MacBook Air - Voltage at the power supply output connector pins

In all my years of servicing Apple laptops I have never needed to measure any voltages on the power adapters. Testing with a known good power adapter is easier. Make sure to disconnect the power adapter from the electrical outlet for a minute to reset the power adapter's internal circuitry since any power related issue could have placed the power adapter into a protected or even odd state. You are more likely to see visible issues with the connector or cable and even the Duckhead adapter than you will with the actual power brick actually failing.

If your MagSafe cable or power adapter isn't working - Apple Support

Pin 3 is a sense pin.

Pins 1 & 5 are ground

Pins 2 & 4 are the power pins

I would expect about 14.5V at most from the power adapter.

Keep in mind that Apple's Logic Boards must do some negotiating with the power adapter before any power is transferred from the power adapter to the laptop which is why it takes several seconds (sometimes longer) before the laptop begins to charge. Most power supplies these days need a load to function. I have no idea about the Magsafe since I've never tried to measure them.

Since you are posting in the MacBook Air forums, it is much more likely you have an issue with the internal I/O Cable. In fact most of the issues I have seen while repairing the older MBAirs usually involves liquid damage to the I/O Cable and the I/O Board.....sometimes even the Logic Board itself.

Gotlib wrote:

According to https://en.wikipedia.org/wiki/MagSafe there should be 6.86V there.

The laptop is in working order.

What could be wrong with the power supply?

I don't know since I've never tried to repair the power adapter. I never saw the need since I would never be able to give one back to a regular user since the power adapter would no longer be safe for them. Like I said previously, most of the power adapter issues I ever encountered were with bad wiring from users twisting & pulling the wires causing breaks at either the Magsafe connector or at the brick (some of it was a design flaw, Apple did replace some under a free repair program at one point....now expired).

How is it possible that there are 5 pins in the plug when there are

only two wires in the cable (photo attached)?

I don't know since I've never opened up the power adapter before. See the next item for a bit more information.

One wire for "+", one for "-", and how is the

data transmitted?

The answer is in the Pinout section of the Wikipedia article you linked....I'll reproduce the relevant section here:

From the "Pinout" section of the Wikipedia article: https://en.wikipedia.org/wiki/MagSafe

* The central pin is used for communication between the computer and the power adapter following the 1-Wire protocol. The computer makes use of this to retrieve information about the power adapter and to change the color of the LEDs on the power adapter's connector. The Apple MagSafe power adapter's 1-Wire communication chip is located inside the MagSafe connector itself; the cable does not carry the data line to the power adapter enclosure.[18]

[18]  Shirriff, Ken. "Teardown and exploration of Apple's Magsafe connector". Retrieved 3 December 2013.