Pry out the back cover with a pry bar after heating it for a few minutes. Take it down.
Apply some protection stickers on the camera module.
Unbolt the screws for fixing the motherboard and take off the fixing cover. Disconnect the flashing light flex cables with this pry bar, the same operation goes to the battery cable, the display cable, the charging cable, the power cable, the earphone jack cable and the sensor cable. Take down the camera module and the motherboard.
Prepare a new screen assembly.
Install the motherboard. Connect the display cable, the charging cable, the power cable, the earphone jack cable and the sensor cable.
Turn it around, power on. Now we test its touching function. It turns out it won’t work. So we power off and try again, the problem still exists.
Disconnect the battery cable, the display cable, the earphone jack cable and the sensor cable. Take down the motherboard.
Observe the display connector and other connectors.
Now let’s see this brief diagram of Huawei mate20 touching connector’ pins
They are: pin 49, 47, 45, 43, 41, 39, 37, 35, 17. Their resistances are as demonstrated.
Next we are going to measure the resistances of each pin with a multimeter and see if there is any abnormality.
J1701 Pin 37, no reading, while its normal value is 450.
Move on, all performs normal.
Measure J1701 Pin 37 again, still 0. This circuit connects a resistor R1722.
Observe the pin, it has fallen out.
Now we measure the resistance of R1722 Pin1, 450V.
J1722 Pin2, 0, so R1722 is where the problem lies.
Put the motherboard onto the PCB holder. Apply some soldering paste on the bonding pad of R1722, grab a new resistor of the same size with tweezers and heat R1722 and weld it to the motherboard(330℃, wind 30).
After the motherboard cools down, measure the resistance of J1701 pin37 again. It’s 450V, pin2’s value is 450V as well.
Take down the motherboard, install it to a new screen. Connect the display cable, the charging cable, the camera module, the earphone jack cable, the sensor cable and the battery cable.
Power on, test the touching function. It works. Problem well solved.
Provided more info: