Thank you for your reply.
My glow plug relay wasn't going on when turning ignition key to pre-glow position. I swap the one from my MB 300SD 1982 and it works OK.
I opened the one that wasn't working and after a good check up on it I found one broken leg of the relay coil at the soldering point on PCB.
After repair soldering I try again and it works fine.
Only one difference that I felt was that after the relay turn off (30 seconds), if turn the key off and on again before two minutes, the relay will not click.
After two or three minutes it will work normally.
The relay of my 1982 does not work the same way.
Is this symptom correct?
Thanks.
Nyck

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Nyck - TX - USA