Continuation of the not-so-shifty gearbox problem

1113 Views 10 Replies 2 Participants Last post by BiffNotZeem, Dec 15, 2020

BiffNotZeem

Discussion Starter · Dec 8, 2020

In the last installment of this adventure, my '08 had experienced a failure where it suffered some kind of gearbox failure. It first stopped running a few hundred feet from my house, then started and drove back to the house fine, then died again and started popping the 40A gearbox motor fuse. I replaced the gear selection motors (the combined M17/3 & M17/4 unit), which fixed the popping of the fuse problem, but the gearbox wouldn't train in. It would report "implausible signals" from one of the motors (don't recall which one) and only go into 1st gear. Eventually, it wouldn't even start, let alone drive.

After that, I bought a used gearbox, one from a '09 with only 39k miles on it. I also bought a clutch kit since the one in the car has 108k miles on it. And I spent Thanksgiving changing out the gearbox and clutch (and cleaning 12 years of dirt out of the engine compartment (I live on a gravel road, so lots of dirt).

Now I am back to where I was before. The MBII diagnostic reader reports that the gearbox needs to train in, it failed to train in, and there is an implausible signal to one of the gear selection motors. It drives fine in 1st gear, but won't select reverse.

As I see it, the problem is either a) the controller needs some kind of reinitialization that only a Mercedes STAR can do (this happened after the car was repaired by a non-Mercedes shop after it was rear-ended - the gearbox didn't quite work right until I took it to a Mercedes dealer to run it through their diagnostic computer) or b) the wiring harness between that the gear selection motor with an "implausible" signal and the gearbox controller is bad.

Which seems more likely? Is the gearbox wiring harness a separate one that can be replaced on its own? Is there a non-STAR diagnostic computer that can reinitialize the gearbox like the STAR can or do I have to take it to Mercedes (I live on an island and the Mercedes dealer is off-island. I had a car towed from off-island to my house and the total tow bill was $700. Can I buy my own STAR for $700?).

alan

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gtdump

· #2 · Dec 8, 2020 (Edited)

Sounds like a shift motor issue to me. I would pull them and check under the motor casings for carbon dust from the brushes. Failing that, it might be worth checking the harness.

There aren't a lot of components connected to the TCU, a couple of sensors, the clutch actuator, and shift motors. I would think any issue will be found on that short segment of the harness. Perhaps double-check your connectors and ensure everything is securely plugged in.

Edit: If I understand you correctly, you changed the transmission? Did you not disconnect the harness then? The harness starts at the TCU under the driver's seat, passes through the body and into the engine bay behind the drivers seat, and winds it's way around the transmission area to connect to the various components. That section of the harness is maybe 6' long.

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BiffNotZeem

Discussion Starter · #3 · Dec 8, 2020

Translations: "gearbox" is transmission and "gear selection motors" is the shift motor assembly. I also use the manual references for components, M17/3 for the 2/4,R drum shift motor, M17/4 for the 1/3,5 drum shift motor. I would use M15/6 for the TCU, but TCU is less to type.

The shift motor assembly/M17 is a new factory part.

M17 has been in and out many times (including when the transmission was changed), so, yes, the harness has been disconnected at the connectors to M17/3 and M17/4. Yes, I fully seat the connectors.

To confirm the harness from M17 to the TCU is a separate harness, right?

As I noted, aside from the teach-in code, a "implausible signal" from the M17 code is being thrown. Fuse 30 stopped popping when I replaced M17. I have been wondering whether the excess amperage that was popping fuses also damaged the wiring harness between the TCU and that side of M17. The wires in that harness are small gauge. Now I am thinking it is possible that it damaged the TCU.

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gtdump

· #4 · Dec 8, 2020

BiffNotZeem said:
Translations: "gearbox" is transmission and "gear selection motors" is the shift motor assembly. I also use the manual references for components, M17/3 for the 2/4,R drum shift motor, M17/4 for the 1/3,5 drum shift motor. I would use M15/6 for the TCU, but TCU is less to type.

OK, doesn't matter to me if we call them Laurel and Hardy or whatever, so long as we're all on the same page is what maters!

BiffNotZeem said:
The shift motor assembly/M17 is a new factory part.

If it's new I would think my guess of it being the shift motors is unlikely. The code you're getting about "implausible signals" suggests to me there's a problem talking to the shift motors. My personal experience (limited as it is) has been that this indicated fouled up motors, but with a new part that seems unlikely. My next guess would be to check the harness for insulation breaks; the wire they used seems to be prone to cracking when it gets old (again, my own limited personal experience). The problem could be elsewhere as errors on the canbus can disrupt other nodes.

