Electrical issues question

I realize that lots of theories may exist, and trouble shooting is a crap
shoot from afar but I am wondering, what would cause a full discharge
indication when I put my landing light on.

This started shortly b4 I had my radio put in, and when I flew it back from
the avionics shop, on the short flight, figured to see if it was a fluke or
was it real. So, this is not related to the radio installation.

Seems that when I put my avionic switch on, amps hold. If I put my nav
lights no strobe, slight discharge indication. Put my nave with strobes,
same load. Put my landing light on and amps show a full discharge.

Would it be my alternator can't take a full load?

I don't get a chance to try this in reverse as when I put the landing light
on without anything electrical on, it shows a full discharge.

The landing light circuit is a 30 amp circuit. Does that seem like a
rather large circuit for something as simple as a landing light?

Or would it be something silly with my electrical wiring? Battery is only
2 years old, starts with one or two turns of the prop, so can't imagine it
being the battery.

Hope to get a hold of my A&P on Tuesday to get this resolved, but hoping
for some wisdom from the group so I know what should be looked at.

Allen