Khan28

Hello,

The 5 speed transmission in my C280 wasn't shifting very well for about 3 weeks in August. When I started up first thing in the morning and leave to go to work I found the transmission would upshift pretty smoothly but if I came to a rolling stop at a stop sign right as I were to move off again the transmission would clunk or hard shift into 1st gear an then the car would move off normally. At first I thought I happened to step on the gas right when the transmission was shifting and I was confusing the computer but it happened way too often. Also, it does that when I slow down from like 70 to 40 km/h and go to accelerate again the transmission would clunk. Also, at times when I let off the gas the transmission will hold the current gear for a bit then clunk into the next gear but that's intermittent. There's also a bit of a jerk when it shifts at times. Other times it's smooth as silk.

I took it to an independent shop and they did a fluid, filter and gasket change. The tech even used extra fluid to flush out the transmission and he said the oil was black and the pick up tube was almost blocked. The mechanic hooked up his computer to the car and it showed 2 error codes indicating gears 1 and 3 were "implausible" or "slipping". Fast forward 3 weeks and the tranny is doing the exact thing again. When I picked up the car the upshifts were silky smooth and the clunking issue was so faint I felt that was as good as it was going to get and was happy with the result but now it's back to shifting poorly at times again.

Could I have a different issue like transmission mounts, internal issues, conductor plate or other issue?

KSKarl

If the fluid was grossly contaminated before, it's likely that there is still a lot of crudd floating around to block the shifting valves. Your transmission shifted smoothly for a while when the fluid was clean.

Khan28

Should I have another drain, flush and fill done again? That's going to be another $420. The mechanic initially flushed the system with extra fluid. He did offer me 2 choices when I initially took the car in. He said he can do a full transmission service which includes a flush, new filter and pan gasket or change the conductor plate along with a tranny service. Flush was $420 and plate was $975. He also inspected the TCM to make sure fluid didn't go into it.

Now I'm thinking I should have paid the $975 initially to have both the conductor plate replaced and service done but I was thinking that "what if that doesn't fix the issue". I tried the transmission reset but I'll try it back tomorrow and see if that makes it shift at least like it did when I just got the car back.

Does bad mounts cause jerky shifts?

KSKarl

IMHO - Bad transmission mounts to not cause jerky shifts. on the contrary, good solid mounts will make it more obvious when the transmission is shifting jerky.
Not sure of your terminology "conductor plate" unless it's what I would call the valve body - generally about an inch and a half thick plate with all the valves and servos that control the shifting, all connected by small fluid passages. The valve body is the same shape as the transmission oil pan and is what the pan bolts to. It hangs below the main body of the transmission that houses the gears and clutches that actually do the shifting. Any little piece of crud can block a fluid passage or jam up a valve in the valve body, causing erratic shifting like delays in upshift/downshift or jerkiness. When there's a lot of crud in the fluid, one flush can't get it all, a little stays stuck in corners of the fluid passages, either in the valve body or up in the clutches or in the torque converter, or even the oil cooler. It works its way out as you drive.
If you had replaced the valve body with your first flush, crud from somewhere else in the system could easily have washed into it and jammed a new valve anyway...
My advice would be
1 - get some advice from the shop that did the flush for you.
2 - put some miles on it to work crud out of the system and repeat the flush. As the system gets cleaner, it should go longer before the jerky symptoms return.
3 - in the future, change fluid regularly so it doesn't get as dirty as it was before.

neurodave

I have the same issue as you and have yet to be able to figure it out. One thing I noticed, is resetting the throttle works for a few hours and then it goes back to what it used to do. Try doing that and see if alleviates the issue temporarily. If it does, I would rule out the trans itself, probably something electrical.

Khan28

I plan on changing the fluid regularly. I bought the car when it had just shy of 100,000kms on it so I was unsure as to how it was looked after before. I'm at 114,000 now. The reason why I bring up the topic of the conductor plate is because tons of people have had to replace theirs. It's separate to the valve body but they work in unison and the electrical components on the plate along with the speed sensor fails. People seem to experience all sorts of issues but quite a few find that the transmission won't shift out of gear when their plate fails. The tranny in my car is shifting when it should, it's how it's shifting at times is the issue so I'm not entirely sure that's the issue.

I forgot to mention that when the mechanic hooked up the computer it showed the charging system was at 14.4 volts which was considered a tad high. Could that have anything to do with it? Funny thing when I was going to the mechanic the transmission shifted soooo buttery smooth.