Continuity testing - Why does swapping DMM probes give differing results?
It sounds as though you are connecting your ohm-meter to a live circuit. This could destroy your meter.







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. Equivelant circuit of ohm-meter.

Most budget digital multimeters use a 0 - 199.9 mV (0.2 V) voltmeter circuit in them. All of the voltage and current ranges on the meter switch in and out resistors so that at full scale for that range (200 mV, 2 V, 20 V, 200 V) 200 mV will be presented to the internal voltmeter.

For resistance measurement a constant current is sent out on the red probe and when attached to the device to be tested this results in a voltage reading across the multimeter leads. In Figure 1 the 200 mV voltmeter is represented by VM1 and the resistance range selected is giving out 100 &mu;A. From Ohm's Law we can calculate that this will give us a reading of 200.0 mV at \$ R = \frac {V}{I} = \frac {0.2}{100\mu} = 2 \ \text k\Omega \$ and that any resistance greater than 2k (including open circuit) will give the over-range indication, usually 1---. You can work out the currents for the other ranges on your meter.

Now it should be clear that by connecting your probes to a live circuit you have (1) messed up the test current and (2) risked applying too high a voltage to the internal voltmeter.

If you have been using the diode test / continuity range the internal circuitry will be somewhat similar and you risk similar damage.

Resistance and continuity checks can only be made on dead circuits.