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Hello all, last night my wife was driving the C43 and started having trouble with the transmission. The car was driven for 45 minutes without an issue, then sat for approximately 1 hour. After that she reversed out of a parking space, and the car refused to go into "D", the dash indicator would just flash and go to neutral. After multiple attempts the car eventually went into "D" but would only use gears 3-4-5. When she got home I confirmed this behavior in the driveway. There was no CEL or any warning messages on the dash and the trans temp was in the normal range. After about 15 minutes I performed the "TCM Reset" procedure I found online (no clue if this does anything), restarted the car and could shift to "D" normally and the car went to 1st gear.
Tonight I took the car out for a drive getting everything up to temperature, driving in all the different modes, manually shifting gears, etc and everything functioned as usual. I scanned for codes before and after the drive and came up with nothing. Is it possible there was some software or sensor glitch that locked out the other gears?
Do the MB specific codes for the trans still trigger the CEL?
The only other thought I had is that the battery went flat about 2 months ago after the car sat for 10 days during winter, and perhaps it is getting weak and causing strange problems (original 2018 battery). Appreciate any insight.
Hello all, last night my wife was driving the C43 and started having trouble with the transmission. The car was driven for 45 minutes without an issue, then sat for approximately 1 hour. After that she reversed out of a parking space, and the car refused to go into "D", the dash indicator would just flash and go to neutral. After multiple attempts the car eventually went into "D" but would only use gears 3-4-5. When she got home I confirmed this behavior in the driveway. There was no CEL or any warning messages on the dash and the trans temp was in the normal range. After about 15 minutes I performed the "TCM Reset" procedure I found online (no clue if this does anything), restarted the car and could shift to "D" normally and the car went to 1st gear.
Tonight I took the car out for a drive getting everything up to temperature, driving in all the different modes, manually shifting gears, etc and everything functioned as usual. I scanned for codes before and after the drive and came up with nothing. Is it possible there was some software or sensor glitch that locked out the other gears?
Do the MB specific codes for the trans still trigger the CEL?
The only other thought I had is that the battery went flat about 2 months ago after the car sat for 10 days during winter, and perhaps it is getting weak and causing strange problems (original 2018 battery). Appreciate any insight.
Your scanner does not turn up any faults that may explain limp-mode. Based on your comments this may be explained by low voltage aka "drained by driving".
You can easily test for this on the same 45mn or longer drive during daytime only. Display the live chassis voltage, then watch when it goes near 11.Volts while driving after 25mn mark... that's a deep drain! (14.9v to 12.6V: ok, not below)
Having an older 2018 battery helps cause this chaos with lower cranking voltage that disrupt CAN-Bus. A fresh AGM will do you well.
Last edited by CaliBenzDriver; Apr 10, 2026 at 01:40 PM.
Not sure what IB and BN indicate, but a resting voltage of 11.6V is definitely poor. I did only check for codes with a basic OBD scanner, not sure if that makes a difference. Does the car have an electronic level sensor for the trans fluid? Not sure how else it could be checked/monitored with a dipstick.
Not sure what IB and BN indicate, but a resting voltage of 11.6V is definitely poor.
I did only check for codes with a basic OBD scanner, not sure if that makes a difference.
Does the car have an electronic level sensor for the trans fluid? Not sure how else it could be checked/monitored with a dipstick.
Great: you have retrieved UB /IB at Key:On Engine:Off directly from the trusted battery sensor.
The battery charge level is low.
This does not mean the battery is bad.
At the very least it means Batt. is discharged.
Another new battery may not help underlying causes leading to low charge condition.
Tranny have no dipstick. They do no consume any oil the way engines do. No active leak means level is ok.
It's unlikely the main issue here.
> UB/IB Test Drive...
-- Setup the UB/IB display and go for uninterrupted 45mn daytime drive to witness what happens after the 12.6V regulation begins...
-- Do you see UB voltage keep going down any lower than 12.3Volts (IB climbing up) ??
> Plan B... Use Float Charger!
