Here's the deal--my friend has a 1991 W126 350SDL. As can be expected, the original 3.5 liter OM603 engine failed, so it was replaced with a 3.0 liter OM603 engine out of a 1987 W124 300D.

Most of the glow plugs weren't working, so I replaced all of them with new Bosch ones of the same length and part number. Even though the original plugs that came with the engine weren't afterglow rated, all new ones supposedly are.

But, the plugs were working intermittently and the relay was making noise, getting hot, and melted a bit. So, the relay is bad.

The previous owner removed the original afterglow-type relay from the W126 and installed the older-style relay from the W124 that doesn't have afterglow. This was a smart move since the old plugs weren't afterglow rated.

But, now since the car has afterglow rated plugs installed, I figure why not put the original afterglow-type relay in? But, I ran into a problem; the temperature sending unit on the head has only one prong, not two (the second one is for the afterglow feature). The glow plug harness was also taken from the W124, so it only has provisions for the one prong sending unit.

So, in order to restore the afterglow, I have to replace the sending unit in the head and replace the glow plug harness with the type that takes two prongs. But, this is going to cost a good bit of money and 350SDLs are never in the wrecking yards here.

My questions are:

1. Can I just put an afterglow-type relay back in the W126 as intended and have it not damage anything, even though the sending unit on the head and the glow plug wire harness are missing the afterglow wire provisions?

2. Will the relay still work, but I just won't have afterglow until I replace the sending unit and wire harness later?

3. Or would it be better to use an older style (W124) relay until I get the parts replaced?

The reason I ask is because my roommate has to buy a new relay and we don't know what type to get. I am thinking the afterglow type as original to the W126 is the best to use (it just won't afterglow), and then the sending unit and glow plug harness can be replaced at a later date to make the afterglow operational.

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1967 W110 Universal Wagon, Euro, Turbo Diesel, Tail Fins, 4 Speed Manual Column Shift, A/C
1980 W116 300SD Turbo Diesel, DB479 Walnut Brown, Sunroof, Highly Optioned, 350,000+ Miles