Troubleshooting Code P2100 on 722.6 Gearbox - 1-2, 4-5 Shift Solenoid

[Martin398]

I have a 2003 C200 CDi with 722.6 gearbox.

Code readers, including Snapon Solus give P2100 1-2,4-5 Shift Solenoid fault and the box is in limp mode (it drives in second gear and reverse only).

I have tested the electrical resistance of the 1-2,4-5 solenoid, the 2-3 solenoid, and the 3-4 solenoid from the connector at the transmission control module (TCM) and all 3 solenoids read 4.6 Ohms.

I have tried clearing the code with the Solus, but it returns. (I had hoped the Solus would allow functional control of the solenoids, but it doesn’t)

Below is the wiring diagram. I carried out some electrical checks but the answers I got have confused me and I am hoping someone can explain it to me.

Looking at the wiring diagram, I was expecting a ground-side-switched system, with Pin 38 (+) showing 12V, and, when the shift solenoids are operated, Pins 14, 15 and 16, would, in turn, be grounded as required. Instead, however, Pins 14, 15 and 16 are grounded all the time (key on, engine off or running, shift lever in park or moved through its positions), and there is practically no voltage on the white wire from Pin 38. With Pins 14, 15 and 16 being grounded all the time, clearly Pin 38 cannot be at 12V, otherwise all solenoids would operate together, so my thinking seems to be wrong. (These voltages were obtained with the TCM connected, by probing the wires.)

I have a spare TCM, and that shows the same behaviour.

(There is power on the red-white wire to Pin 20 from the front SAM. And the ground at Pin 30 brown wire is good.)

I’d be most grateful for help understanding when and how these solenoids are powered and grounded so that I might proceed with the diagnostics. There’s no point in my going any further if I don’t understand the operation. What am I missing?

 

[Martin398]

Having thought about it away from the car, there are a couple of further tests I need to do.

i need to check if there is a short to ground on the wire from Pin 38 to the transmission.. So I need to:

1. Disconnect the plug at the TCM that contains Pins 10-17 and 33-38. Then, at the TCM, I need to see if I now have 12V on Pin 38 (assuming the TCM is not blown). Also, I need to see if Pins 14,15 and 16 are no longer grounded.

2. Then I need to check if, on the now disconnected connector, there is a short to ground on Pin 38.

If the wire from Pin 38 to the transmission is shorted to ground, then I have a short to ground in the wiring or in the connector plate. If not, then I’m really confused!

I’ll report back in a couple of days with the results.

I’m assured by the owner that this transmission has had the modification done to prevent oil seepage past the connector O-ring, and that the harness was renewed. We’ll see.

 

[mattkh]

Just checking.
Are you aware that on some Mercs, the oil from the gearbox travels up the wiring loom upto the engine control unit.

 

[Martin398]

Just checking.
Are you aware that on some Mercs, the oil from the gearbox travels up the wiring loom upto the engine control unit.

Hello Mattkh,

Many thanks for looking through my posts and replying. Yes, I came across that defect quite early on in my Googling. That’s why I added my final sentence in bold on my second post. But I added it during an edit, and it’s quite possible that you posted your kind piece of advice whilst I was making my edit, because your reply came through quite soon afterwards.

But I really appreciate your reply, because, from what I read, that oil leakage did indeed cause the symptoms like I’m seeing, so even though the owner assures me the modification was carried out, and that he has never seen any transmission fluid coming out of the wiring, I keep an open mind. So I’m really grateful for your reminding me.

 

[chopperovski]

I had similar problem with the 722.6 gearbox on a 2005 E Class. Turned out to be the electronic conductor plate which I duly changed plus ATF and filter. This seems to be a common problem and lots of info on forums.

Not sure if C200 has dipstick (E class didn't ) but ensuring the ATF level is correct is very important maybe try that first?

Hope that helps.

 

[mioba]

I would be getting under the car if you can to check the wiring loom for oil contamination. The rubber seal should be replaced at ATF change (not a service item) but needs to be maintained.

 

[Martin398]

I had similar problem with the 722.6 gearbox on a 2005 E Class. Turned out to be the electronic conductor plate which I duly changed plus ATF and filter. This seems to be a common problem and lots of info on forums.

Not sure if C200 has dipstick (E class didn't ) but ensuring the ATF level is correct is very important maybe try that first?

Hope that helps.

Many thanks. I forgot to ask him today about how the level is measured; I think he told me there’s no dipstick, but I need to double check. The Mercedes wiring diagram doesn’t say what sensors n1 and n2 are (see above), and I figured they may well be fluid-level detectors - I can’t think what else they could be. Thanks for reminding me. I’ll make a note about checking the level.

