Testing an RJ45 Cat5 wire?

Joined
May 25, 2005
Messages
1,006
I got an RJ45 cable tester because my home network has been slowing down randomly (I've tried changing the router and connecting one thing at a time to see if anything specific causes it.. no dice).

The house had Cat5 cables put in ~15 years ago I believe (I may be off on that number), and I figured the cables may have degraded.

I tested an unused Cat5 cable I had lying around, all eight lights came on sequentially. Same with a Cat5e cable and a Cat6 cable I tested.

Then I tested the house cables - lights 4 and 5 don't come on at all.

I read that pin 4 and 5 on ethernet cables aren't used in Cat5, but how come another Cat5 cable I tested still had those lights go on?

Could cables be the source of my issue?
 
While it's true that wires 4-5 aren't used in 10/100-base-T, they would be used in a gigabit connection, so depending on what hardware you have at each end, they could cause trouble...... Even so, most gigabit gear will fall back to 100 fairly gracefully if the gigabit connection can't be established...

It's possible that the cable itself is bad, though it's more likely that there's a problem with the jacks themselves. (I'm assuming that the cables are terminated to jacks....) Sometimes the damage to a jack is pretty easy to see (look for bent pins or ones that don't spring back into place when the cable is removed). You might wish to try re-terminating the jacks and re-testing.

All that said, the basic cable tester you describe (with simple lights for each wire) is only going to show the most egregious problems (no connection). Even when a wire passes a wire-map test, there can still be problems that only a more capable tester will find.
 
I would suspect your networking hardware long before any cabling. Unless it has been abused, network cabling doesn't "degrade". I would double check the source of the information that is leading you to believe that the network is "slowing down randomly".
 
I would suspect your networking hardware long before any cabling. Unless it has been abused, network cabling doesn't "degrade". I would double check the source of the information that is leading you to believe that the network is "slowing down randomly".

I did experience similar problems when a family of mice decided to turn a 3" conduit with 14 runs of cat5e in it into their home. They chewed the insulation right down to the copper so the pairs were shorted. Network transfer speeds to my NAS were all over the place, hitting highs of 80MBps and lows of 12MBps. Drove me crazy until I found it last month when re-wiring my house.
 
While it's true that wires 4-5 aren't used in 10/100-base-T, they would be used in a gigabit connection, so depending on what hardware you have at each end, they could cause trouble...... Even so, most gigabit gear will fall back to 100 fairly gracefully if the gigabit connection can't be established...

All of my gear is gigabit capable.. so maybe that's an issue.. When you say 4-5 aren't used, how come a Cat5 cable I tested showed all lights sequentially, while the ones running through my house don't show 4 and 5?

I would suspect your networking hardware long before any cabling. Unless it has been abused, network cabling doesn't "degrade". I would double check the source of the information that is leading you to believe that the network is "slowing down randomly".

Well, that's the thing. I've tested a ton of possibilties by pulling out all devices and plugging them back in one by one. The only thing I can think of is that the cabling is screwed up.

Multiple times I've had what I assumed were router problems, since computers that were hardwired directly to the router (but from across the house through the walls with jacks) would lose connection to the router randomly, dropping packets.

But I've since tried multiple routers, and I'm still having issues.
 
All of my gear is gigabit capable.. so maybe that's an issue.. When you say 4-5 aren't used, how come a Cat5 cable I tested showed all lights sequentially, while the ones running through my house don't show 4 and 5?



Well, that's the thing. I've tested a ton of possibilties by pulling out all devices and plugging them back in one by one. The only thing I can think of is that the cabling is screwed up.

Multiple times I've had what I assumed were router problems, since computers that were hardwired directly to the router (but from across the house through the walls with jacks) would lose connection to the router randomly, dropping packets.

But I've since tried multiple routers, and I'm still having issues.

How are you testing the cabling inside your house? pins 4/5(the center pins) are wired for dial tone in houses prewired with cat5 usually.
 
I attached one side of the tester to a cable going into the wall in one room, then the other side of the tester to the cable that comes out of the ceiling in the storage room where the router is.

Also, ethernet was installed way after the phone system, so I don't think that would be the case.
 
When you say 4-5 aren't used, how come a Cat5 cable I tested showed all lights sequentially, while the ones running through my house don't show 4 and 5?

Wrong terminology ;)
When you're talking of CAT5, CAT5e, CAT6, etc, you're talking about the physical cable.
They have 4 pair, or 8 wires total.
The equipment is what determines which pins/wires will be used.

100BaseT (ie 100megabit network) does not use pins 4 and 5. Those are not needed for Fast Ethernet.
They are used for 1000BaseT (gigabit Ethernet.)
When testing with the cable tester, chances are it's only testing for physical connectivity between both ends.

Since pins 4 and 5 are not showing as connected, then either they are not punched down on one or both ends, or there's a break in the CAT5.

Regardless, missing those pins, you should still be able to get 100Mbit connectivity.
 
slowing down internet traffic? slowing down transfer between two pcs on a lan?

A cable tester won't tell you anything about the quality of the connection, it will only tell you if the cable itself is working or not. In other words it will let you that the cable is broken if there is no connectivity, but it isn't going to tell you that the cable is having issues with signal, noise, etc.
 
I'd try transferring files between the systems on each side of the cable, but if it is slowing down and speeding up, I think it's more likely that some system on the network is causing the issue unless you are rebooting. I've seen and heard of older routers slowing down, but I don't think they usually speed back up.
 
slowing down internet traffic? slowing down transfer between two pcs on a lan?

To be clear: A cable tester won't tell you anything about the quality of the connection, it will only tell you if the cable itself is working or not. In other words it will let you that the cable is broken if there is no connectivity, but it isn't going to tell you that the cable is having issues with signal, noise, etc.

Just to be clear: A simple cable tester like the OP described won't tell you anything about the quality of the connection... A professional (AKA expensive) tester will tell you all kinds of info about the quality of the connection (various types of cross-talk, noise, reflections, etc)... That high quality tester could also likely pinpoint the point where the cables lose continuity (TDR to the point of breakage).

Anyhow, to the OP: As I stated before, in a home environment, I'd still recommend looking at the jacks. Aside from animal damage and someone accidentally pulling wires off a punch block or patch panel, broken pins in cheap jacks aren't uncommon (in my experience).

It sounds like you do have some cable issues to address.. I hope once you get them sorted, that solves the issues
 
Back
Top