1984 MB 300D - Pre-ignition rely/coil some time fails to activate

What is your question?

The pre-ignition rely/coil that sends the spark to the glow plugs fail to activate some times but it is totally random. Is there some way to activate it manually and/or reset it so it will work normally? Seem that if you can get the car started [in warm weather] then the rely starts working again by itself. But car won't start in cold weather without it!!! Help Please email address davidjsloan2@yahoo.com