 

[Martin398]

I would be getting under the car if you can to check the wiring loom for oil contamination. The rubber seal should be replaced at ATF change (not a service item) but needs to be maintained.

Thank. Certainly, if there is a short to ground, we’ll have to get underneath to discover if the short’s in the harness or inside the box. And even if there is no short, that oil-contamination possibility really must be double checked regardless of what his garage told him. The owner has access to a four-poster lift, so there’s no excuse for not checking this thoroughly.

Many thanks

 

[Martin398]

SOLVED:

It was a short to ground on the control side of the 1-2, 4-5 shift solenoid.

The resistance measurements from the connector (disconnected from the TCM) to battery negative:

Pin 38 to ground 5.7 Ohms
Pin 14 to ground 1.2 Ohms
Pin 15 to ground 9.1 Ohms
Pin 16 to ground 9.2 Ohms
Pin 17 to ground 7.8 Ohms

of course, the resistance to ground should be OL through the solenoids (with the connector disconnected)

Looking at the wiring diagram above, these readings show that the short to ground is on the right-hand side of solenoid y3, the resistance of each coil previously having been measured at 4.6 Ohms.

It is, of course possible that the short to ground is in the harness, but as the harness was apparently recently replaced, and the wires are not free to chafe against metalwork, it’s safe to presume the short is in the box.

On disconnecting the connector, the voltage on Pin 38 at the TCM came up to 3.6V, so not the 12V I had guessed at, but clearly there is protection inside the TCM should the power to the solenoids become grounded. Similarly, Pins 14-17 on the TCM, with the connector removed, went from being grounded to very high resistance.

The owner will take the car to a transmission specialist, and if there’s anything to add as a result of the repair, I will add it.

Thanks to everyone for their suggestions and advice.

 

[Martin398]

Whilst looking for a pin-out diagram for the 13-pin connector at the transmission, I identified n2 and n3 on the wiring diagram.

n2 is labelled RPM Sensor 2 (Y3/6n2), and n3 is RPM Sensor 3 (Y3/6n3).

b1 is the temperature sensor and s1 is the starter lockout contact.

(so n2 and n3 are not fluid-level sensors as I incorrectly guessed.)

Another piece of information: I looked on Haynes Autofix and, normally, there should indeed be 12V on Pin 38, the power feed to the solenoids when the engine is running. I measured 3.6V with the connector disconnected; this, I believe will be the bias voltage that the TCM uses to check for faults eg an open or short circuit, and only if it sees no faults will it then provide 12V on cranking. This would explain why the TCM is not damaged by the short to ground.

 

[MinionBob]

I don't think the 722.6 has a fluid low sensor in it. Dont ask me how i know....

Thanks for taking the time to reply and update, always very ueful to know this stuff.

 

[Martin398]

I don't think the 722.6 has a fluid low sensor in it. Dont ask me how i know....

Thanks for taking the time to reply and update, always very ueful to know this stuff.

Many thanks. It’s a pleasure. I figure someone else is bound to have this problem in future.

I keep asking the owner how he knows the fluid level is correct and he can’t give me an answer! Other than that it’s possible to add a dipstick retrospectively (eg from ebay). I guess he uses his driveway to determine the level, assuming the transmission was initially full.

Anyway, to prove beyond doubt that the short to ground is inside the gearbox (and not in the wiring loom), I got the owner to disconnect the connector at the transmission, and as soon as he did, the bias (test) voltage went from zero (short to ground) to 3.6V (open circuit). My multimeter is a Fluke and it has a low-impedance optional setting ( 3k Ohm input impedance instead of the usual 10M Ohm).. When set to that setting, the 3.6V fell to 2.9V, proving that this voltage of 3.6V prior to cranking, is from a very high impedance bias circuit used for the onboard diagnostics prior to startup - thereby preventing the inadvertent input of a high current 12V supply into a circuit that is shorted to ground. To figure out how the onboard diagnostics can so accurately determine which solenoid is faulty , I jumped the supply (3.6V) from Pin 38 on the connector to Pin 38 on the TCM, and then measured the voltage on Pins 14, 15, 16, 17, 36 and 37 on the loose connector. On Pin 14 (1-2, 4-5 solenoid) I got 2mV; on all the others I got 23mV, thereby showing how the OBD knows which solenoid circuit the short to ground is in.

The owner is going away on Friday, so it’ll be at least 2 weeks before the car will go in for repairs.