Resistance to ground increase when key is on

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zakimodo

zakimodo

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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?
 
To clarify, the "additional" resistance is being measured between the block and battery not from the sensor its self.
 
Not knowing what yur working on, except maybe the 74 Duster, you do know that turning the ignition to run, connects the field windings in the alternator to B+, right?, and disconnecting the regulator opens the field windings to ground, right?
IDK if this has anything to do with your "issue" tho.
 
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?
Click to expand...
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
 
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?
Click to expand...
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.
 
Correct. The problem with a votlage drop on the sending unit is the current is very low.
Voltage drop only occurs when and where current is flowing through resistance.
 
Mattax said:
Correct. The problem with a votlage drop on the sending unit is the current is very low.
Voltage drop only occurs when and where current is flowing through resistance.
Click to expand...
I agree but if there was a main ground connection issue, he may see more than 0.2 volts at the sender. Not the senders fault, just a grounding issue, and the voltage will be from everything using that ground, not just the sending unit circuit. Less than 0.2 puts his mind at ease that no problem exists.
 
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