How to simulate simple on/off, Single-Pole-Single-Throw switch?
I get the error:


  Error on line 3 :
    s1 1 4 sw on
  Unable to find definition of model  - default assumed


The definition of your switch is incorrect.
It should be of the type SXXXXXXX N+ N- NC+ NC- MODEL &lt;ON&gt;&lt;OFF&gt;.
You should:  


define the controls NC+ and NC-
define the model (which has the name reserved name SW in your case) by using either .model mySwitch CSW(...) or .model mySwitch CSW(...)
not using the name SW as it is reserved as said above (I think that's why it crashes). 


So, using the fictional nodes 98 and 99 that control the switch:
s1 1 4 98 99 mySwitch on
.model mySwitch SW vt=1 vh=0.2 ron=10u roff=1G

Do read the manual, section 3.2.14 Switches and 3.2.15 Switch Model (SW/CSW).



In addition to Sorenp's answer, you can also implement the switch by making the value of Rwire1 conditional:
Rwire1 6 5 R={if(time&lt;2,1G,if(time&gt;3,1G,10u))}
Between time is 2 seconds and 3 seconds, Rwire1 has its original value. Before and beyond these time stemps the resistance is 1&nbsp;G&Omega;.