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Marty, Mercedes Technician
Category: Mercedes
Satisfied Customers: 53937
Experience:  Bosch Certified Master Tech. Over 35 years experience.
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I have a 200 ML320 with P120 and 202e code. Voltages from

Customer Question

I have a 200 ML320 with P120 and 202e code. Voltages from throttle body are good Supply is 5.9V Photometers are good. I have just cleaned them as well. Open and closed position voltages are within spec. Sometimes the car runs good, code goes away and sometimes it wont go, with Throttle only at fixed 4% position. I have carsoft 7.4. I checked the wiring harness and even resodered the pins in ECU. Same problem. Can my data get corrupted? Can something else affecting the ECU to throw codes and putting the car in Throttle limp mode
Submitted: 1 year ago.
Category: Mercedes
Customer: replied 1 year ago.
Posted by JustAnswer at customer's request) Hello. I would like to request the following Expert Service(s) from you: Live Phone Call. Let me know if you need more information, or send me the service offer(s) so we can proceed.
Customer: replied 1 year ago.
I just replaced the throttle valve with another one of same part number. Same problem. Sometimes good and sometimes limp mode. I know that the excessive current (6 amp) at the throttle motor can stop the ecm from giving power to it. I have tried two throttle valves with same results so I kind of rule that one out. I need to test it with an amp meter.
Expert:  Ron replied 1 year ago.

Hello, my name is ***** ***** I am a professional here at Just Answer. I have noticed that your question was not getting a response and thought I would see if you still need help with this.I apologize for the delay and I hope I can still assist you with this here.

The code P0120 can be set due to any of the following,

  • Air Intake Tube
  • Charging Voltage
  • Mass Air Flow (MAF) Sensor
  • Pedal Sensor
  • Throttle Body
  • Wiring

    In order to determine which is causing the problem you are going to need to do some testing, I have enclosed the steps in the link below, copy and paste the link to your browser to open and view them.
Customer: replied 1 year ago.
As mentioned previously, my specific Mercedes code was P202E under p0120 which is potentiometer 1 and 2 value from the throttle valve or voltage supply. I checked all that and they are within spec. I have carsoft 7.4 multiplexer and WIS. I went through all steps for re learning throttle control several times. I checked the wiring harness several times for continuity and shorts, re-soldered all 6 pins in ECU, tried a spare voltage regulator for alternator and tried another throttle valve and paddle hall effect potentiometer (by the firewall) from another car. My battery voltage is 12.5V and alternator voltage is 14 volts. Sometimes the car starts fine and drives really good. and sometimes it goes in throttle limp mode where I have no throttle output between black and blue wire for the throttle valve motor but I have 4.9 volts supply for pots in the valve. The checks in document you send me was one of the first things I did. I checked the pots in the throttle valve. They are good and within specs with no discontinuity. Closed and open throttle position voltages are good. What can be wrong?
Expert:  Ron replied 1 year ago.

Okay then I will have to open the question back up for you as this is all the information I have. If someone has something different to offer, they will post back.

Expert:  Marty replied 1 year ago.

Hi, I've been following your post but haven't had time to reply. It looks to me like you have ruled out everything except the ECM. At this point I would either replace the ECM or send it to Beckmann Technologies for testing and repair. I've attached a link to their website below.

Let me know how you make out.


Customer: replied 1 year ago.
ECM is not a simple thing to replace. I have done it before and I hate it. I had to get another one with the same part number, swap the EEproms for driver authorization. Its tedious and risky. I used low temperature (eutactic) flux which mixes with existing solder and lowers the melting temperature. I used foil with window cutout for the chip and use a heat gun and picks, and clean with solder wick. I have access to a spare ECU. This would be my last resort. I am hoping someone could offer me an easier way to get around this problem. When in 'on 2' position (not running), I measure all the right voltages if I move the valve by hand. I just now I drove the suv. It ran fine. But if I start it again I will get the same problem. I am guessing either one of the voltages is dropping during drive and sets the code. Maybe (1) I can have an external 5V supply for the pots referenced to the negative supply from ECU or have a big capacitor like 10,000 uf with the supply for the pots so the voltage don't get interrupted. (2) Check the traces for pot 1 and 2 voltage detection circuit in ecu and see if everything is ok. (3) Use a subwoofer speaker choke in series with the motor to avoid current spike and stay away for 5.95 amp limit. I will have a slower throttle. Any more ideas.
Expert:  Marty replied 1 year ago.

I've read through everything you have done and you've done a lot! It's time to send the ECU to Beckmann or swap it with another one. I wouldn't go through all of the trouble of swapping EEproms I would have Beckmann unlock a used one for you. I'm assuming that you have checked the battery and the alternator already. Overvoltage or undervoltage can cause the problem.

Customer: replied 1 year ago.
Hi Marty. My alternator voltage is about 14V when running. My off battery voltage is 12.5V. My problems started when I let the battery drain. I was installing an aftermarket radio with GPS, backup cam, DVD, wifi etc. Bypassed the bose amp. I was using dome lights to work at night. I was wrong about adding a large value capacitor. When powered down, voltage regulator chips cannot tolerate positive bias at output. Cannot use a cap without a diode. But a diode will drop voltage below the spec. I will test some more before swapping the ECU. I will hook up several multimeters and monitor throttle pot supply, pot1 and pot 2 voltages and motor current when the car is running.
Expert:  Marty replied 1 year ago.

Good morning, in reading your posts it sounds like you have been very thorough in checking the wiring and you have already swapped the throttle body. You could try overlaying wires from the ECU to the throttle actuator to see if it still acts up. The connectors at the throttle body would be the most likely place to experience a wiring fault or high resistance. You could also try swapping the pedal value sensor but I don't see it causing this fault code. Aside from that there isn't much left other than the ECU.

Let me know how it turns out.


Customer: replied 1 year ago.
Hi Marty. I was away all day. One thing I noticed was that when I turn off, the 5V power to the pots still remain for some time. I guess 10 sec. Is that normal? Maybe there is signal from pot 1 and 2 when there should not be any? Can this trigger the P0120, p202E code and put the SUV in throttle limp mode? One more thing I remember, When I was installing the radio which came with no instructions, there was a wire called amplifier power. I first hooked it up to Acc. When I turned on the car I saw in some dim light in position 1 and I immediately turned off and disconnected the wire. Turned out that it was an energizing signal for an external amp such as a subwoofer amp turn on trigger. I left it disconnected since I did not have the subwoofer amp on hand. Could this have fried the ECU? The car still runs fine one third of the time. This is so frustrating. I got this SUV used less than 2 months ago, installed front shocks and rare struts (quite some work), new brakes plugs, complete fluid change, new tires, alignment and now this problem.
Expert:  Marty replied 1 year ago.

Hi, I can't see where tapping into the accessory wire would harm anything. I've never checked the 5V signal after shut off but I would expect it to go away with the key off. I'm not sure if that is normal or not but the 5V comes from the ECU so if it isn't normal I'd still say you have an ECU problem. If anything I would suspect that a bad throttle actuator damaged the ECU.

Customer: replied 1 year ago.
Hi Marty. I was away for few days and did not have a chance to reply or test. I got back today. I also got a spare ECU with same part number for only $65 on ebay. Tried it. I hooked up 3 volt meters to pot 1, pot 2 and 5v supply. I have start error as expected but I could move the throttle in position 2 (on) with this ecu and monitor the voltage changes and throttle position change on carsoft. Against your advice to get a verginalized ECU for a good sum of money and resync for even more cost, I just swapped the EEPROMs. I used Chip Quik kit. It took me half an hour to do the swap. Car starts. No more throttle limp.