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Wife's SLK 350 stuck in 1st gear

8547 Views 32 Replies 8 Participants Last post by  i860
So she was driving to work this morning pulled up to traffic lights, pulled off and the car wouldnt shift out of 1st gear. I got there thinking maybe she had accidentally hit the manual shift control. But no it wouldnt budge out of 1st. Limped home some 2 miles, checked battery voltage all was fine and on target.

Connected my bluetooth adapter up with Carly App, scanned it to find

"Pressure Control Solenoid B Electrical

Code: OBD00 P0778

Fault: There is a fault in control unit ''Transmission''. There is a fault in control unit


Code: 001737"

Read up on some threads on here that similar symptoms were related to the speed sensor on the conductor plate.

Also found a definition

"P0778 - Pressure Control Solenoid 'B' Electrical

Possible causes
- Low transmission fluid level
- Dirty transmission fluid
- Faulty line pressure solenoid 'B' valve
- Line pressure solenoid 'B' valve harness is open or shorted
- Line pressure solenoid 'B' valve circuit poor electrical connection What does this mean? What does this mean?
When is the code detected?
Engine Control Module (ECM) detectes an open or shorted condition on the solenoid circuit
- Engine Light ON (or Service Engine Soon Warning Light)
- Transmission shift shock
P0778 Description
The line pressure solenoid valve regulates the oil pump discharge pressure to suit the driving condition in response to a signal sent from the Transmission Control Module (TCM)"

So im somewhat lost

Any help would be greatly appreciated

Many Thanks
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One thing is for sure: you will not fix it using diy.
It has to go to a technician who knows the box.
Sounds like the conductor plate issue that is common with the early version of the 7G transmissions.

Click the link that Jeff provided in the post above.
Thanks Jeff & Akonion.

I was unsure to start as to whether that was the case as i did read that they fail over time and basically the symptoms get worse until the finale, however in the meantime the same symptom has occurred approx 4 times within the last 6 months she tells me

So i think your correct in that its the conductor plate. Thanks for your help

Now to find out how to remove it.....anyone?

Again Thanks Jeff & Akonion

PS no recalls on this obviously wide spread problem by MB is there?
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this is not a diy part as I have heard even MB does not sell the parts to Indy mechanics. (have only heard of one that was able to purchase) as it also needs to be 'married'/das/star done
How many miles on the clock, may i ask?
105K nakulan7
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repair runs about $1600
My understanding was that these issues happen at about 50kish and are unlikely to happen if higher mileages are achieved.
Is that at the Dealer Jeff?
Is that at the Dealer Jeff?
yes at Tampa Mercedes.
My understanding was that these issues happen at about 50kish and are unlikely to happen if higher mileages are achieved.
can happen lower. former member 2005 amg had it at 30k

and you are thinking of balance shaft issue. conductor plate can happen anytime
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Yes Jeff your correct the conductor plate can happen anytime at any mileage. The speed sensor on it i believe is actually the culprit. I found a couple of videos on youtube (infact its a set of 4 i believe) about the whole process and removal of the valve body and plate etc. I realize its on a Mercedes 722.6 Transmission where as mine is the 722.9, but im tempted to have a go at removing it...ooooooh dear lol. I do like a challenge tho.

This is the basics of it so dont blast me for missing stuff out

Jack the puppy up and support safely
Drain fluid via tranny drain plug
Remove the tranny pan
Remove the valve body
On bench disassemble the conductor electrical plate from the top side of the valve body
Pack it up nicely and ship it off to the repair company in California (modulerepair, 1-2 business days needed....Ebay 99.5% positive feedback)
No additional programming is required. $269 + $16 shipping

Reassemble in reverse order

Not sure though, perhaps your right Jeff just bite the bullet and take it into local MB here in Melbourne
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well it is not so much as doing it yourself but of getting the part from MB. the decision will be made when you call the parts department up and order it and they say sorry, no can do.
plus you have the mb warranty on the part and the work provided it is the part that will solve your issue
Your correct there Jeff
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I had my conductor plate replaced recently. Got the part from Mercedes for £340 with £50 back once you return the old one. These are theft protected parts, so have to be coded to the car via DAS. Local Indie fitted the new part, new fluid and coded for £290, so £580 all in. Just over 90k on the clock.
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So I researched and researched with regard to the conductor plate issue and I honestly don't think that's my problem. So I've gone ahead and purchased the materials etc to go ahead a do a transmission oil change

Heres a few reasons that lead me to believe in that its low on tranny oil.

1. We have only had the car about 18 months, wife has only put about 12 k on it from when we got it with 93k on the clock. We haven't had a transmission oil change on it and I cannot find anywhere stating the previous owner has
2. I went under the car and disconnected the circular electrical plug to find tranny oil in it ( I believe the oring fails). I've read this can cause problems not only with loss over a period of time of tranny oil but with the electrical part too
3. My sons manual clutch on his car went the very next day after the SLK, so his car had priority in the garage. Just finished it. All this time the SLK has been on the driveway (slight incline) nose pointing up. Now last night we had to move it to get my sons car out. Now before from park to drive or reverse it was lunging into gear, last night smooth as babies bottom. So I decided to take it a gentle ride around the block. It changed thru all gears and even in triptronic. So put it back in garage. What I did notice though was a smell of heat like oil heat

So from that I reckon its low on tranny oil??

Tranny oil change starting weekend….a lot of info found here

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Also as the thread above mentioned I'm also going to go ahead and change out the tranny mount

Mercedes Benz Transmission mount HD 1 Year Warranty 1000 sold
Mercedes Benz Transmission Mount HD 1 Year Warranty 1000 Sold | eBay

There's a bunch of videos on YouTube that I've seen for assistance too


mercedes 722.9 7 G tronic adjust transmission fluid level

The only thing I'm not sure of is, I believe the torque convertor holds around 4 liters or so of fluid it, but not many people seem to drain and refill that They seem to just drain the oil pan and valve body area.

So I will report back early next week after the fluid change Again many thanks for your input
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all your signs pointed to conductor plate. Once the CP ready to go, sometime it will work, sometime not.... Low tranny oil will cause the car to shift abruptly.... unable to upshift is CP. You can just replace the fluid, but at $20/quart (7-9 Qts, don't remember exactly.. that is almost $200 just for oil), if it doesn't sort out, you will have to drain the fluid again.. wasting time and $$.
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Drained the oil, popped weir pipe off removed the cover. Measured the exact amount of oil drained out to be 3.25 litres
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