This is supposed to be changed every time the chain is replaced/removed.

1) Why spend $90 for the whole tensioner when a $3 spring will do the job?

2) If a $3 spring will do the job, why would anyone NOT do this?

Found this info regarding the tensioner;

Engine Failure On 1978-89 Mercedes Benz 3.0L Diesel Engines

The American Educational Research Association (AERA) Technical Committee offers the following information regarding engine failure on 1978-89 Mercedes Benz 3.0L diesel engines. This failure has been reported on vehicles with high mileage or shortly after a partial engine repair.

The damage reported has ranged from lack of compression to catastrophic component failures. The cause of these failures has been related to a lack of tension on the engine's timing chain, allowing piston to valve contact. The reason for insufficient tension on the timing chain is a broken chain tensioner spring. The spring fatigues and breaks after the outside
diameter of the coil wire wears excessively.

The timing chain tensioner requires removal and re-installation anytime the cylinder head is removed. It is therefore advised to replace this spring, Part # A1109933901 anytime the tensioner is removed.

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83 300D 328k mi (623) Light Ivory - daily driver
82 300D 166k mi (881) Thisle Green - fixer upper
82 300D 282k mi (473) Champagne - parts

If the spring fails on the earlier non ratcheting tensioners I can see this. The ratcheting tensioner was introduced at some point and cannot back off the chain totally without taking the tensioner apart. It is always semi pretensioned by strong non reversing mechanical design.
Have you found the spring available separatly? At least wear of the springs circumferance as a cause sounds reasonable. The spring could be inspected for the wear before making a decision on the replacement. You posted really good helpful information for the members to think about. Good thinking about saving dollars as well if possible.

Logically, it appears that taking the tensioner apart, replacing the spring and resetting the ratcheting should do it. I was thinking this would be the normal procedure when rebuilding the motor or replacing the timing chain. One could go ahead a buy all new parts but sometimes that's not practical or even necessary.

Chain Tensioner Spring
http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=2A80PDXLE2A900LJ1Y&year=1983&make=MB&model=300-DT-001&category=A5&part=Chain+Tensioner+Spring

Timing Chain Tensioner
http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=2A80PDXLE2A900LJ1Y&year=1983&make=MB&model=300-DT-001&category=A5&part=Timing+Chain+Tensioner

__________________
83 300D 328k mi (623) Light Ivory - daily driver
82 300D 166k mi (881) Thisle Green - fixer upper
82 300D 282k mi (473) Champagne - parts