1987 300DT A few weeks ago, my preglow timer relay fuse went. No big deal, cheap fix that only requires a screwdriver and remembering to disconnect the battery first

The glow plugs, starter, and battery are all less than a year old (2007 was an expensive year). I've replaced the fuse, and it goes in 1-3 days after a new one is in. So, what's the troubleshooting process? What I understand of the system leaves me thinking it's either the relay itself or the wiring harness. Is there something else I'm missing and how could I test instead of just throwing parts at it?

Thanks in advance!

Welcome to the forum.

If the 80A fuse is burning up prematurely, it's typically a sign that too much current is going to the plugs. This can only happen if one of the plugs has a partial short to ground.

Do a resistance check on the plugs. Pull the connector on the side of the relay and use a proper meter that can read accurately below 1 ohm. The plugs should be all approx 1 ohm.........but, if one of them is dramatically lower than the others, you found your culprit.

You need an accurate meter that can go down to such a low range for this test..........or it's not valuable.

It's still not a firm conclusion because the glow plugs can have a different resistance when hot.