722.6 transmission RPM sensor problem - PeachParts Mercedes-Benz Forum

ace · #1 12-08-2005, 06:42 PM

'97 E420 Transmission is giving me error code p0730, which indicates "gear ratio problem" .
There are several possible causes for this , some of them related to the transmission actually not shifting into the desired gear thus not attaining the proper gear ratio, but in my case, appropriate gear is reached.
The transmission controle unit derives the gear ratio by comparing input from RPM sensors N2 and N3.
These are hall effect sensors that are part of the electrical conductor plate which is mounted on top of the valve body.
During test drive with scanner connected, N3 sensor is sometimes reading 0 rather than a "normal" value. Incidentally, I only observed the bad reading from N3 when transmission was in 5th gear.
I measured the sensors electrically (resistance) and N2 and N3 behave similar. I realize that they are hall effects and that just measuring their resistance may not be telling all.
So, to rule out the sensors, I bought the new electrical conductor plate from MB, which cost only $150 so not too wildly expensive. It comes with the 2 new RPM sensors.
With the oilpan and valve body removed, when I look up from the bottom of the transmission, I see the two holes through which the RPM sensors stick into the main transmission housing. I can see the two encoder wheels from which the RPM sensors pick up their respective signals.
Now here comes my question:
The front encoder wheel ( the one closest to the engine) is solidly mounted on its shaft. The rear one, which seems to be part of clutchpack K1, has about 1/8 inch axial movement.
Is this excessive and could it be that N3 is losing it's signal because the encoder wheel is not lining up properly with the sensorhead?
Or, is the axial play a logical result of the fact that there is no oil pressure in the system when I am inspecting it this way ?
I have attached a picture to illustrate what I am talking about.
I would appreciate your opinions.

Arthur Dalton · #2 12-08-2005, 07:19 PM

I am not a tranny guy , but I would certinly expect to see any hall effect sensor to require a solid set position/gap to maintain a true reading....
.. and .125 varient may be enough to even drop the signal.

I am sure a trans tech will drop in....

duxthe1 · #3 12-08-2005, 11:12 PM

What's the mileage on the unit? These are not the most durable trannies. Without looking at one I don't remember exactly how the sensor wheel rides but the exessive play there could indicate impending failure. Is there much sign of metal inside the pan?

ace · #4 12-09-2005, 12:05 AM

The Mileage is around 110K. There is no metal in the pan but the transmission was serviced last June by someone else ,so evidence might have disappeared. Question, if the clutch packs are worn , would I see metal or friction material in the pan? Or are you suggesting something other than clutch packs has worn out?

mbdoc · #5 12-09-2005, 08:12 AM

To start with "P" codes from the engine computer aren't that exact on what is happening with the transmission.

You need to read codes from the trans controller! THERE are 96 codes possible & another 96 intermittant codes!

ALSO P0 730 means that the trans is slipping!
Again a scanner that can read live data from the trans controller will show if the trans clutches can still lock-up.

P0 700 or P0 715 can mean that the RPM sensors have a problem, but can also signal MANY other problems as well!

ace · #6 12-09-2005, 12:44 PM

Thanks kindly MB Doc,

According to the information I have , P0730 series codes are
P0730 Incorrect Gear Ratio
P0731 Gear 1 Incorrect ratio
P0732 Gear 2 Incorrect ratio
P0733 Gear 3 Incorrect ratio
P0734 Gear 4 Incorrect ratio
P0735 Gear 5 Incorrect ratio
P0736 Reverse incorrect gear ratio

I agree with you that a slipping clutchpack would cause a change in the RPM ratio between in put and output shaft. This may cause the TCM to report that the gear ratio is incorrect.
However, as I mentioned in my posting, I monitored the live sensor readings for N3 and saw them go to 0 at times. This was with the transmission in 5th gear and not much torque applied. That is why I started pursuing the RPM sensor path.

Do you happen to know if the axial play I have observed on the encoder wheel for N3 is normal or excessive?

mbdoc · #7 12-09-2005, 12:47 PM

Since you have looked at live data with scanner, look at the shift adaptation tables both up-shift & downshift.

THE RPM readings that you are seeing are erroneous & don't have any bearing on your problem!

C32AMG · #8 12-09-2005, 09:53 PM

There are two causes of gear ratio codes, electrical faults such as bad sensors, wires, controllers, etc.And mechanicals problems, transmission slippage, the wrong ratio parts and or wrong transmission Installed, internal parts failure. . What are your adaptations numbers?

The exciter ring should never move, and if it does you should see an erratic speed sensor reading, not ‘0’. No reading means no signal, and then you should have a code P0715.

It can also mean that 5th gear is in fact slipping. Does your scanner show a gear ratio number if it does, in 5th gear your ratio is 0.83. With a lose exciter ring the transmission needs to be repaired.

The TCM is linked thru a CAN Bus to ALL the modules such as ABS, ASR, etc, you have to scan all the systems, not only the TCM.

ace · #9 12-10-2005, 12:38 AM

Gents,
thanks for your input. I will check the gear ratio readings per your suggestion.

kashmir320 · #10 06-05-2009, 09:00 PM

Soo I am having a problem

. I am doing my ex-wife a favor and working on her 97 E320 with a 722.6 tranny and have encountered some issues. After her original transmission went bad (front pump went out and kept going into limp mode) I installed a known good tranny from the wrecking yard out of the same year/model car along with a new filter and MB synthetic fluid through the dip tube using the MB fluid level tool. After installation the transmission will not shift out of 1st (or 2nd..not sure which one its in) and the check engine light light is on giving off a code of P0730. That code reads as "improper gear ratio" on the web but the dealer told me that the book just states that it indicates there are codes that need to be read in the Transmission Control Unit which I didnt read yet. I have heard different stories and do not know what to believe...one shop says I may need to have them clear the codes and initialize the software to take it out of limp mode....another says that the valve body in the new trans may be a different version than the control unit...and another said that solenoid plate or something might be bad. Like I said I am doing this as a favor for my ex because she is very short on money and couldnt afford the labor costs for the tranny install and now we are still in the same boat with no real answer on what is going on. Any ideas out there? What sounds like the likely culprit? What should I do first? BTW, when the old tranny was in it would work fine when it was cold shifting through all the gears, but once it heated up it the front pump would start grinding and then it would go into limp mode and eventually wouldnt move until it cooled off again. Now it doesnt shift at all cold or hot and it also bangs into gear when putting it into DRIVE or REVERSE.

whunter · #11 06-22-2012, 05:49 PM

for customer

spit64 · #12 06-22-2012, 11:22 PM

Early 722.6 transmissions had a poor valve body conductor plate design. This part has the starter lock out switch and a transmission VSS sensor built into it that powers the shift solenoids. I am not sure but could this be the problem.

Not hard to change the conductor plate you could do the job with tranny still in the car. Could it be possible that you have a leak in the A/T plug adaptor and it find it's way up to the electronic unit.