The 1990 300d when cranking I do not have a glowplug light. After it cranks glowplug light stays on for a couple of minutes after car has cranked. or maybe a little longer. What is going on.

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Joseph Blackwell
1985 300SD
1990 300 2.5 Turbo

When 'cranking' do you mean before you turn the starter- i.e key in position 2 (pre glow)? Does the car start ok or roughly?
No light at all before start normally means 1 or more plus out or relay fault. Light coming on after start normally means 1 or more plugs faulty.
Check plug resistances at harness and if all ok check for 12v at relay. I think your relay has a strip fuse on top too.

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1993 320TE M104
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past:

1983 230E W123 M102
1994 E300D S124 OM606 (x2)
1967 250SE W108 M129
1972 280se 3.5 W108 M116
1980 280SE W116 M110
1980 350SE W116 M116
1992 300E W124 M103
1994 E280 W124 M104
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"music and women I cannot but give way to, whatever my business" -Pepys

It is pretty quick to check the Plugs with an Ohm Meter and a Meter from Harbor Freight is $5 or less when on sale.

I think if your Car has the Glow Plug Relay with the After Glow it is supposed to keep they Plugs on for a certain amount of time or temp. But, I am not well read on the details and don't know what years and models have that.

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84 300D, 82 Volvo 244Gl Diesel

Yes, the key on in the #2 position, no glowplug light, car cranks smoothly but after car has started and running the glowplug light stays on and does not go off. I understand the glowplug light not coming on when 1 is out but not understanding the light staying on after engine is running.

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Joseph Blackwell
1985 300SD
1990 300 2.5 Turbo

The light coming on after start is to tell you there's a fault in the system somewhere.

Have a look through this, it shows you how to troubleshoot all the components in turn:

Mercedes Diesel Glow Plug Repair

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1993 320TE M104
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past:

1983 230E W123 M102
1994 E300D S124 OM606 (x2)
1967 250SE W108 M129
1972 280se 3.5 W108 M116
1980 280SE W116 M110
1980 350SE W116 M116
1992 300E W124 M103
1994 E280 W124 M104
----------------------------------------------
"music and women I cannot but give way to, whatever my business" -Pepys

The glowplug light says on exactly 60 seconds after engine has cranked.

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Joseph Blackwell
1985 300SD
1990 300 2.5 Turbo

On all of the 124.128 Chassis
(300D in America,250DTurbo on the rest of the Planet)
OM602.962 Engines Is ELECTRONIC and has a "Sensing" feature that Protects
and "Trips" like a breaker if "Shorts" or "Dead Shorts" are sensed until the
the problem is rectified.SO NO GP 80Amp Aluminum Strip Fuse!!!

This relay is telling you that there's a problem with one or more GP s
(No Light,at "Glow" setting of Key)
AND Possibly a problem with the wiring of the Temp Sensor on the Block
that instructs the relay's afterglow function.[Mayhaps a bad sensor]
(Full 60 seconds of "Afterglow" after engine Ignition)


Always check your GP relay's Separate Ground!

Your GP Relay is doing it's assigned tasks...
AND also Informing you of Yours.

Attached Thumbnails

       

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'84 300SD sold
124.128

That is a sign of one or more plugs gone bad - its a very rudimentary check glow system lamp. I believe the lamp not illuminating at all on preglow position and then illuminating after start is an indication of a shorted glow plug, an open glow plug shows preglow lamp before start as normal but will illuminate the lamp after start too.

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)