1985 Merc 300TD Wagon.

I could write a multi-volume history of me troubleshooting why I keep burning up alternators. I'm still not sure if it is crappy alternators or (da da DAH) the Bosch glow plug relay.

I just upgraded from O'Reilly's crappy "Ultima" (yeah whatever) 55A rebuilt to the crappy 65A rebuilt. And I'm sure a bunch of you will tell me that's the problem. And you might be right. But enough on that. Let's talk Bosch relay.

This character in question is the Bosch 0 281 003 096 glow plug relay. And my what a character.

I've found that unlike the glow plug relays in my other 2 W123s this Bosch relay stays on for awhile after the car starts. "Ah HAH!" I think. "This MUST" be the problem." But after a bit of googling I discover 2 forums threads (can't remember the links) that say this part number is a Bosch Duraterm relay which is supposed to stay on after the car starts and that this change is a design improvement. Ok whatever.

So I rewired the glow plug light in my car to show when the plugs are actually ON rather than showing the silly suggested-time-to-try-to-start the car. And I am finding that the plugs are on for not 1 minute, not 2 minutes, but almost 3 full minutes *after* the car starts, even though it's about 60*F out and the car started easily and smoothly after 5 seconds of glow plug on-ness.

Is this really how these Bosch Duraterm relays are supposed to work? And if so, is this sane? Or is mine broken?

I opened up the relay box and suspect that, by just adding a diode in the logic circuitry, it seems like it would be pretty easy to modify the circuit to shut off the plugs when the starter starts. That isn't what the relays in my other cars do. They shut off when the engine starts i.e. when the starter stops. But I'm tempted to try this easy mod anyway, although I thought I'd check in and get some advice first.

Advice?