1995 C280 Transmission Woes
 

Replaced leaking slipping transmission and torque converter with one from a wrecked vehicle with 90k miles on it. Also, replaced the engine wiring harness and rebuilt the transmission wiring harness. Added 5 quarts of fluid on the initial refill (capacity is 7) and then another quart after warm up. The level seemed fine.

Put it all back together yesterday and all seemed fine. Transmission shifted fine and the car ran well yesterday. This morning the car was accelerating onto the interstate and suddenly the transmission disengaged. No warning sounds or vibrations or anything that would indicate the impending problem. Now no power transfer to the engine regardless of which gear is selected, nothing. Fluid does not smell burnt or show unusual discoloration. When going from PARK to reverse there is a slight but distinct reduction in idle speed but the car will roll as if in neutral.

Is there anything in the transmission wiring harness that could disconnect and cause such an instant failure?

Any chance that a fluid overfill could cause this problem?

Nothing electronic on that trans.

Over fill OR under fill could cause that issue!

I have a sinking feeling that the last quart of fluid may have been too much. Perhaps the fluid wasn't warm enough when we added it and accidentally went over. I assume if this is the case we have busted seals inside and that's the reason for the no go?

I came across a thread with a similar issue where the modulator was identified as a potential cause. Could a non functioning modulator cause this symptom? A mityvac on the modulator line holds vacuum FWIW.

Modulator can't cause that.

Taking a SWAG that a gunked up or broken valve body might be the culprit, we dropped the pan and popped in a valve body from a known good (but leaky) W202 tranny last night and got the same result. No power to the wheels. BTW, no shavings or debris to be found in the pan.

The interesting and I think telling discovery with this though, was the fluid level on the dip stick. After putting it all back together and attaching the pan we put about 2.5 quarts of fluid in, start the car and go through the gears in typical fashion. After a few minutes of no success I checked the fluid level and the level was well past (1" +) the full mark on the stick. :eek:

So with the fluid level being this high with so little actually in there, is it safe to assume that the pump is kaput and not moving the fluid to the places it needs to be?

Try checking fluid flow thru the trans cooler on the pass side of the trans.

This saga continues - The pump was definitely shot in the replacement transmission. So, back in with the original tranny and lo and behold we have power to the wheels again.

The problem now is, there is always power to the wheels regardless of where the selector is............Park, neutral, reverse and everything in between the transmission is in gear. Power to the wheels in the forward direction all the time. Linkage is good and the selector moves to the proper locations as it should. Initially, the tranny behaved as expected but after 10-15 minutes of run time it developed this strange symptom.

Suspect that swapping the valve bodies around may have introduced some dirt to the innards and we now have a clogged passage that won't allow for the gear to change. Any thoughts on the proper way to clean the valve body?

Update for anyone following this thread:

The 'feature' of always being in gear was indeed due to an issue with the valve body but it wasn't because it was gunked up or anything. The plastic tab/slot on the manual valve (this attaches to the gear selector arm) had broken leaving the gear selector useless and the tranny stuck in go.

The fix was reasonably simple since I have 2 valve bodies. Rather than digging into the innards of the bottom half of the valve body to replace the broken piece, I chose to just take the bottom with the good manual valve and matched it to the top of the other (after a thorough cleaning of course) and put this back in the tranny.

Now that the original leak is repaired (pump gasket) all is well except for the strange shifting issue. What was originally described as a slipping transmission is actually a different issue. When shifting from 2nd to 3rd it actually wants to remain in 2nd longer than it should, but a slight lift of the accelerator pedal will allow the shift to occur and then it's off to the races. If anyone has bothered to read this far perhaps you could offer an opinion on the cause of the difficult 2/3 shift.

Thanks

I just installed a junkyard 722.3 transmission into my car.

All was well except for flaring on 2-3 and 3-4.
I replaced the K1 and K2 shift kits with new ones - however no improvement on the flaring.

Next, removed the vacuum connection to the modulator. Flaring disappeared however at the expense of rough shifts. Spoke to a Benz transmission specialist. Modulator needs to be adjusted correctly using pressure meters. - Adjusting by hit and miss technique wont cut it.