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HELLO ALL, JUST RECENTLY THE GLOW PLUG LIGHT ON MY 1979 300SD DOES NOT ILLUMINATE DURING REGULAR START UP ON COOLER MORNINGS. IT STILL CYCLES BECAUSE SHE STARTS THE SAME AS WHEN THE LIGHT DOES FUNCTION. AFTER THE VEHICLE IS WARMED UP THE GLOW PLUG LIGHT WORKS AS NORMAL. I HAVE CHECKED ALL CONNECTIONS. EVERYTHING SEEMS TO BE FINE. DOES THIS MEAN I NEED TO REPLACE THE GLOW PLUGS IF THE LIGHT FUNCTIONS ONLY WHEN THE CAR IS WARMED UP? THANKS FOR YOUR RESPONSES.

CLARK
 

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Clark, you may have one or more glow plugs that are not working. If you have not replaced the plugs in awhile, you may want to do that job. You might have a glow relay problem, though I would personally start with the plugs if you have not done them in awhile, particularly if you are starting in colder temps. I have done my plugs twice without removing the injectors. I have a 126 chassis and am not sure how tight of a squeeze it is on your chassis (I believe that you are the 123).

Definitely go with the Bosch plugs. You should be fine starting it throughout the summer, though you wanna take care of it before winter for sure. I have started mine in -23 C, roughly -10 F.
 

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Once you get the plugs out, bore out the carbon from the holes that the plugs came out of. You will notice a ton of carbon on your old plugs..I am pretty sure that this is what causes the plugs to be bad or go bad in the first place. I dunno what your chassis looks like in detail (my dad had a '78 SD), but the more stuff that you remove the more accessible it all is, the easier it is to take the plugs out, clean out the carbon, etc...
 
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