BiffNotZeem said:
M17 has been in and out many times (including when the transmission was changed), so, yes, the harness has been disconnected at the connectors to M17/3 and M17/4. Yes, I fully seat the connectors.

I preume you've checked, so will disregard.

BiffNotZeem said:
To confirm the harness from M17 to the TCU is a separate harness, right?

The transmission components connect back to the car separate from the engine harness. The trans harness passes into the car through a grommet in the body behind the driver's seat, then it connects to the TCU under the driver's seat. There are perhaps 25 wires in the harness and it's maybe 6' long. I believe you should be able to feed the TCU end of the harness back through without lowering the sub assembly. I don't have a spare of that harness, so cannot provide a good clear picture in it's entirety.

BiffNotZeem said:
As I noted, aside from the teach-in code, a "implausible signal" from the M17 code is being thrown. Fuse 30 stopped popping when I replaced M17. I have been wondering whether the excess amperage that was popping fuses also damaged the wiring harness between the TCU and that side of M17. The wires in that harness are small gauge. Now I am thinking it is possible that it damaged the TCU.

I think any and all of the above are possible. I would test the harnes first if it were me, costs nothing but time to test it.

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BiffNotZeem

Discussion Starter · #5 · Dec 9, 2020

Thanks. I found an eBay listing for the harness, which has a bunch of photos, so I now know what it looks like. I am kinda tempted to buy a harness and TCU from eBay just to have spares. Or I could just swap them in and test/repair the ones that come out of the car at my leisure. My wife would like to get her car back.

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BiffNotZeem

Discussion Starter · #6 · Dec 11, 2020

Pulled the TCU this morning. The foam attached to the carpet above it was soaked. While there was no water around the TCU, it looks like it had been submerged at some point. I have a used one on order.

Here are photos of the inside of the TCU. Surprised that it works at all.

Electronic component Electronic engineering Circuit component Electronics Space

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gtdump

· #7 · Dec 11, 2020 (Edited)

I think you've found your problem!
I don't know it will work here, but I have had mixed luck soaking/rinsing in distilled water on other PCB before and going at it with a toothbrush.

I'm curious to see if you'll need a STAR system to marry the TCU in or if it will be a drop-in.

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BiffNotZeem

Discussion Starter · #8 · Dec 12, 2020

gtdump said:
I think you've found your problem!
I don't know it will work here, but I have had mixed luck soaking/rinsing in distilled water on other PCB before and going at it with a toothbrush.

I restore vintage computers and have stuff for cleaning PCBs. I gave the TCU a good cleaning as well as cleaned up the ground connection next to the TCU. No change in behavior.

I only cleaned the exposed side of the PCB. It looks like I need to desolder the connectors from the PCB to remove it to check enclosed side.

I'm curious to see if you'll need a STAR system to marry the TCU in or if it will be a drop-in.

So am I. The iCarsoft box says the TCU has the VIN in it and I recall another value on that same page says something about "interlock", so I may have to find a STAR. I wish that one could just buy a compatible system like I can for my VW stuff.

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BiffNotZeem

Discussion Starter · #9 · Dec 14, 2020

Got the new-to-me, used TCU and installed it. It seems to work. No codes thrown in the TCU, in particular, no "train-in needed" thrown. No three bars of death. It seems to select R now (which it didn't do before) and all shift positions can be selected without it flashing on the instrument cluster. There are no seats in the car while I continue working on drying out the carpets, so no test drive yet.

It looks like the different VIN in the TCU is not an issue.

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gtdump

· #10 · Dec 15, 2020

I have heard reports that powering things up with the seats removed can trigger airbag codes (for the in-seat airbag) which are reputed to be difficult for some scanners to reset. I have always just left the battery disconnected when the seats were out, though I look forward to seeing if this is just superstition or not.

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BiffNotZeem

Discussion Starter · #11 · Dec 15, 2020

gtdump said:
I have heard reports that powering things up with the seats removed can trigger airbag codes (for the in-seat airbag) which are reputed to be difficult for some scanners to reset. I have always just left the battery disconnected when the seats were out, though I look forward to seeing if this is just superstition or not.

That is OK. I was already getting an airbag code that I could never reset after I had to replace the driver seat seat belt latch years ago.

With the car2go forTwos, it seemed like every one of them had the airbag light on.

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