300 SD Intermittent Glow Plugs
 

I have a 1985 300 SD that the glow plug light does not come on each time the key is turned on. The engine does not fire indicating it is just not an indicator problem. Is there a procdure to diagnose if this is a relay or switch issue?

The Chilton Manual has an illustrated procedure for checking glow plugs involving resistance measurement. These problems can be deceiving: One bad glow plug can cause the indicator not to light leading to the assumption the module is bad. In my case, a 1985 300TD, I found two bad glow plugs via the resistance test and not the module, which was my first assumption. The test is quite simple and requires an accurate ohmeter or multitester capable of reading resistance.
I would also check the wiring from one plug to another, since they seem to be wired like an old Christmas tree string. The problem is very nasty in cold weather when a bad plug can leave you stuck.