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DIY Fix - 205C16 Adblue Temperature Sensor : Short circuit to Earth
Here’s a DIY to ‘fix’ this issue. Please note that this ‘fix’ is only for areas where Adblue will never freeze at -11 degrees. Here in Sydney, Australia, this car will never experience those temperatures.
Symptoms:
- 2013 W166 ML250 Orange Check Engine Light comes up.
- Blows a lot of whit exhaust smelling like ammonia
- Adblue consumption high. Uses remaining tank within 3 days of light appearing.
Diagnosis:
- Error Code 205B98 Adblue Tank Temperature : Excess Temperature
- Error Code 205C16 Adblue Tank Temperature : Short circuit to earth
The proper fix:
Replace Adblue Preheater assembly Part No. A1664710775
This includes the heating element, level sensor and temperature sensor.
Aftermarket Part costs AUD$1700
Wreckers selling used ones for AUD$1000
Did not enquiry with Dealer
Background information:
From research this issue is common in these Bosch units. There are detailed DIYs for VW Golf TDIs and BMW X5s, but not Mercedes ML250 which has slightly different wiring.
The temperature sensor (and level sensor) use resistance to feed information to the control module. This DIY can be applied to the Adblue level sensor issue which also seems to be quite common.
It seems that a 10K resistance is the normal temperature level. From my research, I was able to find this info.
Resistance value at a temperature of -10°C: 55 k
Resistance value at a temperature of 0°C: 32 k
Resistance value at a temperature of 10°C: 20 k
Resistance value at a temperature of 20°C: 12.5 k
Resistance value at a temperature of 25°C: 10 k
Resistance value at a temperature of 30°C: 8 k
Resistance value at a temperature of 40°C: 5 k
As I live in Australia where the temperature never will never freeze Adblue (-11 degrees), I figured that feeding a 10K resistance will solve the issue.
DIY - Sorry for the lack of photos, didn’t think about writing this up at the time. But was frustrated with the quoted cost of repair and thought the community could benefit from this.
Step 1
Remove the rear right side (driver side in Australia) wheel.
Step 2
Remove the wheel arch cover
Step 3
Remove the plastic cover under the Adblue tank
Step 4
Remove the two right side bolts holding the Adblue tank. This will allow the tank to drop a few centimeters to allow access to the harness. I didn’t remove the tank as I’d just filled it with 28 liters of Adblue.
Step 5
You can now see the Adblue tank harnesses. The two you need sit next to each other at the rear of the tank. One has 4 wires, the other has 3 wires. It is a little tight, but I managed to unclip the connectors from the tank. One clips by pulling to the right direction of the car, the bigger one unclips by pushing to the left direction. Disconnect the connectors.. you need the connections that connect directly to the heating element in the Adblue tank, not the connection that runs to the car.
** This image is of a BMW X5 harness. Note that on the W166, the black thin wire is missing, so the white thin wire needs to be connected to a resistor and jumpered onto the grey base wire. **
The temperature sensor wire is the white third thin wire on the three wire connector. (The other wires are thick). I measured 0 ohm resistance between this wire and the grey wire base on the other connector. This was the cause of the “short to ground” error.
Cut this white wire and connect a 10K resistor to the short wire from the connector. The other side of the resistor needs to be connected to the grey base wire on the other connector.
I tried soldering, but couldn’t maintain the connection after bending the wires, so I ended up using a screw terminal to connect the resistor to the wires, and heat shrinking, then some silicon to seal it against the elements.
After retesting the resistance and confirming that there was now 10K Ohms between the white temperature sensor wire and grey base wire, I reassembled everything.
The Check Engine Light went away after two restarts of the car.
If you’re getting the Level Sensor Error, you can use this method to connect 10K resistors to the shorted connections, but need to be sure to top up Adblue as you will lose the ability to monitor Adblue Levels.
If you live in a climate where it reaches -11 degrees, XModex say they are working on the kit for the W166.
Hope this helps people stuck with this issue, and don’t want to fork out $1.5k+ for the fix.
Total cost for this fix was under $10, and 45 minutes of labour.
Just follow the instructions from you and the problem solve. Thank you so.
Just done this job. I bring down the tank. May I know which wire suppose to connect with 10k ohm resistance for level sensor? Since I already bring down the tank, I would like to do it together. Thank you
Hi GL350,
Just a confirmation please For ML250 W166 with only temp sensor short circuit. Cut both thin white and base grey and jumper with 10k resistor OR should i keep base grey and just have thin white jumperd on it (T JOINT)??????
12-16-2018, 05:37 PM
