Please can someone tell me if I am looking at this the wrong way. I feel that my local dealership has poured oil onto my TCU plugs in the hope of earning a quick buck! After sending the car in to have the codes read I opened the footwell to have a look at the TCU.

The wires coming up from the transmission are bone dry. I pushed the wires apart and ran a cotton bud between them - bone dry! The pins on the TCU are bone dry but the plugs are soaked. The wires from the plugs are soaked for no more than 10cm (4 inches). There is a large puddle of greenish oil in the footwell directly under the plugs of the TCU. No codes point to the TCU being faulty.

I have not yet looked under the car to see if there is any evidence of oil wicking from the transmission.

This was my diagnosis and the reason for going to Merc in the first place;

My 2002 C270CDI (270 000km) went into limp mode a while ago. All the signs of limp mode discussed on this forum were present, plus the display on the dash would blank (normally shows S/W and P/R/N/D). Once I also saw that the display stayed in P when I moved the gear lever to D. Sounds like a floor shift module to me.

I took the car to the dealership to have the codes read and cleared. These were the transmission codes:
P2502 - The gear is implausible or the transmission is slipping
P2600 - The voltage supply of circuit 87 has undervoltage
P2402 - The right front wheel rpm signal sent from the traction system via the CAN bus is implausible
P2403 - The left front wheel rpm signal sent from the traction system via the CAN bus is implausible
P240C - The selector lever position sent from control unit N15/5 (Electronic selector lever module control on it) via the CAN bus is implausible

I think the codes in italic are issues that date back quite some time and have been resolved. The underlined items seem to add weight to my initial diagnosis.

The Merc dealership wanted to remove/install the transmission and replace the floor shift module, electrohydraulic controller and TCU. When I asked about the TCU they said that they found evidence of oil on the TCU. The quote was R46 000 ($3 700).

Am I not seeing the full picture? Can the ATF wick up the wires and leave complete sections of wire totally dry?

 

When I've seen that happen on cars, the harness usually remains dry. The oil actually pushes along the wire inside the insulation, so unless you cut back some of the covering on the affected wire you wouldn't see it until it gets to the plug where it can leak out. Sometimes the connectors are so well sealed to the outside that the computer will fill up with oil. It's rate but it does happen.

 

Thanks ItalianJoe1,
That makes sense. I think I was seeing red when I jumped to my conclusion. I am glad that there is a better explanation than the one I came up with!

If the pins on the TCU are dry and I have no error codes connected to the TCU, am I wasting money by replacing it?

 

Probably, but if it got any oil in it or on the connector enough to bridge the pins, it could have caused damage. I'd start with the selector module as you said and see if that clears it all up. Also reset all those codes and see what returns to rule out some of the circuit faults.

 

Thanks.
I'll start with that. Off to eBay.

 

agree 100%. put a new wiring seal in thne tranny and clear codes. then if anything shows up you know it's recent and needs to be checked. i don't know the reason but many mechanics fail to clear fault codes after they repair the problem. i specifically asked the tech at the dealer to clear codes after he fixed the problem. after the fix i went home and plugged my own star c3 in and all faults were still there. i cleared them via star and none have come back. good luck!

 

Primary reason is just laziness, if they don't have to re-connect the scanner for whatever reason and they are hidden codes that will not turn on a CEL, it just doesn't matter enough to them to actually clear them all. Terrible but that's what most techs have become at this point. It doesn't help that dealerships make it harder on them to earn a decent check, but that's no reason to be a slacker and not take pride in your ability to complete the job fully.

 

Do I need to replace the wiring harness as there is already oil inside the wires?

I will do the job myself and will then have them clear the codes. There were pages of codes since the last time I had them cleared. Some laziness on my part for that one. I am amazed at the amount of data the ECU stores.

 

Just having oil in the wire in theory doesn't hurt, but if it gets into the connectors it can do strange things.

 

Thanks. I'll keep an eye on it.

 

ItalianJoe1 knows his stuff1

 

Thanks, I am an ASE L1 master tech, I have a pretty solid background in this kind of stuff so I try to help out when I can.

 

Sorry, am I missing something? How did fix the problem? I got exactly the same

 

Hi BlackMammoth,

I PMed you but here is the short version for those interested.

I replaced the floor shift unit only. Got a second hand part off a C240 from a scrap dealer. This worked for many miles but then gave the same symptoms, so I thoroughly cleaned the optic sensors in the original unit with electric cleaner and it works. I know that this will eventually give in, so I'll replace it with a new unit next time.

If you do buy a new unit, note that Merc have added a safety feature to the software. They, or an indie with the right tools, will have to code the unit to your VIN.

Hope this helps!