Power and Ground Distribution: Testing and Inspection

Measuring the voltage drop over signal ground

When fault-tracing wiring where a fault is suspected to be due to a ground terminal, check as follows.







Equipment:
Measurement tool FLUKE 88, 951 0060 (or similar).

WARNING: Do not measure the voltage drop of a ground terminal for SRS. The cable normally has no voltage. No procedures may be carried out other than the instructions given in the Service Manual or in VADIS.

Method for checking the ground terminal
- Check which power consuming components are connected to the ground terminal
- Switch on the ignition
- Activate as many of the connected power consuming products as possible
- Connect the test probes. Read off the value

NOTE:
- The test probes must be sharp to obtain perfect contact.
- The whole cable terminal including the cable cores should be tested.
- If the cable cores are not accessible, the test probes should be positioned as illustrated, A.

Permitted voltage drop
A voltage drop of maximum 5 mV is permitted on ground points for motors.







A voltage drop of maximum 20 mV is permitted on other ground points in the car.

Corrective action for excessive voltage drop
Excessive voltage drop can be due to insufficient crimping in the cable terminal or oxidation.

If the voltage drop is excessive, remove the ground screw. Clean oxidation and dirt off the ground terminal using an emery cloth.

Check that the cable terminal is intact. Check that the cable is correctly crimped in the cable terminal.

Secure the cable terminal using a new ground terminal.

Torque for ground screws:

Cylinder block M8: 22 Nm.

Bodywork, M6: 10 Nm.

Carry out the test again. Check that the value lies within the permitted range.

NOTE: When the test value is correct, rustproof the ground terminal using rustproofing agent, P/N 1161422-9.