Ethernet wall socket works only with powered LAN-switch

Question

First of all, please excuse my english, I might make some mistakes.

So I have this problem for a year now, I can't find a solution.

I have ROOM A, there is my main router. ROOM B is my study room. And there is ROOM C which is empty right now.

So I have a wall socket in every room, with 2 connectors. So every room has direct Cat 6A connection to both other rooms. It is like a triangle.

I want to connect my Desktop PC from ROOM B to my router in ROOM A.

CASE 1: The Ethernet cable from the rooter is connected to the wall socket in ROOM A. In ROOM B the PC is connected via ethernet cable to the wall socket. In this case the PC doesn't observe the cable, like there is absolutely no cable connected (red X on the Network icon).

CASE 2: ROOM A is like above. In ROOM B the cable from the wall socket is connected to a simple LAN-Switch which is powered through an AC-adapter. With another Ethernet cable I connect the switch to the PC. In this case everything seems to work fine.

CASE 3: The Ethernet cable from the router is connected to the wall socket which goes to ROOM C. In ROOM C I connect the socket from ROOM A to the socket from ROOM B with a short Ethernet Cable. In ROOM B the cable goes from the wall socket in the PC. In this case everything connects fine.

So my thought was that the connection between ROOM A and ROOM B is the problem. The connection works only through the AC-powered switch. I am not an expert on Networking, but it's strange that only the extra electricity is missing. With this extra electricity from the Switch the connection is working. Let me clear that I also tried other devices instead of my Desktop PC, with my laptop it's the same.

So yesterday I went to buy an Ethernet connection tester. This is a pair of devices which have LEDs on them. If the connection is OK the 8 LEDs light up from 1 to 8. Everything works fine! The connection is good. All 8 LEDs light up and in the same order on both devices. Only the LED G (shield) won't light up, so the connection is non-shielded. I wanted to test if that could be the problem so I went to test the connection between ROOM A and ROOM C (which I knew that it works perfectly). The LEDs light up the same way, no LED G, it is also non-shielded. So there is apparently no difference between the perfectly working connection from A to C and the faulty connection from A to B.

Anybody has an idea? If all the connections from 1 to 8 are working (or at least the same like in the other room) what could cause the problem?

Thank you in advance!

Answer 1

This might not solve your problem as I am unable to reason if there is some mechanism in play related to how Ethernet works. However, I had some years ago a similar problem with my Ethernet wall sockets.

Very brief: it was not crimped well. In my particular case I could connect, but was unable to get full duplex speed.

My solution was to open the wall sockets until I could reach the Ethernet connector itself (black box with golden contacts connected to the cable that went to the other room). With the backside of the point of a knife (the side that does not cut) I pushed very firmly on all contacts until I felt how some of them went a little deeper into the wires they were connected to. For this you may also use a small flat screwdriver.

I mounted everything back together and then I made sure that the contacts inside the connector (the ones that touch the cable that goes to my PC) were not bent upwards. These are like springs, and might make a bad contact with the cable if bent a little upwards. I pulled at them with a needle whose tip I had bent into a hook.

This solved my problem. No weird Ethernet voodoo nor problems with my switch. It was a simple bad electrical connection. Its been working now for several years flawlessly. Keep us posted if you solve it :)