The Haynes manual mentions that before checking the timing, I need to short 2 pins on a service socket located under the dash on the passenger side. I wonder if this is really necessary and why? I checked with my timing light without doing this, with spark plug wire # 1 clamped to my timing light and the timing seems to be correctly set up as I could see it aligned with the red line on the crank pulley.

What exactly will this 'shorting pins' procedure do?

Thanks for any inputs....

 

It is necessary because it puts the ECU is limp mode where the timing stays at a constant and you can set the base timing. It makes it much more difficult to accurately set the timing without shorting the service connector, but still possible. It isn't much more work, so I would do it right the first time to be certain the timing is correct.