Volkswagen Workshop Service and Repair Manuals

Note:

Check establishes whether Mono Motronic control unit (J257) recognises the engine operating modes (idling, part throttle, acceleration/full throttle enrichment, full throttle and overrun).

Special tools, testers and auxiliary items

• ◆ Fault reader V.A.G 1551 or vehicle system tester V.A.G 1552 with cable V.A.G 1551/3

Test conditions

• ● Coolant temperature at least 80 °C

Test sequence

• ‒ Connect fault reader V.A.G 1551 (V.A.G 1552) and select engine electronics control unit (address word 01); carry out selection at idling.
  

• ‒ Input 08 for "Read measured value block" function and confirm with Q.

• ‒ Enter 04 for "Display group number" and confirm with Q.

• ‒ Observe display zone 4 -8 digit number block-:

Display in operating mode

• ◆ Idling:

• 2nd position from right must display 1
       Display: 00000010

• ◆ Part throttle:

• ‒ Rev up evenly.

• 3rd position from right must display 1
       Display:: 00000100

• ◆ Acceleration/full throttle enrichment:

• ‒ Quickly open throttle valve slightly.

• 5th position from right must display 1
       Display: 00010100

• ◆ Full throttle:

• ‒ Floor accelerator pedal and release.

• 4th position from left must briefly display 1
       Display: 00011000

• ◆ Overrun mode:

• ‒ Raise engine speed to above 3000 rpm.
• ‒ Close throttle abruptly.

• As long as the engine speed is above 1500 rpm 1st position from left must display 1
       Display: 00000011 (briefly)

Note:

Below 1500 rpm idling speed is again recognised.

• ‒ Press ⇒ key.
• ‒ Input 06 for "End output" and confirm with Q.

If the specifications are not attained:

• ‒ Interrogate fault memory. Eliminate faults if necessary => Page 01-6 .
• ‒ Check throttle valve potentiometer
  => Page 24-45 .
• ‒ Check idling switch => Page 24-37 .