Measuring the resistance of large cables and connections using an ohmmeter is not a very accurate process. The resistance of the test leads is usually larger that the resistance of the item you are measuring. That adds a great deal of error to the results.
For that reason, using the voltage drop method is the preferred technique.
Voltage drop testing of wiring and grounds.
Use a Digital Volt Meter (DVM) to measure the voltage drop across a connection or wire. Adding length to the test leads may be required, and does not affect the accuracy of the test. Use 18-20 gauge wire for the test leads if you have to lengthen them.
1.) Most grounds use the negative battery post as their starting point.
2.) The voltage will be small if the connection or ground is good: less voltage drop = better connection.
3.) Be sure that the power to the circuit is on, and the circuit is being used in its normal manner. For instance, if it is a light circuit, the lights on that circuit should be powered on.
4.) To measure grounds, place one DVM lead on the battery negative post and the other on the wire or connector that goes to ground. Place the other DVM lead on the wire or connection you want to test.
5.) Read the voltage drop: Light duty circuits (1-5 amps) should show .1-.25 volts. Medium duty circuits (6-25 amps) should show .25-.5 volts. Heavy duty circuits (26 amps up) should show .5-.75 volt drop. A voltage drop lower that spec is always acceptable.
See http://www.fluke.com/application_notes/automotive/circuit.asp?AGID=1&SID=103 for help for help troubleshooting voltage drops across grounds.
