Check quality of wiring to relay and grounds, especially engine ground. All glow plugs should test res< 1 ohm. Also, see if current is actually going through relay. If supply wiring or ground is not near perfect, you'll get a lot less current to those glow plugs. I would not suspect your relays so easily.

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'82 300SD - 361K mi - "Blue"

"Good judgement comes from experience. Experience comes from bad judgement."

listen, look, .........and duck.