I hope there is a solar guru out there to answer my question.

I removed the solar panel from my uhaul because it was putting out only three volts no load. Upon examination I found a loose connection in the box under the panel. Once I did that, it put out 18 volts or so. Good news so far.

I then hooked it up to the controller and when I did I got almost zero volts out of it.

Then I hooked it directly to the battery and read 13+ volts.

Now I am questioning if the controller is bad ( shorted ) or---- when the battery is in the charged condition does the controller regulate its output by sinking the solar output?

Speak to me o-mighty guru!

A solar panel is a fairly simple device really.
They either work or they do not.
Individual cells can weaken and load down the collective output but you are describing the voltage as where it should be.

You can connect a meter directly to the panel indoors and then go cell by cell with a flashlight and determine roughly which cells work and which ones may be weak.

I think though that if you are getting 18v out of the panel it sounds OK.

You didn't mention how big a panel it is and what exact control you have?
Some controls will shunt the panel output to avoid overcharging and some are very inexpensive and would be easy to replace.

If you connected the panel to the control but then no battery to the control it may not output voltage either without an output load on it.

Ed
It sounds to me that you may have found the problem though already?

I bought an inexpensive digital meter just to use with the solar panel but it only reads the output voltage for an instant and then shows mothing. I now use an old analog meter that I had and have no problem since.