What am I doing wrong with this voltage regulator?

I have this to try to get a regulated -15V DC:

^{simulate this circuit – Schematic created using CircuitLab}

But my 7915 outputs a (positive) low voltage in the 1-3V region.

I've checked the input voltages and they are good. I've tried two 7915s and they operate the same way. I've tried a higher input voltage and get the same behavior.

Datasheet excerpt:

What am I doing wrong?

Update: pics (the 7915 is the reg on the right)

Answer

It's likely you're measuring the voltage between the yellow and blue wires, on the bottom right part of the board.

However, the closeup of your 7915 shows that the blue wire is the central terminal of the 7915, which, unlike 7815, it's the input and not the ground.

Therefore you're measuring \$V_{OUT}-V_{IN}\$, which is about \$-15\ V-(-18\ V) = 3\ V\$.

You should measure the voltage between the yellow (output) and the red wires (ground) that are connected to the 7915.