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?

You may be leading yourself down a rose colored path. You did not specify exactly where and how you connected the meter. If you had the meter directly connected to a good ground point on the engine block, and you had the sender wire DISconnected, with the meter connected directly to the sender terminal, then it is almost impossible for what you claim to be happening In other words I'm thinking it may be your measurement connections/ methods at fault rather than some actual problem.

On the other hand, and certainly this can be a problem...........

Main starting battery ground must be to the block, and the block/ battery neg must ALSO be firmly grounded to body. This is minimal in many older, OEM cars. A better ground is to connect a "Ford style" starter --to--solenoid cable (eyelet to eyelet) perhaps a foot--foot and a half long to the rear of the driver side head, and to the firewall, such as master cylinder stud