go away, plug the yellow line going to the door locks at the firewall under the hood. If it goes away the leak is under the hood. If it doesn't go away, look under the hood for the leak. All these are tests which don't require a Mity Vac. If you have a Mity Vac you can use it to trace down the leak.

[psaboic]

After reading more on it I now understand that in no stretch of the imagination could the B2 piston be bad (embarrassed smile)

I tried the turning off the climate control unit yesterday and it still clunks, so that apparently is not the issue. All of the central locks seem to work just fine, but I will plug the line just to be sure. Just so I understand, once I plug the line at the firewall, that should effectively isolate the lock system, and once plugged, if the problem goes away, the issue lies in the central lock system....correct. Otherwise it is most likely under the hood or at the modulator itself

Sounds like I have a job to do tomorrow night.

[Brian Carlton]

If you have a vacuum gauge or a Mityvac, install a T in the line from the vacuum control valve down to the modulator. Put a hose on the T and route the hose into the cabin and connect the gauge.

Read the gauge at idle and note the reading.

Then, take it for a drive and note the readings at about 1/2 pedal, 3/4 pedal, and at full pedal (floored).

Report back with results.

[psaboic]

It is literally POURING rain here right now, and of course the garage is loaded with winter clutter that needs to go. Needless to say, I WILL be working on it today, but after the monsoon stops! Thanks to all for getting me started in the right direction.

[psaboic]

FIXED IT!! transmission shifts up and down fine now. The culprit was one of the three way connectors for the transmission vacuum circuit. It had a small split on the underside. I hooked the vacuum gauge up to both tees that come off the main vacuum line from the vacuum pump to the brake booster and I had a solid 21' of vacuum on both, so then I used the mity-vac on the climate control, shut off, central lock, and vacuum reservoir circuits and they all held vacuum fine.

Sooooo, as soon as I pulled a vacuum on the transmission modulator circuit, it leaked rapidly. A little bit of looking and I found the problem. Start to finish, including a trip to the auto parts store for some vacuum hose was about 3 hours.

Thanks for the help and tips all.

Glenn

[TnBob]

Glad to hear it was so simple. Nice that the prior posts of experience headed you in the right way too.