Have a few outlets that test as a slightly dim open ground

[swampwiz]

I've got a METERK MK16US outlet tester with the 3 lights like any other tester. There are 3 outlets that I have tried that are important to me to have a good ground since I am running some pinball machines, and I have done some requiring with switches (I think I've done it right, but you never know).

Anyway, for one of the outlets, the tester shows a very bright pair of green lights indicating CORRECT. However, for a pair of other outlets, which are not on the same breaker, and with nothing else plugged in, I get the proper pair of green lights for CORRECT, but the light that is the determinant of OPEN GROUND vs. CORRECT (i.e., if this light is on, that it's correct, but if it is off, it's OPEN GROUND) is neither as strong as the other light for the tester, nor when compared to that mentioned above. I wouldn't say it is dim, but it's definitely not as a brilliant as the other light.

So what's the deal here?

 

[Tom.G]

Believe the tester.

The most likely is an open, or high resistance, Ground and there is other equipment connected to the same breaker that is supplying a sneak path good enough for partial illumination of that tester lamp.

If possible, disconnect, (just turning them off is not enough) everything else on that circuit and test again. If the problem clears up, then there may be TWO problems:
(1) a faulty Ground connection
and
(2) another device on the circuit in questionable condition with a sneak path

Be sure the breaker is Off before getting fingers in the way!
Then it is a matter of physically checking every connection on that circuit for loose or wrong connections. Pay attention to the wire colors too, if they are not consistent they are a problem. Check every outlet box and junction box on that circuit and inside the Breaker panel.
If you are not sure what you are doing, call in a professional and let them do the dangerous part. A lost finger is worth more than a service call.

Cheers,
Tom

 

[swampwiz]

For one of the outlets, the only other stuff that is on that circuit is a light switch (it was off when the outlet was tested).

 

[sophiecentaur]

a light switch (it was off when the outlet was tested)

Light circuits (single pole switches) are often dodgier and could have caused problem is it was not actually disconnected (L and N conductors).