The salt shaker is by-passed with the knob in "starting position". If you see no voltage going to the ballast resistor when somebody is pulling the knob in "pre-heat" ...but voltage is present when knob in "starting",then the salt shaker is at fault(you can just by-pass it).Never by-pass the big ballast resistor on the firewall with the series type (old type)glow plugs.


There is no strip fuse on this model. Upgrade to fast-glow plugs is easy (get Bosch) .The ballast resistor now needs to be by-passes and the ground wire (from GP of first cylinder to the engine) removed. Wires need to be min.6mm2.
May want to install a small box with min.50A strip fuse before the newer type glow plugs.

Click on the link,open w115.
http://www.peachparts.com/shopforum/3803537-post1.html

..select 220D.Engine-->Elctrical System(pre-glow). Relevant to om621's too.