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1999 C280 (sport), also 2001 SLK230
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62 Posts
Discussion Starter #1
Hey folks! I'm looking for some wisdom on my transmission. Any guesses about how to solve this problem would be much appreciated. Fortunately I can live with it, but I’d like to fix it.

In brief, transmission won’t upshift if car has sat in dampness for a long time. Problem disappears when warmed up and driven a few miles. It will start in 2nd if the switch is set to “W.” Transmission otherwise works fine. As described below, transmission is in mechanically good shape, had fluid changed at 73K mi, and has zero sign of abuse. Photos below show codes.

Detailed Background:
1999 C280 Sport, purchased in Vallejo, east of San Francisco in June 2017 with 73K miles and in overall excellent condition. Shortly after purchase, I did a complete transmission fluid change (including torque converter) and replaced filter, pan gasket, electrical plug, etc. Car ran perfectly before and after. Used fluid was a normal pinkish color, no burned smell, normal amount of clutch residue in pan, no metal flakes or “sparklies;” everything basically normal. Prior owner was a very conservative driver (older guy), who probably never once floored the accelerator. When I test drove the car, he marveled when I used the shifter lever and said that all he ever used was “R” and “D.” I’m 3rd owner of the car and a former mechanic. (In the 1980's.)

Details on Current Problem
For the first 20K miles or so after purchase, the car performed excellently. In fact, the more I drove it, the better it got. I spent many weekends driving between Redding California and San Francisco, a roughly 450 mile round trip through the often-hot Sacramento Valley. Car was happy cruising at 85MPH, and happy negotiating traffic, hills, and stop-and-go in San Francisco. In short, an excellent performer.

In January 2019, I left the car for a couple of months at a friend’s house in San José, CA. Unusually, this was a very rainy period. When I got back, the car wouldn’t upshift. Since I had had the car for two years, and didn’t know the vintage of the battery, I replaced it with a fresh MBZ battery purchased at the dealer. This did not help. Since the car ran fine except for lacking upshifts, I contemplated driving slowly back to where I was living in Redding by using the “W” setting on the transmission and limping along in second gear the entire way. But I first decided to drive it around San José a bit, and took it to an independent MBZ mechanic. (Normally I do my own work, but was far from my tools.) They said they didn’t work on transmissions, but would be happy to read the codes for me. (No CEL light was on.) The codes were inconclusive, though they seemed to indicate some kind of communication problem between the shift lever and the transmission. See the photos of the Star Diagnostic printout below.

After I left the MBZ mechanic with a recommendation for an MBZ transmission shop, I headed back to my friend’s house. Within five minutes, the transmission started to function normally again. I drove the car around the Bay Area for a couple of days just to be sure, and it continued to work just fine, so I drove back to Redding. After that the problem vanished for most of a year.

A year later, I drove the car back to my home in Boston. There, after sitting for six weeks, the problem returned. But once I had driven the car around for a while and gotten the engine and cabin warmed up, the problem vanished again.

So the problem seems to be caused by dampness and/or disuse. If I drive the car regularly, there’s no problem, even if temperatures are well below freezing. But if it sits in a damp place for a week or more, then the problem returns. Unfortunately, Boston is the very definition of a damp place, and my cars often sit for weeks or months while I travel. I just came back from a 23 day trip, tested the car, and the problem was back, but disappeared after I let the car idle in the driveway for about 12 minutes and then took a 1-mile drive. Oh, and it initially didn’t engage first until I had revved the engine a bit, and then engaged with a thunk or jerk.

