Note, there is a pre-made adapter cable to connect a 20pin Irritrol RainDial timer into a 16pin back terminal board.  If you need one, let me know, I have several in stock.

To go the other way, using an older 16 pin Irritrol RainDial timer in a newer back terminal board, there’s a way to do it, but it’s sort of a hack job.   I’ll show  you the steps, but recommend you purchase an extension cable so that you can hack up the cable instead of damaging the back terminal board itself.  That way if you later decide to use it with a 20pin timer, you can just remove the extension.

Here’s a screenshot of an extension cable that should work; I found it on Amazon… you don’t need a very long one, the shorter the better.

You would do the following steps to the cable above, instead of the backboard.  Here are the steps.

The following picture shows where you need to snip out a wider slot, so that the 16pin ribbon connector can slide down into the backboard’s connector housing. See the red-X.  Just cut this out with a small wire cutters, or other tool.

You’ll then  have to bend or break the four pins shown with the X below.

Wondering about these pins?  They’re used  for the rain sensor connection that is  not supported by the older timers that have a 16pin connector.  You can still install a rain  sensor, but it’s wired in series with the sprinkler wires instead of having dedicated connector screw terminals.

Again, it’s better to modify the extension cable instead of the back terminal board.

When you insert the cable, be sure that it’s making contact with all the pins except the 4 that we’re not using.

That’s how  you connect an older 16pin  timer cable into  a newer 20pin connector back-terminal-board.