1980 300D won't start - Rain got inside the glow plug relay
 

One night I had the battery on the charger and left the hood open. It rained heavily that night . Water must have gotten inside the glow plug relay because after the rain, starter would not turn and I could hear a buzzing whistling noise from under the hood. Upon investigation the noise came from a black box on the driver side close to the front of the car.
Question: Why does the starter care if glow plugs are on or off? Is there an interlock? Can I by pass or short the interlock?
Some years back I remember someone had a schematic using a Ford solenoid to replace the whole glow plug circuitry. There was no interlock and it was independent of the whole car circuitry. What am I missing? Thanks.

Ah yes , the joys of running 36 yr old electrical components in the rain.< Smile> Yes you can custom build a < Ford Starter Typical> circuit that will work well, and it is easy to do. However my friend I am a bit suspicious that what you are experiencing is road salt corrosion . You may have a corrosion issue on the starter post where the positive wire from the battery attaches.
I live in an area of Canada where this is not uncommon. The first common symptom on older vehicles here is things simply stop working. Or conversely work but poorly. Starters fail or crank slowly ETC.
My advice is this, Disconnect both your battery posts, clean the terminals and posts, then lightly grease them with dielectric grease. Leave disconnected. Remove the nut on your starter positive wire connection , Clean /grease reconnect. Follow the negative cable to its attachment point remove the bolt and clean/grease. Reconnect battery and see if symptoms remain If so the next place to look is a voltage test at your fuse box. If all is well the next known issue with most cars is a failing/ ignition key switch The quick way to test is to SAFELY use a mechanics remote starter switch between the starter solenoid and the battery. If the starter operates fine this way, its probably your key switch. I just finished assisting a neighbour with his dead 1983 Ford . He went through 3 starters in 10 days. In fact it was a bad body ground and an ignition switch issue. All 3 starters were fine. Now his tail lights are bright again and his alternator charges properly as well. The original starter works fine. Good luck Allan