I replaced my glowplug harness and glowplugs about 20K miles back. Just lately they don't come on until AFTER the motor is running. Is this a relay or fuse issue?

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1995 E300 diesel

You might have a timer in there which could be an issue.

606's have a small monitoring circuit that basically checks there is continuity on all six GP's.

It is most likely you have a failed plug or a socket has come off one of the plugs.

It is correct for the GP lamp to not illuminate during pre-glow if there is a failure. The GP lamp will come on once started to notify you of the problem for a short (maybe 3 minute) period.

Your other [working] glow plugs should still be operating: depending on your outside temp when starting, you may notice a shaky start for a minute and possibly with some white smoke. If no plugs were working and your outside temp is cold then it should be difficult to start.

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1995 E300 Diesel

Do you mean that the glow light on the dash comes on after the engine starts? What brand glow plugs did you use? Did you ream the holes with the appropriate reamer? How is it starting in colder weather? Clack, clack, clack, until all cylinders are hitting?

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2009 E320 Bluetec 117,000
1995 E300D 306,000 Sold
1996 Ford Taurus LX 130,000 Sold
1985 300TD Still 225,000 Sold
2016 Ford Fusion 24,900

One or more glow plugs are inoperable. Maybe from wiring or connector issues. Or possible bad glowplug(s). If you turn the key on without turning the motor over and let it set. The glowplug light will come on, but it takes about 1-2 minutes. This is to tell you that there is an issue in the glowplug circuit.

this is what is occuring, its been down to about 25-30F at night. Guess I need to check my plugs.....

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1995 E300 diesel

Yeah, that's a bummer especially because you've changed out the plugs already and more of a bummer because you have to pull the inlet manifold off to get to the front and rear plugs.

I think you can get at one or maybe even two plugs with just the cross-over pipe off and you might be lucky that it is one of the easy access ones at fault.

If it is a plug that failed so early then let us all know which brand you used.

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1995 E300 Diesel