Disappearing continuity from power pin to backshell

Thread Starter

bquad

Joined Oct 24, 2019
3
Testing a D38999 connector (big metal, mil-spec connector), I do a continuity check between a power pin and the backshell of the connector. There should be no conductivity (as the backshell is tied to EMI braid and grounded). When I check the resistance, there is about 300k ohms that goes up higher about a second later. If I recheck immediately, it's still high. If I wait about a minute, I get the 300k and the it goes up again. Clearly whatever it is, it builds up over time and then dissipates through the multimeter when connected. Static? Capacitance? Something wrong with the cable? The power is off. Anyone seen something like this?
 

Thread Starter

bquad

Joined Oct 24, 2019
3
I'm not touching anything. The cable has 8 connectors (all metal backshell's are connected to the EMI shield), so there's a chance another connector's backshell is touching metal somewhere.
I probably should clarify that the backshell/emi shield is normally tied to ground. As the cable is disconnected, it is not grounded. Could there be something wrong with my dvm (it's a fluke 77, but it's not been calibrated recently and it's been banged around)?
 

Tonyr1084

Joined Sep 24, 2015
4,705
I've seen a meter (not a Fluke meter) change its readings. A friend and I were looking to find what circuit was having a parasitic draw on the car battery. We'd check the voltage. While watching the voltage we'd pull one fuse out. If the voltage was unchanged we'd surmise it wasn't that circuit. When we found the right circuit the battery voltage should show an increase. Well, we thought we had found it. The battery voltage (engine off) began to rise when we pulled a fuse. So we marked that fuse and then put it back in. But the voltage didn't fall. In fact, after a few minutes the voltage was reading well over 14 volts. And with the engine NOT running - that shouldn't be happening. So I grabbed a different meter and sure enough the battery voltage was normal. The meter I was using at the time had a 9V battery in it. That battery apparently was going dead. That's what accounted for the change in reading.

Since you're measuring continuity, the meter is sending a small current out through the probes and measuring the returning result. Hence, if your battery is weak or dying it MIGHT show up on the display.

Two things you can try - - - Touch the meter leads together and hold them there for a few minutes. If the resistance starts changing then you've isolated the problem to your meter. The other thing(s) you can do is test (or replace) the battery in the meter.

That's all I got. Other than what others have said - parasitic capacitance. But if both ends of the cable are disconnected then you shouldn't see that at all. And as for static electricity - no, that wouldn't affect the readings.
 
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