Auto Trans. "limp mode" and relays

[Doug Yoder]

So, the starter wouldn't engage on my '91 100 the other day (turn the key,
fuel pump runs, but no cranking of the engine).
I did the tests in the Bentley (for power at the solenoid) and found out
that the starter solenoid wasn't getting +12 when I turned the key (it was
getting the +12 supply voltage, but not the signal telling it to turn on).
I jurry-rigged a line to give the starter power when I turn on the
ignition, and was able to start the car, but discovered the transmission
was in limp mode.
After pouring over the wiring diagrams for several days, I think I finally
understand them and have determined that my Automatic Transmission Relay
is bad (supply power to it, it doesn't click, and the terminals that are
supposed to be connected still show infinite resistance on an ohmeter).
So, while I am tracking down a spare, I have some questions: what can
cause a relay to go bad? do I run the risk of blowing the new relay when I
put it in?  There don't seem to be any problems with the power getting to
the relay (ie, it's not 20 volts or anything like that).

Thanks,
-Doug