97 E 320 ABS ETS light On Tranny in Limp - Benzworld.org

s491ja · 01-29-2008, 09:55 AM

My wife just called and said the ABS and ETS light is on the speedometer is not working and the transmission is not working. Does this tranny have a speed sensor. Computer code is PO720 "Output Shaft Speed Sensor".

I think I need to give a little history. The previous owner said the ABS light would turn on about once a month, he would turn off the engine and then back on and it would go away. This tranny was rebuilt a year ago and was running shifting great. When I bought last saturday it turned on like every other day. We turned off and on and it would go away. Last Tuesday we had the tires rotated. Discount tire guy says tires are almost to their limit. We did not buy new tires. It was after the rotation of the tires that the ABS light started coming on more often and the next day the speedo started to not work turned off and on and worked again. This followed by the ABS light staying completly on and the check engine light on (code PO720) and the transmission not shifting out of first (i beleive this is the Limp mode).

In reading through some of the forum their is indication that the ABS system can trigger the tranny to go into limp mode, is this true?

If so would it also initiate a PO720 code?

The forum threads also mention that the tires if not in good condition can trigger the ABS system to initiate, is this true or possible?

I also understand from the forum that I need to get the codes cleared in the tranny computer of which only a tranny or stealership can do.

Here is my question, if get new tires, get the codes in tranny cleared and clear the car computer (OBD II) will the car operate correctly?

(again the only thing we did was rotate tires and the issue developed)

Also I found this statment on another forum, is this true?

"The ABS part of it is where the speed of all 4 wheels is known, then ABS knows what to do when its needed. The same signal is required by the gearbox to know what gear to be in, when the signal is missing it goes into the limp home mode (2nd)."

Thanks

I checked tire pressure, turned on and abs and ets light did not go off. I cleared codes in OBD II let it warm up and went for a drive. The tranny never came out of limp, the abs ets light stayed on, the check engine circuit came back on. I cleared OBD II again same thing happened. I disconnected battery no change, I disconnected and touched negative and positive cables together for two minutes, no change.

I am going to check resistance and voltage on the front sensors and would like to check voltage and fuse on the OVR. Can some help locate it on the 1997 e320 straight six? thanks!

Ok I checked all four resistance on wheel speed sensors, these are the results:

All four wheels resistance were: 1.65 ohms, when I rolled the wheel the resistance increased some.

I checked voltage to all wheels it was: 4.6 volts

Are these good numbers? Also how do check the voltage reading back when the wires are connected?

I checked the brake light switch. The connection with two wires was normally closed when brake pedal was pushed it opened there was no resistance.
The connection with three wires was normally open and closed when the pedal was pushed.

Does this sound like the brake light switch is working properly?

I check all fuses in the engine compartment on the right and left side. All good

I check fuses on the side of dashboard. All good

I check fuses under rear seat. All good

Can someone tell me where the OVP is found so that I can check this?

The ABS and ETS lights remain on and the Check Engine Circuit light is on. The code for this is PO720 Output Speed sensor and Tranny is in LIMP mode. Please Help

E-Class Owner · 01-29-2008, 12:42 PM

as that is the code being thrown. The transmission has input and output sensors mounted in a plastic housing sandwiched between the valve body and the transmission housing. The valve body is removed to access the sensor array.

Once these transmission codes are solved, then you can move on to other issues, if any are present.

Usually ABS/ETS problems will throw control unit or throttle actuator codes as well, so I am hoping that curing the sensor issue will solve your problem.

Good luck,

JR

s491ja · 01-29-2008, 02:40 PM

Thank you JR! My thoughts were the other way around. Since the ABS light was turning on and would clear after restart of car. So I am thinking that if the ABS, ETS is Cleared or corrected the tranny would come out of limp mode. What do you think?

Do you know if the E320 1997 has a Over Voltage Protection relay? I can't find it.

Where does the speedometer get its single from?

