The relay is on the driver side fender -- trace the wires from the glow plugs back to the black box about 4" square mounted toward the front. You can pull the cover off the relay and check the 80amp fuse 1st. But if you have 2 bad glow plugs you can get the same symptoms even with a good relay and relay fuse. Check the glow plugs, even though yours are relatively new. Quick and easy glow plug check: use 12v test light. Remove the relay cover. Attach aligator clip to the 80amp fuse link under the relay cover and then touch the probe to the nut on each glow plug end. The light in the test light should glow on each plug. It will not glow on any bad glow plug.

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1984 300SD 326,997 miles and counting . . . No wait, my odometer is still dead