Preliminary conclusions:
  • There’s almost certainly nothing wrong with the mechanicals of the transmission. When it works, it works great, no slipping, no hesitation to downshift, etc. I’m willing to hear otherwise, but this is my preliminary working conclusion. That said, the springs in the valve body are notorious for breaking. Maybe one of those is broken? I’ve looked into the Sonnax kit, and will likely do this soon as I’ve now got 30K on the “new” transmission fluid, and it's due to be replaced once more. While I've got the transmission pan off, it would be criminal to not at least look at the springs in the valve body. Counter to the spring theory is that they wouldn't be affected by sitting around, e.g., a spring problem would likely be more consistent, or at least not triggered solely by sitting.
  • There’s likely some electrical gremlin plaguing me. I’ve already ruled out the battery, but am wondering if my shifter position sensor is bad. If I recall correctly, I could get a new one for about $125 USD, likely not much more than a dealer code reading. Everything else electrical in the car works fine. The codes indicate some intermittent electrical faults, but since many were “stored,” I have no idea of how relevant they are.
  • Otherwise I’m at a loss. Any thoughts you all have would be MUCH appreciated.
Further Steps Planned:
Get codes cleared (I’m not sure if last mechanic did this; they didn’t charge me for a half hour of work), and then try to recreate problem and read codes again.
Maybe replace shift position sensor; depends somewhat on what you all have to say. Otherwise, I'm at a bit of a loss. And as noted, the problem is annoying, but not really keeping me from using the car, especially once it's back in more regular use.

Attached are photos of the codes from the Star Diagnostic system that the mechanic kindly gave me.

Thanks for reading and have a great day!
2618654


2618655



2618656


2618657
 

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Registered
1999 C280, 2000 C280, 2000 C230K,2011 C300
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679 Posts
I don't have much diagnostic skills. If I have a problem, I'll check the internet for the most common causes and hope my problem is a common one and not something esoteric.

I've had limp mode twice. Once on our 2000 C280 from spilling a drink on to the shifter console. Solution? New shifter, $600. Another recent poster was able to find a used one at a junkyard so the cost of the repair was minimal.

The other time was on the 2000 C230K. It was the limp mode and very hard shifting that would sometimes disappear with a shut down and restart. I had not done the plug and fluid change so I did that and while I was there I changed out the valve body. Whether it was the new plug or the valve body that fixed the limp mode, I don't know. But it is common for the valve body to fail, (something about constantly being bathed in scalding oil). Nothing obviously wrong with the old valve body but there are online pics of others with cracks. I had extra springs but they all looked OK.
 

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Registered
1999 C280 (sport), also 2001 SLK230
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62 Posts
Discussion Starter #3
@glenmore -- thanks for the reply. I'm leaning toward the shift lever assembly being the problem, but would rather feel more confident before I spend ~$500 USD on a part. Apparently, there's a sub-assembly that is the switch that tells the system which gear the lever is in which is less than $200, which I'm more tempted to replace. I've personally never spilled a drink in there, but I did buy the car used, so who knows?

I'm coming around on needing to change the transmission fluid again, so when I do that, I'll check the springs in the valve body, cleanliness of the electronic plug and harness, etc. But I've done a lot of research on this problem, and it's unusual. I'm not even sure I'm in "limp mode" either, as it pops out after the car warms up a bit. Is there a way to know for sure that the transmission is in limp mode? It has never illuminated a warning light on the dash.

Thanks again.

Kim
 

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Registered
1999 E430; 2005 ML500
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1,869 Posts
I would first do some basics regarding electrical connectivity. The 722.6 transmission has a common problem with the connector plug at the transmission leaking, so I would replace that first.

Then I would pull out inspect and clean the TCM itself:
 

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Registered
1999 C280 (sport), also 2001 SLK230
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62 Posts
Discussion Starter #5
Hi @Hirnbeiss,

Thank you very much for the help. I have yet to follow your suggestion, but I now know that the transmission computer is relatively easy to get to. I did replace the electrical connector that screws into the side of the transmission 25K miles ago when I did the fluid change, but who knows? Maybe it is leaking. I'm certainly going to pull up the rug and see if the transmission computer is, for some reason, damp.

Meanwhile, I drove the car today with nary a single problem. But it tends to happen if I've left the car to sit for a week or more in cold, damp conditions, and I've been driving it every couple of days lately.

Thanks again, and have a great day!

Kim G
Boston, MA
Where at least it isn't snowing, even though it's supposed to.
 

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Registered
1999 C280, 2000 C280, 2000 C230K,2011 C300
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679 Posts
In my case, the effect of the spilled drink was immediate limp mode on the first start.

In the C230K, limp mode would start typically after getting off the freeway. So driving fine on the highway and then slowing for an offramp exit would throw it into limp mode, stuck in one gear and if there were a shift it would be with a big kick in the pants. For a while, it would return to normal with a shut down and restart.

