Your model does not have a separate or modular tach amp as older models do - it is in the EDS - electronic diesel system.
The ELR is the trimmer for idle speed control - an assortment of metal film resistors in a 7 position adjustable control by the fuse box.
Position four I think is the normal setting.

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1986 300SDL 201K Daily Driver
1984 300D In Progress
1989 240GL 196K Swedish Brick
1984 300SD -- Sold 289K
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