I had the same problem. I don't believe the relay in the W115 can handle the voltage of the bosch duraterm plugs...I had 3 plugs working in the block, but number 4 would always draw too much voltage. My father had an EE work the circuit out, and I really don't think the relay can support that amount of voltage reliably.

Regards,
Kyle

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1976 300D
190,000 Miles
Colorado Beige

1975 300D
Parts Car
78,000 Miles
Rustbucket
Also Colorado Beige

1984 190D 2.2 (Dad's)
156,000 miles
Champagne Metallic Clearcoat