# Manually test computer Ethernet port with a multimeter

I have updated the drivers for the ethernet port an a laptop at home, yet it is only letting 10 mbps through for every cable that is connected, even though Speed/Duplex was set to Dual 1 Gb. I now suspect that the port may have dead pins, so I wanted to see if I can test the pins. If possible, I would like to test with equipment that I already have, such as a multimeter. How can I go about testing this?

Edit: I should quickly mention that all cables were tested and were working fine with other laptops

Edit 2: The Ethernet device I have on the laptop is the Realtek GBE Family Doing some more extensive checks, the Realtek Ethernet Diagnostic utility reports that each pair has a "normal" connection when a cable is plugged in, however pair 4-5 reports "short" when no cable is plugged in, while the other 3 pairs report that they are "open". Manual continuity testing shows that each pair does have continuity between the two pins of each.

@jstola The cables that I have been working with were observed to have download speeds greater than 20 mpbs on various alternate devices. I also attempted to connect the laptop directly to the router with another cable (which was labeled cat 5e and worked the right speed on another device) and there was no such luck there.

No matter what setting Speed & Duplex is set to, it seems to fall back to 10 Mbps (as reported in link speed - under the networking tab in Task Manager)

• Needs special test equipment, a multimeter isn't going to cut it. Jul 1, 2018 at 1:39
• are you sure that it is the laptop that is causing the issue? .... have you tried another computer? Jul 1, 2018 at 2:08
• @pericynthion can you be a little more specific? what "test equipment" is required? I would prefer to wait for a full answer rather than a quick comment that does not properly answer my question. Jul 1, 2018 at 2:31
• @micsthepick, your question does not contain enough information about your lan connections .... please add information about what you are connecting the laptop to .... all cables were tested ... how were the cables tested? Jul 1, 2018 at 3:50
• Full duplex often fails try Half Duplex Auto then test bandwidth or view status Jul 1, 2018 at 5:18

You can do some tests with a multimeter but you can't fully test functionality.

simulate this circuit – Schematic created using CircuitLab

A correctly working gigabit ethernet socket will show continuity (low resistance) betweens pins 1 and 2, between pins 3 and 6 , between pins 4 and 5 and between pins 7 and 8, it will show no other connections, including from pins to ground.

The easiest way to test an ehternet socket is to connect it to another known good ethernet socket using a known good cable, and then check the link lights at both ends.

• This is a good answer. Unfortunately testing in this way has not revealed any issue (however I am unsure about what it means when Realtek's Ethernet Diagnostic Utility says "short" on the status of pair 4-5 when there is no cable connected) Jul 3, 2018 at 7:12
• that diagnostic could indicate damage to the transformer or the circuitry behind the transformer on that pair, or it could be bent pins in the socket, but I guess you checked for that already. Jul 4, 2018 at 10:53

the Realtek Ethernet Diagnostic utility reports that each pair has a "normal" connection when a cable is plugged in, however pair 4-5 reports "short" when no cable is plugged in, while the other 3 pairs report that they are "open".

Ethernet uses transformers, here is an example image of an fast ethernet port (100Mb) implementation. Gigabit has two more transformers.

(source)

If the diagnostic utility reports a short on pair 4-5, and there is no visible damage to the socket, then perhaps the transformer is damaged.
This is often embedded into the socket and not user replacable.

Would ethernet still work without that pair, and would it be forced to use a low speed?

Ethernet has Autonegotiation, it will aim high and detect if a link can be established. Downgrading if the link if bad or unstable. So yes, if there still are some pairs, it could still run 100 Mb or 10 Mb.

• would ethernet still work without that pair, and would it be forced to use a low speed? Jul 3, 2018 at 8:11
• without pair 4,5 you still have compatability with 10/100 like that in the image. Jul 4, 2018 at 10:55