Ok

Having a litle trouble finding info on that chassis b/c it is MAS controller Cal version w/16 pin diagnostic box. Kinda rare combo.

But , I did find TN signal code and I would first check this sensor and the wires from/to it.. and take fuse #7 out and clean the ends and the holder.. [ or spin the fuse a few times in the holder]

http://catalog.peachparts.com/RenderScriptTemplate.epc?_cmd=epccat_VehicleAAA&cookieID=2GN17VPXQ2GN17XWT3&yearid=1990%40%401990&makeid=MB%40%40MB%40%40X&modelid=300%2DTE%2D001%40%40300TE+Wagon&catid=F%40%40Engine+Electrical&mode=PA&subcatid=F6010@@Reference+Sensor&source=www.peachparts.com&clientid=catalog.mercedesshop

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A Dalton