If it is the sensor in tranny can a local transmission shop do the work?
Thanks
ja

texholdem · 01-29-2008, 02:45 PM

I don't think we have an OVP on our cars, just wondering what kind of protection against overvoltage is there?

s491ja · 01-29-2008, 03:27 PM

Oh wow! Hopefully someone can confirm this to us. If we don't then something else is being used to take care of Voltage. If so does it still go out and does it still the primary reason for the ABS ETS light to come on?

ja

E-Class Owner · 01-29-2008, 09:05 PM

that your particular problem, i.e. an out of parameters relationship between input and and output revolutions can trigger a limp mode condition. Additionally the ABS system monitors a number of inputs to determine if the car has a wheel that is slipping, or experiencing a loss of traction; the output turbine speed may well function as a variable in the parameters processed by the ABS/traction control ECU. All of these functions: the throttle position sensor, transmission control ECU and ABS/traction control, etc., communicate over the CANBUS.

Good luck.

JR

ohlord · 01-29-2008, 09:20 PM

a great deal(like free)on this car.What on god's earth would possess you to purchase a mercedes benz with an issue like that going on?Brake system faults will trigger limp mode in some cases,start there.The issue that appeared after rotation---do you have staggered sized tires on the car?Are the tires the same height?
You need to check these items and if none test out then a trip to a good shop is warranted and above all,next time get the car inspected by a qualified inspector or shop before you buy it.With so many really nice examples for sale never unless you can do the repairs and you get the car for far below market value,NNNever buy one with known problems

This is almost like the guy who bought the car and is now having a post auction inspection done on it.Inspect it before you Buy it

ohlord

s491ja · 01-30-2008, 05:52 AM

JR All the tires are the same size and brand. I have not performed a roll out.That what it sounds like. Like the transmission sensor is not liking not getting a sensor reading from the ABS. Do you know if this car has a OVP relay?

Yes, I did buy way below the blue book, but now that I know they are so sensitive, I would reconsider.Ouch!

ja

E-Class Owner · 01-30-2008, 06:52 AM

Sorry.

Looking at this history of all this, I wonder if the ABS fault started being triggered after the transmission work was done by the former owner...?
You might want to ask him.

Ohlord is right that one should never buy a Mercedes with an issue like that, but the purchase is now water under the bridge, as the old saying goes.

On my W124, 400E, the ASR/ABS light would occasionally come on and would clear with a restart for a while, then the light appeared more and more often, until it would not clear/reset and the car stayed in limp mode. That seems to be the progression. The culprit was a bad throttle actuator coupled with a bad neutral safety switch.

Your issue may be tires related, OR is could be a coincidence that the problem got worse after the rotation. With transmission work having been done on the car, and the only code being thrown is the "PO720," personally I would start there. The car is not likely going to go out of limp mode until that fault is corrected.

Ohlord may be right that other issues may be involved, and a quick change of the stoplight switch might not be a bad idea as well...

I hate to say this, but some professional assistance may become necessary, because if you start to throw parts at the problem, it is going to get expensive.

I would, if possible, find out from the previous owner what was wrong with the transmission, who did the work, and when in the sequence of things the ABS warning light began to appear.

Good luck,

JR

s491ja · 01-30-2008, 04:59 PM

Thanks JR, you are right it is water under the bridge. Lessons learned. Nonetheless, let me give you an update. I started up the car today to limp to the tire company to install 4 new tires. Praise the Lord! The car was not in limp mode! Yes the ABS and ETS light is on still. I put 4 new tires. Took for a spin and the lights remained on. I decided to take to an MB service center (bite the bullet). After $110.00 the computer identified the front right speed sensor as failed. Two different controllers identified it. They said this would take care of the ABS light. They said the replacement of the brake switch would take care of the ETS light. I so thankful that there were no other serious codes like the tranny. By the way out of limp mode the tranny works like a charm, the whole car for that matter.

They wanted $525.00 for parts and repairs, I told them I would get back with them. I feel I can do the installation. Where can I buy these parts at a reasonable price and good brands?

After I replace I will post the results.

Thanks again!

ja