Resistance to ground increase when key is on

zakimodo said
I came across this by accident while checking the resistance of some new sending units. What I noticed is that when the key is on (Run), the measured resistance from the sender to the block was ~10 ohms less than the the resistance from the sender to the negative post of the battery.
Initially, I assumed there was a grounding issue between the engine and battery. After turning the ignition off, I started back at checking resistance from the block to various spots on the body and negative battery terminal. To my surprise, the measured resistance was now no more than a few 10ths of an ohm. I then turned the key back on to run and now have an additional 10 ohms of resistance. I tested a few other things and eventually decided to disconnect the voltage regulator. With the voltage regulator removed, the resistance measured between the block and negative post is now the same regardless of key position (this is repeatable).

The car has been in its current configuration for some time,I have not had any charging or other electrical issues. Should I just leave things as they are or is there something else I should be investigating?

It is not a normal test to check for resistance that way, through that path, so any results are unreliable because there is no specification and will vary with the quality of the meter used.

Any time you want to test a DC electrical circuit while it is in use, just do a voltage drop test. Basically checking voltage between two points. For checking grounds, select the DC voltage scale on the meter. Place black lead of meter on battery’s negative post. Use red positive lead to probe/touch different connections. It should always read less than 0.2 volts while operating. Any higher reading means there is resistance to current flow between the two points you are checking. So look for poor or loose connections, rusty or corroded connections, or missing or incorrectly positioned ground cables or wires.

Circuit being tested must be complete and in use when tested this way. Normal clean ground paths are usually lower than 0.2 volts.

Sorry for the long post.