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Discussion Starter #1
Ok first post. Here goes. I have searched and come up without an answer to this problem.

I recently purchased this car knowing it had a transmission problem. Feel free to call me an idiot or whatever names you wish. I got it cheap though and I really like the car so I would like to get it usable.

I am a long time shade tree mechanic. Not a professional by any stretch of the imagination but I have rebuilt a few engines in my day and even a manual transmission or two. But I have never owned a Mercedes before this car.

So on to the symptoms:

When I purchased the car it had an engine light on and was stuck in 2nd gear limp home mode. If the car was driven until it warmed up (while it was still in limp home mode) it would eventually stop moving as if it were shifted into neutral. No movement forward or reverse once warm. And by warm I mean a tick over 80 on the engine coolant temperature gauge. The display would show the proper gear selected but the car acted like it was in neutral.

I read the code using my sons Snapon code reader. It was p0715 input turbine speed sensor. Since the fluid looked fine (and at the proper level) I decided to change the conductor plate based on what I read online. I also checked the TCU which was dry, and I replaced the electrical plug even though it was dry. After the new parts were installed, the car acted identical as before (stuck in 2nd gear at least). After the fault code was cleared from the TCU the car ran and shifted fine. I thought I had fixed it. I put about 50 miles on it the first day.

I drove it a few more short trips before trusting it enough to take it to work. On my way into work the temperature got a tick above 80 on the coolant temperature gauge and the car again acted like it was in neutral. Since I was in busy traffic I had to try to get to safety. I put my hazard flashers on and shifted the car into "actual" neutral and coasted to the side of the road. On a whim I shut the engine off while I was coasting and restarted the engine. When I popped it back into gear it would accelerate again and shift into another gear but only for a few seconds until it acted as if it were shifted into neutral again. I was able to make it into work and park it. My son brought his snapon reader to me again and I was able to read the codes in the TCU.

This time the codes were different. P2500 (transmission has impermissible transmission ratio), P220A (Y3/6N2 - Y3/6N3 speed compar implausible) and P2502 (gear implausible or transmission slipping).

So after I got the car back home by trailering it I checked the fluid. It was a bit low so I filled it to the proper level (even with the 80 mark when warm). I cleared the codes and drove the car again. It drives and shifts fine when cool. But as soon as the gauge gets a little over the 80 mark it goes into false neutral again.

I don't even need to be driving the car for this to happen. If I let the car warm up in my driveway as soon as it goes over 80 degrees by a small amount it will not shift into any gear.


So I am looking for guidance from the masters. Anyone have any suggestions on what I may be up against?
 

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1998 E320 base sedan @ 160kmiles
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Hello :)

Have you ever observed the rpm sensors at different gear settings ? (one driving, the other reading the values with your scanner).

I do not think the coolant temp has much to do with the transmission. It seems to be more related to to the oil pressure inside the transmission dropping after start up, causing ATF starvation. Worn oil pump, plugged filter, filter not properly installed.........

I am sure you checked the ATF contamination at the adapter bushing, and at the TCM.

Have you checked if there is coolant contamination in the ATF when you drained it ? at the bottom of the pan ? Metal shavings ? Anything unusual ?
 

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Discussion Starter #4
Hello :)

Have you ever observed the rpm sensors at different gear settings ? (one driving, the other reading the values with your scanner).

I do not think the coolant temp has much to do with the transmission. It seems to be more related to to the oil pressure inside the transmission dropping after start up, causing ATF starvation. Worn oil pump, plugged filter, filter not properly installed.........

I am sure you checked the ATF contamination at the adapter bushing, and at the TCM.

Have you checked if there is coolant contamination in the ATF when you drained it ? at the bottom of the pan ? Metal shavings ? Anything unusual ?
First off, thank you for taking the time to reply. It is appreciated.

I have not had the scanner connected while I drive the car. I guess I will try that. But what puzzles me is the fact that the car doesn't even need to be in motion for the problem to occur. Just parked in my driveway idling will cause the issue. How can there be an implausible or slipping when the car isn't even in gear?

The main reason I mention the coolant temperature is it seems consistent as far as when the transmission stops engaging. I assume there may be a correlation between temperature and fluid pressure? Maybe the transmission fluid is getting to a certain temperature and flowing somewhere it doesn't belong when it gets to a certain temperature? Or maybe it stops flowing altogether when it gets warm?

There is no visible contamination of water in the ATF. And likewise I see no ATF in the coolant.

Also once the car is warmed up and in false neutral, if I shut the engine down and wait 10 or so seconds and restart the engine the transmission will engage again albeit very briefly.
 

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1998 E320 base sedan @ 160kmiles
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The reason I suggested to look at the n2 and n3 sensors in gear is to see if there is a discrepancy in readings when both are active. If there is, then the "loss of drive" condition occurs, which is defined in the ATSG technical service information as:

Loss of Drive
If the TCM detects a situation that has resulted in or
may result in engine or transmission failure the
transmission will be placed in neutral. Improper
Ratio, Input Sensor Overspeed, or Engine Overspeed
DTC's will create a loss of Drive.

With the P0705 as a generic summary code, your car complains that there is a problem with the rpm sensor readings. If one sensor fails (no output), the transmission will get into electrical limp mode, i.e no recovery till the code is received. The only other possibility I see is the TCM code 110 (P0715) which indicates that the readings from n2 and n3 are implausible. This problem will not show when not in gear, but as soon as you put the car in gear, and if the conditions are right, you will get the code 110, and the loss of drive condition.

So, I believe when the slipping occurs, this is detected by the rpm sensors, and the transmission is put in neutral as explained above.

Why would the transmission start to slip after some driving ? It can be due to underfilling / overfilling of the ATF, ATF pump issues, blockage issues in the transmission or filter, etc. etc.

Regarding the coolant temp issue, yes the ECU broadcasts the coolant pump temperature over the CAN bus, and the TCM uses this information, so it can determine the shift points when the engine is cold. It will not directly impact the slip condition you are having.

Also scan the other modules like the ECU and the ETS (traction) to see if there are current and stored faults.
 

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Under filling or over filling? So exactly how much should be in there? Every time I’ve drained mine a different amount comes out compared to the first time I did it and it was burnt. Each time I’ve refilled it I used new fluid. So how can I tell if it’s over or under filled?
 

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Discussion Starter #8
Thank you mrboca. Your explanation is very clear and easy to understand. A true master can take the complex and make it understandable to us less informed types. I appreciate the help.

I will plan on getting the reader from my son again and will scan it while the condition occurs and will update this thread for anyone who stumbles across it in the future. It seems there are lots of threads on the internet that complain of similar (but maybe not exact) problems but don't seem to follow up with their solutions.
 

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Thank you mrboca. Your explanation is very clear and easy to understand. A true master can take the complex and make it understandable to us less informed types. I appreciate the help.

I will plan on getting the reader from my son again and will scan it while the condition occurs and will update this thread for anyone who stumbles across it in the future. It seems there are lots of threads on the internet that complain of similar (but maybe not exact) problems but don't seem to follow up with their solutions.
Sadly this is going to be another thread where the solution does not get posted. I have decided to sell the car as is and get something ready to use for the winter.
 
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