This being your wife transportation, you may want to involve your local MB Specialist at your discretion
or buy a BATT. FLOAT CHARGER such as CTEK or NOCO.
It will charge up your battery perfectly every time.
Last edited by CaliBenzDriver; Apr 11, 2026 at 02:49 PM.
Well the battery did go completely flat about 2 months ago after a trip. I figured after all this time of driving if it's not charged up then it is probably weak.
Today I bought a battery and put it in. Went out for another drive, about 30 minutes. Stopped at a gas station and drove the last 2 minutes home without a problem. As I was straightening the car in the driveway, it did the same thing as before and would not engage "D". The trans temp had reached 53 degrees Celsius (just past the point of changing to white from blue). Still no dash warnings or engine light. Eventually it clunked into 3rd so I finished parking. I guess I will be taking it to the dealer.
Car has about 128,000km, I have not performed a transmission service yet, but I believe it is due around this mileage.
Well the battery did go completely flat about 2 months ago after a trip. I figured after all this time of driving if it's not charged up then it is probably weak.
Today I bought a battery and put it in. Went out for another drive, about 30 minutes. Stopped at a gas station and drove the last 2 minutes home without a problem. As I was straightening the car in the driveway, it did the same thing as before and would not engage "D".
The trans temp had reached 53 degrees Celsius (just past the point of changing to white from blue).
Still no dash warnings or engine light. Eventually it clunked into 3rd so I finished parking.
I guess I will be taking it to the dealer.
The local MB Specialist will have this sorted in no time.
Speaking of flat battery, what about the AUX Batt?
When MAIN gets flat, so does AUX.
AUX does interact with the ISM shifter, so that would make complete sens.
Use a scanner or volmeter if you have a true battery instead of "AUX Module".
Last edited by CaliBenzDriver; Apr 11, 2026 at 06:18 PM.
Tried again after car sat for about 45 minutes, shifted to "D" again without an issue. The trans temp had only gone down to 52 degrees Celsius in this time.
Well the battery did go completely flat about 2 months ago after a trip. I figured after all this time of driving if it's not charged up then it is probably weak.
Today I bought a battery and put it in. Went out for another drive, about 30 minutes. Stopped at a gas station and drove the last 2 minutes home without a problem. As I was straightening the car in the driveway, it did the same thing as before and would not engage "D". The trans temp had reached 53 degrees Celsius (just past the point of changing to white from blue). Still no dash warnings or engine light. Eventually it clunked into 3rd so I finished parking. I guess I will be taking it to the dealer.
Car has about 128,000km, I have not performed a transmission service yet, but I believe it is due around this mileage.
I did my transmission fluid at 5 year mark so in 2022 I have a 2017 since new but my mileage was low at that time at 36000 KMs. I believe the interval was 5 years or 50,000 (or was it 60,000?) miles whichever comes first, I like to half my intervals.
Great: you have retrieved UB /IB at Key:On Engine:Off directly from the trusted battery sensor.
The battery charge level is low.
This does not mean the battery is bad.
At the very least it means Batt. is discharged.
Another new battery may not help underlying causes leading to low charge condition.
Tranny have no dipstick. They do no consume any oil the way engines do. No active leak means level is ok.
It's unlikely the main issue here.
> UB/IB Test Drive...
-- Setup the UB/IB display and go for uninterrupted 45mn daytime drive to witness what happens after the 12.6V regulation begins...
-- Do you see UB voltage keep going down any lower than 12.3Volts (IB climbing up) ??
> Plan B... Use Float Charger!
This being your wife transportation, you may want to involve your local MB Specialist at your discretion
or buy a BATT. FLOAT CHARGER such as CTEK or NOCO.
It will charge up your battery perfectly every time.
Quick question, what does float charge do? It takes a long time as well, 10 days according to CTEK? Is it worth it, is it fine to interrupt it half way in or it is better to not float charge at all if I can't keep it plugged in for 10 days non stop?
