Trying to diagnose fault on CAT6 cable

[AndrewT1275]

I'm not great with techy data stuff but know how to troubleshoot electrics, but need some help from the PH experts with the following problem.

I've run a CAT cable from my downstairs router to the top floor in order to put in a hardwired wifi access point. This cable is terminated with an RJ45 at the top end and a keystone jack in the patch panel near the router.

When I test the cable with a cheap tester it shows a fault on 2 and 4. I replaced both the jack and RJ45 but got the same fault which suggested it is a cable fault.

So I disconnected the jack and twisted each pair of bare wires together (i.e. blue/white connected to blue etc) then put a multimeter across each pair of matching pins on the RJ45 to check for continuity. This worked fine for all the pairs and there was no continuity where there wasn't meant to be any. As a double check I then paired random wires (eg blue with green, blue/white with green/white etc) and did the same continuity check across the pins on the RJ45 with no issues.

This suggests that both the cable and the RJ45 connection are fine so it must be the jack. I tried another keystone jack at the bottom end but got the same fault on 2 and 4 again. It's too much of a coincidence that all 3 jacks are faulty with the same fault when I've not had any problems with the other jacks in that batch.

The tester works fine on other cables so it's not that either.

It's driving me crazy.

Is there anything I migh t have missed? Is it possible that the tester is finding a fault that a continuity check is missing? I assumed the tester was just doing a basic continuity test anyway.

Any suggestions as to what might be up?

[.:ian:.]

Theres two ways to wire the colours, A and B, they need to match at each end. The keystone is hopefully labelled up with both ways.

https://2020cadillac.com/568-b-wiring-diagram/

[AndrewT1275]

Thanks but it's not that.

They are both wired up in B configuration. A mismatch would just put the lights out of sync on the tester but I'm getting no lights on 2 or 4.

[Captain_Morgan]

First step might be to check which of these two cabling schemes you used.

If you didn’t then choose one and apply to both ends then retest.

[Captain_Morgan]

Okay if you have spare cable cut off the ends, at the far end link the pairs (brown to brown/white, green to green/white, etc) then use a multi meter on continuity and check brown to brown/white etc have continuity if so it’s a patching issue, if there’s no continuity it’s likely a cable fault.

[mattlovescars93]

That is strange, and would be too much of a coincidence for all 3 jacks to be faulty. Is there much play on the cable? For how cheap cat 6 cable is I’d be inclined to replace the cable if possible.

[Mandat]

Have you tested the tester?

Lights 2 & 4 might be faulty on the tester itself.

[Vsix and Vtec]

Also check the pins on the tester. I've had them stick in an "up" position and not make contact with the RJ45 plug tracks before.

[AndrewT1275]

Captain_Morgan said:

First step might be to check which of these two cabling schemes you used.

If you didn’t then choose one and apply to both ends then retest.

Done that, as above. If it was a mismatch of cabling scemes then all the lights would come on but in the wrong order. It's not that.

[AndrewT1275]

mattlovescars93 said:

That is strange, and would be too much of a coincidence for all 3 jacks to be faulty. Is there much play on the cable? For how cheap cat 6 cable is I’d be inclined to replace the cable if possible.

Unfortunately replacing the cable is not an easy option.

[AndrewT1275]

Mandat said:

Have you tested the tester?

Lights 2 & 4 might be faulty on the tester itself.

Tester works fine on other cables.

[AndrewT1275]

Captain_Morgan said:

Okay if you have spare cable cut off the ends, at the far end link the pairs (brown to brown/white, green to green/white, etc) then use a multi meter on continuity and check brown to brown/white etc have continuity if so it’s a patching issue, if there’s no continuity it’s likely a cable fault.

As above, I've done that and it is fine as there is continuity. But when testing the patching of the RJs in isolation it's fine too. So the only problem could be the jacks but I've got the same fault on 3 different ones now which suggests it's not that either.

I'm stumped.

[AndrewT1275]

Vsix and Vtec said:

Also check the pins on the tester. I've had them stick in an "up" position and not make contact with the RJ45 plug tracks before.

Hmmm. I'll give this a go in the morning. It's unlikely though as I keep switching between known good circuits and this dodgy one just to eliminate that kind of problem. The tester continues to work fine on all other circuits.

[AndrewT1275]

Thanks to everyone who replied above. Still no further along though.

It's very confusing.

[dxg]

Any kinks in the cable that might have damaged the cable?

Cut a little too deep when stripping the sheath and accidentally cut those two lines at one end - or line 2 at one end and line 4 at the other? That can be hard to spot.

[AndrewT1275]

If that was the case then the multimeter continuity test would have failed wouldn't it?

[Mr Pointy]

Is it screened cable? If it is, have you checked for faults between cores & screen?

[Pearcyy]

If you want to check the cable properly if not already done so twist all cores together on one side and check for continuity between each core on the other side, then separate and check they are open between all the cores also.

If the cable tests ok it could be a bad batch of RJ45’s i’ve had it before a few times. Try buying some from somewhere else.

Have you checked if the cable actually works as intended?

[OldGermanHeaps]

Are you crimping on rj45s or going into jacks with a punchdown tool? I have worn out a few rj45 crimpers in my time and they show up as open circuit faults.

[redstar1]

Keystone Jack is 6 as well? You'd be surprised.