If you've been having glow issues and just can't figure out what is wrong after testing everything and all seems good, try replacing your glow plug relay. I replaced all my plugs, redid a couple wires, proper voltage everywhere, good strip fuse, and it all seemed good, but just didn't work quite right. Symptoms were a slight stutter at start or difficult when very cold (running the full glow cycle a time or two), and the glow light would only go on for a couple tenths of a second.

This morning it was 20 something degrees out (maybe single digits overnight), no block heater, turned the key after the glow light turned off (what a feeling seeing that thing actually work for once) and it started right up. Purred like a big cat.

Oh the glory!

Oh yeah, and it's an afterglow relay.

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1983 300SD