Always a good idea to revisit a previous repair, i.e. the plug. It takes some fiddling to get it in there correctly.
 

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Registered
1999 C280 (sport), also 2001 SLK230
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62 Posts
Discussion Starter #7
@glenmore - thanks for the tip. I'm uninclined to think it's the transmission plug, though I could certainly be wrong. The problem started 2 years after I replaced the plug, and as noted, seems to be a product of disuse over long periods in cold, damp weather. But I definitely appreciate the feed back.

Cheers,

Kim G
Boston, MA
Where there's nothing but cold, damp weather.
 

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Registered
1999 C280 (sport), also 2001 SLK230
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62 Posts
Discussion Starter #9
------------------------------------------------------------- Problem Finally Solved -------------------------------------------------------------

Hey Folks,

This is an old thread, but I think I may have finally solved my problem. Yeah, it's been going on two years, hehe. But the problem always disappeared after a mile or two of driving, and I never believed there was anything mechanically wrong with the transmission.

Anyway, here's what I found out. I always drive with my headlights on, no matter how sunny, as my car is black and numerous studies have shown that if your headlights are on you're less likely to be involved in an accident. So when the headlights are on, the shifter assembly lights up the gear that the car has been put into. Well, not too long ago I noticed that when I was having my failure-to-upshift problem that those lights which indicate PRND4321 weren't lighting up. After the car warmed up and the transmission started working properly, those lights again worked.

So last week, I decided to disassemble the shifter assembly and look at the Auto Trans Gear Recognition Control Unit. Based on some web sleuthing, I had expected to find an optical recognition system with some failed solder joints. Instead, after I disassembled it, what I found was a mechanical position sensor with a couple of shaped wires which slide along a circuit board, making contact with different pads depending on the position of the shift lever. (Sorry, I didn't take any photos.) I carefully cleaned this assembly, and slightly bent the wires so they'd make firmer contact with the circuit board and reassembled. Oh, and I also put some synthetic grease on the mechanical bits, the latter of which has made the action of the shift lever silky smooth.

There's a good video on YouTube by user "Fupabox" on how to disassemble the center console. It's quite easy, and then removing the shifter assembly is also pretty easy. Frankly, the hardest part is getting the electrical connectors unplugged.

Oh, you should also order a new shifter bushing before you start this. Mine was completely gone, and having the lever out makes for very easy replacement of the bushing. While that bushing is hard to insert, what I did was take a big socket and a long bolt with a big washer and basically used the washer and bolt to apply pressure to the bushing, with the socket on the other side to receive it. Works like a charm and you don't have to buy a special tool or risk damaging the bushing with pliers, etc.

It's been about a week since I made my repair, and so far, so good. It's definitely cold and damp here in Boston in January. The problem was always intermittent, so I can't be 100% certain that I've got it, but I think this is it. So if you have a similar problem, check and see if those indicator lights are showing. If not, you may just need that gear recognition control unit, which as of this writing sells for about $100 at AutohausAZ. Or, if you're adventurous, you can take it apart and try to fix it yourself. I basically got my repair for free, save for the roughly $5.00 I spent on a single bushing from an eBay vendor.

I hope this helps someone out there.

Cheers,

Kim G
Boston, MA
Where, despite having a near-perfect '99 C280, we are lusting after a W126.
 

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1995 C280 (165K mi)
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1,669 Posts
Strange problem with a simple solution. Great write up. Thanks.

Let us know if this solution stands. In 2 years! :)
 

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W163 and General M Gremlin
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5,520 Posts
GREAT Sleuthing Kim.
And keeping members updated/ resolved with your post. (y)

Happy Driving.
 

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Registered
1999 C280 (sport), also 2001 SLK230
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62 Posts
Discussion Starter #12
I've been so much helped by this site and others that I feel like I should "pay it back" whenever I can. Even a well-recommended independent MBZ garage didn't have a clue on this one. And one could easily have been suckered into an expensive transmission rebuild or replacement, (or at a minimum, a new conductor plate) so I'd like to save someone that heartache if I can.

Thanks for noticing @khomer2, @MB-W202

Kim G
Boston, MA
Where cold and damp are continuing, so I should have more good testing weather.
 
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