Old geek encounters a basic resistance testing conundrum

[PCGUY]

Kind of embarrassed to ask this question, but I've never seen this before and it happened on two different wiring harness plugs. I was trying to see if the expected resistance between two given conductors was present or not.

Having some difficulties with the refrigerator/main board (pic attached) or the thermistors or the fresh food damper.  Was trying to determine where the fault lies.  Won't go into all the details, but I wanted to test the thermistors and the damper control conductors and both plugs failed to produce any continuity.

In the sample photo below where you see the test points A, B & C, consider that A is the common conductor to B & C (themistors).  B & C should have some kind of resistance, the fridge is pretty much working sans a deep freeze on the freezer side and the display panel registers temps that are in keeping with a manual/digital meter.

I should be able to test A to B and/or A to C (plug disconnected from main board) by probing the contacts you see in the photo, but all I get is an open line.  Tried to check the test points on a couple of them by probing the corresponding conductors from both the wire side on the left and the pin side on the right.  Results were intermittent.

I can see where that could possibly happen on a single conductor, but this issue spans 9 conductors on two plugs.

FTR, the Fluke meter tests properly when I short the probes and when I measure the resistance between my hands.

Never seen this before and am looking for some ideas from some of you geniuses...

TIA, Nick

[jwet]

Trying to understand- do you mean going across these terminals "in circuit" or "in circuit with power on". 

In circuit with power off should give you something reasonable but the polarity of your ohms measurments might matter- there is like an active circuit on B or C with that doesn't conduct in both directions.

Measuring ohms in an active circuit is useless, that sensor is part of a voltage divider or a bridge and putting an ohm meter (current source and volt meter) will give non sense answers.  Almost anything depending on how the meter and the DUT interact.

Unplugging the connector should give reasonable answers.

Is this what you're after?

[Poroit]

G'day PCGUY,

With the connector removed, use your multimeter to check continuity of the internals of the connector pin to the outside point of the same pin where you took your measurement.

I have seen some terminals and connectors that have isolated parts.

[PCGUY]

The plugs are unplugged and the power is off.  The resistance/load is on the other end of the harness and should be present.  The conundrum is that I can not use the external access points (A, B & C) to take a reading; the last time I performed this same test what I am describing worked and now it doesn't.

Worse, I can not get a continuity reading from any one of the access points to either the wire on the left or the pin on the right.

FTR, I'm a long time computer geek; networks, board level fixes, programming in multiple languages and otherwise a jack of all trades/and, as the saying goes, master of none.

This just has me stumped and it's the reason I registered on this forum.

I'm going to set up this test again in an hour or so.  I'll let you know what I find, even if, as I suspect, I've made some false assumptions.

Many thanks, Nick

[Conrad Hoffman]

I've often had trouble measuring those Molex type connector pins. My probe looks like it should hit the contacts, but it doesn't. I've gone so far as to make up meter leads with sharper narrower points that can get by the connector housing, or probe from the top. The only other advice I can offer is to break things down into simpler measurements, even stupidly so, say from the under side of the PCB to the top of the connector. Once you start to measure something, it should be easier to see if there's a measurement problem or a component problems. Also, watch out for dried flux insulating your measurements. And remember, we've all been there!

[PCGUY]

I think we're going to chalk this up to "stuff happens".  Reran the test and got the same results; no continuity between the pins (20ga wires inserted) and the external access point.  I was able to find the resistance reading I was after by measuring the resistances via the 20ga wires.

If I had read the above message re some sort of insulation on the access points I would have tested with another meter with sharp points.  Thanks for that.

Sorry for the intrusion, but this has been confounding and probably something I'll have trouble forgetting.

I do have a photo of the 20ga wires inserted in one of the plugs in question if anyone is still confused...

Thanks again, Nick

[tooki]

I've often had trouble measuring those Molex type connector pins. My probe looks like it should hit the contacts, but it doesn't. I've gone so far as to make up meter leads with sharper narrower points that can get by the connector housing, or probe from the top. The only other advice I can offer is to break things down into simpler measurements, even stupidly so, say from the under side of the PCB to the top of the connector. Once you start to measure something, it should be easier to see if there's a measurement problem or a component problems. Also, watch out for dried flux insulating your measurements. And remember, we've all been there!

Yep, exactly this. To reliably make contact, you need needle-point probes. The beveled points on standard probes aren't pointy enough to reliably make contact through the windows. (I won’t call them “access points” because that isn’t their function; they’re simply for the retention barbs on the contacts to grab into to keep the contact in the housing.)