Did some reading and found others who had this issue, sounds like it could be the "Conductor Plate". About half them said this started happening after having the transmission serviced. Sounds like an expensive repair. Never had such a major issue on a vehicle with such low mileage.
Did some reading and found others who had this issue, sounds like it could be the "Conductor Plate". About half them said this started happening after having the transmission serviced. Sounds like an expensive repair. Never had such a major issue on a vehicle with such low mileage.
The 4th link you posted sounds similar to my situation, but the others are not. I am only having the trans go to Neutral when shifting from Park to Drive, or Reverse to Drive. It has not jumped to Neutral while driving. I just dropped the car off the dealer, 30 mile/50km drive with a mix of stop and go and highway, no issues. Could not duplicate the problem once I arrived at the dealer parking lot. They will look at the car on Monday.
The 4th link you posted sounds similar to my situation, but the others are not. I am only having the trans go to Neutral when shifting from Park to Drive, or Reverse to Drive. It has not jumped to Neutral while driving. I just dropped the car off the dealer, 30 mile/50km drive with a mix of stop and go and highway, no issues. Could not duplicate the problem once I arrived at the dealer parking lot. They will look at the car on Monday.
I thought you had the same problem after re-reading your original post and I noticed the keyword neutral, either way glad it isn't the same issue because looks like that other issue I previously linked is more tricky than yours. Hopefully they figure it out for you, keep us posted. Hopefully DTCs will tell what it might be.
PRND is not related to the valve body conductor but to AUX low voltage locking ISM.
When I look this up it sounds like that is more related to the 7-speed than the 9-speed.
Anyway the dealer called me, they don't know what's wrong (shocking) but they are suggesting valve body replacement, and if that doesn't work, full transmission replacement. Doesn't give me a lot of confidence that they think it "might" be a $4k job. I feel like they should be suggesting a trans service first.
When I look this up it sounds like that is more related to the 7-speed than the 9-speed.
Anyway the dealer called me, they don't know what's wrong (shocking) but they are suggesting valve body replacement, and if that doesn't work, full transmission replacement.
Doesn't give me a lot of confidence that they think it "might" be a $4k job. I feel like they should be suggesting a trans service first.
I think you understand what's happening when your back's against a wall.
Zero diagnostic... you can expect better than "wet finger" from factory trained dealership.
What is the warranty of A TRUE FIX on this $4,000 conductor repair ??
Can this be simply electrical chaos on CAN-C: free reboot + float ?
Last edited by CaliBenzDriver; Apr 13, 2026 at 04:47 PM.
Here is a snippet of the scan data the dealer sent me. Funny that they also told me "both needed to be replaced" strictly based on the stored fault code from this scan. They did not bother to physically check the battery, or else they would have seen it was new. That makes me suspicious of their "diagnostic" capabilities. I hate to think this way but it seems like a tech has simply read some codes and guessed at what the problem could be.
Here is a snippet of the scan data the dealer sent me. Funny that they also told me "both needed to be replaced" strictly based on the stored fault code from this scan. They did not bother to physically check the battery, or else they would have seen it was new. That makes me suspicious of their "diagnostic" capabilities. I hate to think this way but it seems like a tech has simply read some codes and guessed at what the problem could be.
the above Xentry does not read like a need to rush towards extensive repairs... let's not say dishonest, perhaps unexperienced factory trained technician.
Look closer at the report:
tranny is FREAKED OUT ON LOW VOLTAGE: 11.9Volts. clear black on white report....
Unable to talk to CAN-Bus partners, it is unable to work and had to safely limp mode.
You have evidence it is not only the tranny module thats gone wild: the CAN-Bus network too.
This is an external issue caused by partners.
The solderless IGNITION SWITCH was reported nuts.
Tempering with tranny is mis-diagnosed.
The first fix is to correct the chassis LOW VOLTAGE soft-crashing VIP modules.
Your car may suffer from "drain by driving" regardless of once young battery.
Last edited by CaliBenzDriver; Apr 14, 2026 at 04:22 AM.
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