Normally diesel vehicle is 24 V battery voltage. So here take 24V as an example, make a brief introduction about pin definition of Euro III and Euro IV vehicle engine diagnostic socket.
1.Euro III vehicle  
This vehicles usually adopts KWP2000 communication protocol, diagnostic socket is usually 4-pin.Convertion is ground wire, fire wire, K line 1,kK line 2.Use high resistance type digital multimeter under the condition of ignition key on.(Here will not introduce how to use multimeter),measure the voltage of 4 pins respectively, normally ground wire 0V,fire wire 24V,2 K lines are about 22V(little lower than voltage of fire wire).
Now connect one end of main testing cable to car diagnostic code reader, another end DB15 connect to jumper box. Select the correct jumper according to pin on vehicle, connect ground wire on vehicle to GND on jumper box, fire wire to VCC. 2 K line connect to 7# and 15# jack respectively. Now Power light on jumper box will be seen on, and then select the correct vehicle model menu in automotive diagnostic scanners to enter. If still unable to test, you can contact Overseas Dept. of our company for assisting test.
2.Euro IV model  
This model usually adotps CAN communication protocol (Not excluding some Euro IV vehicles that still adopts Euro III standard, Such as FUSO) Diagnostic socket is normally 4 pins: ground wire, fire wire, CAN_H, CAN_L, voltage measured is: 0V, 24V, 2.5+V, 2.5-V.The pin that little higher than 2.5V
Node relation of jumper box is:
1#:K line; 2#:K line;
3#:K line; GND:Ground wire(negative);
7#:RSR232+, J1708+, K line;8#:K line;
9#:K line; 10#:K line;
11#:K line; 12#:K line;
13#:K line; 14#:CAN‐L;
15#:RSR232‐, K line, J1708‐, L line
VCC: power line(Positive); POWER:Power indicator.
Remark: If lack of vehicle electrical equipment knowledge or unable to confirm electrical equipment power supply, please don’t operate jumper, for error operation of jumper may lead to malfunction of electrical failure or FCAR F3-D main unit and wire.   
(Auto diagnostic socket related information reconfirmed as below)
● Digital high impedance multimeter (can not use Analog Multimeter)
● When ignition switch to ON Gear, there is supply voltage of vehicle on diagnostic socket. Voltage difference between voltage on diagnostic socket and vehicle battery side can not be above 2.5V.
In diagnosis, if the signal line used is single-line communication (K line), voltage on signal line lower than battery voltage,(if vehicle supply voltage is 12V,then voltage on signal line is 11V, + 0.25V;if 24V,signal voltage is 21V,+ 0.25V)
If double signal line adopts CAN communication, 2 diagnostic lines use multimeter to measure ground wire voltage under the condition that ignition switch open, the sum of both voltages is 5V,average 2.5V,the one voltage little higher than 2.5V is CAN-H, the one voltage lower than 2.5V is CAN-L.
After understanding of the above, then you can use car diagnostic scanners to read DTC. Now make a brief introduction of operation steps for FCAR auto scanner to read DTC: Confirm diagnostic socket connect properly and then turn on power switch of auto scanner, and select Diagnostic menu.
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