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My LAN cable seems to work (indicator LED lights up) but the computer can't find a connection. What's wrong?

Setup:

  1. I had to run a network cable from a router in one room to a computer to another room, through a hole in the wall that was too small to pass the RJ-45 plug through. The plug was cut off and the cable passed through the wall.
  2. Then a new plug was crimped on using this detailed explanation.
  3. The connection didn't work because the (factory-made!) plug in the other end used a non-standard wire order.
  4. I crimped a new plug on again, using the exact same order of the factory-made plug.
  5. The LED indicators*) lights up on both ends, but the computer cannot find a connection.

*)
Router only has 1 green indicator: ON means OK.
Computer has the usual 2 LEDs: 1 green (solid=link?) and 1 orange (blink=collision?).

What can be wrong? How can I find out?

I don't have a cable tester. By visually inspecting my new plug, I think it's good; the wire order matches the other end, and all wires are all the way inside the plug and reach the connector piece. I've used the cable before (with both factory-made ends) so i don't think that the cable itself has a defect.

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Can't you just use a 'multimeter' (not sure of the correct term) to check the resistance of each individual wire on the cable? –  Hello71 Jun 12 '10 at 1:29
    
I can't do that because the cable goes through a wall, so the wires of the multimeter would have to be 10 meters long.... –  Torben Gundtofte-Bruun Jun 12 '10 at 17:39

2 Answers 2

If the cable seems okay, I'd suggest rebooting your computer and unplugging the router for a few seconds and plugging it back in, if you haven't already done that.

Some motherboards have network diagnostics built into the BIOS. Sometimes the diagnostics are even available via a utility installed along with the driver software. The diagnostics should be able to tell you if a pair is crossed, or if one or more wires are not making contact. Usually they can even tell you roughly how long the cable is.

If all else fails, double-check the cable again. Are you sure you got the wires far enough in? You should be able to look through the very end of the plug and see the shiny tips of the conductors flush with the plastic. If they're not all flush with the end of the connector, I'd try crimping on a new end. Also, if you didn't crimp them hard enough, it's possible the RJ45 connector's didn't pierce the individual wires' insulators and make contact.

When I make a cable, I give myself a little extra wire to work with when stripping off the outer jacket. Then I line up all the wires straight, in the correct order, trim them (leaving about 1/8" extra), and push them into the RJ45 connector. After that, I pull them back out and trim them down to the right size before finally pushing them into the RJ45 connector one last time and crimping the connector.

For a short cable, you can often get by with any wire order (and you should at least get a link), as long as both ends are the same. But FYI, the link you provided doesn't make it clear that the green and green/white wires straddle the blue and blue/white wires. In fact, I think the page you linked is trying to illustrate the EIA/TIA-56A spec, which is only used on one end when you're using a crossover cable. You want to use the EIA/TIA-56B pinout on both ends.

As for the indicator lights on your Ethernet card, they could mean different things. Sometimes the amber light just means you're transmitting, not necessarily that there was a collision. If the amber light is on solid, it could also mean that your connection is either maxed out (e.g., 1000Mbps if you have a gigabit card) or your connection has fallen back to a slower speed (e.g., yo have a 1000Mbps device on one end but only a 100Mbps or 10Mbps device on the other end).

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The link describes (on its following pages) how to crimp correctly, except that in my case I'm not using the standard wire order but the order of the other end, to make two identical ends. Anyway, I crimped a new plug, and powercycled both the pc and the router. Still no fix... –  Torben Gundtofte-Bruun Mar 21 '10 at 11:13
up vote 1 down vote accepted

There must definitely be a problem with the cable, somewhere. Throw out the cable (don't know what part of the cable can be trusted) and get a new one, then carefully crimp both ends. And get a cable tester to check it.

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