I think it's called a "pre-glow time delay relay" or something. I almost bought one for my 1983 SD which I am 98% sure is the same as your 1981 SD.

I think the dealer wanted something like $155 for the part back in 1999, so the $120 sounds like the right ballpark. The cheaper one probably is for a 240D possibly.

I had my indy put in a used one last year; he only charged me $30, and I've been glowing happily ever since. Hope this helps, Bob

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Bob Roe
Lehigh Valley PA USA
1973 Olds 88, 1972 MB 280SE, 1978 Datsun 280Z, 1971 Ford T-Bird, 1972 Olds 88, 1983 Nissan Sentra, 1985 Sentra, 1973 230.6, 1990 Acura Integra, 1991 Volvo 940GLE wagon, 1983 300SD, 1984 300SD, 1995 Subaru Legacy L wagon, 2002 Mountaineer, 1991 300TE wagon, 2008 Murano, 2007 R320CDI 4Matic 52K, some Hyundai, 2008 BMW 535xi wagon, all gone... currently
2007 Honda Odyssey Touring, 2014 E350 4matic