My glow plugs have decided to not light until I crank the car over. The light does not come on and the plugs to not receive power (checked with a test light) when the key is in the normal glow position. This has happened once before, but the weather was warm enough to start anyway and it has worked fine ever since then.

What could be the problem? Faulty switch in the key?

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1979 240D- 316K miles - VGT Turbo, Intercooler, Stick Shift, Many Other Mods - Daily Driver

1982 300SD - 232K miles - Wife's Daily Driver

1986 560SL - Wife's red speed machine

Well I got it started (probably because the plugs were glowing while I was cranking. Drove around, shut it off, and tried the plugs again. Now they work fine. Just like last time.

I would still like to know what could be causing this.

Thanks.

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1979 240D- 316K miles - VGT Turbo, Intercooler, Stick Shift, Many Other Mods - Daily Driver

1982 300SD - 232K miles - Wife's Daily Driver

1986 560SL - Wife's red speed machine

A few different things could cause this....The relay has been known to get cracked solder joints on the board...that can be resoldered ....
Of course connections along the feed line from the battery all the way to the plugs....including the fuse...should be cleaned and tightened.....Also the grounds....
Is this the series or parallel system..?

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1978 Yellow 300D (The Mustard Toad)
1980 Blue 240D (The Iron Toad)
1989 Grey Mitsu.4WD Mighty Max Pickup (Needs a Diesel transplant bad)

(Open the pod bay doors HAL)

It is a series system that has been converted to parallel using diesel giant's method. http://www.dieselgiant.com/mercedesdieselloopglowplugrepair.htm

I checked for power on both sides of the strip fuse and there was none. For some reason either the relay was not putting out power to the plugs or the switch was not sending power to the relay.

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1979 240D- 316K miles - VGT Turbo, Intercooler, Stick Shift, Many Other Mods - Daily Driver

1982 300SD - 232K miles - Wife's Daily Driver

1986 560SL - Wife's red speed machine

If the glow plugs work properly on occasion, the relay is most likely not the culprit.

A worn ignition switch can certainly pose the symptoms that you have.

Setup the test light and cycle the key five or six times. See if it fails to glow the plugs on one or more than one cycle.

I think you have a relay under the steering column in an aluminum box....I have a 78 300D...That relay has trouble keeping up with the new plug current....I bypassed mine with a manual starter relay mounted on the fender and push button switch inside the cabin....
If the relay you have is the one under the steering column..you can take it out and look for the cracks on the circuit board if you still want to use it.....patch it with a solder iron...
I also ran seperate wires to the Gplugs for less resistive drop along the wire.....I can send you a pic of the relay if you need to be sure of the correct one...

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1978 Yellow 300D (The Mustard Toad)
1980 Blue 240D (The Iron Toad)
1989 Grey Mitsu.4WD Mighty Max Pickup (Needs a Diesel transplant bad)

(Open the pod bay doors HAL)

That is the relay I have. Silver and under the dash. I'll try to pick up another at a junk yard and see if it fixes anything. If not I'll do the pushbutton rig.

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1979 240D- 316K miles - VGT Turbo, Intercooler, Stick Shift, Many Other Mods - Daily Driver

1982 300SD - 232K miles - Wife's Daily Driver

1986 560SL - Wife's red speed machine

Brians post about the switch could also be it....That is why I just changed out the whole thing....Nothing better than holding that switch in and hearing my 125 amp relay go (Thunk)!....I know they are glowing then..

If you open up your relay you will see the little cracks that can be patched up...just bend the crimped edge out enough to slide the board out...look on the solder pad side and touch them up a bit...I started troubleshooting with the relay because is easier to get to first...

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1978 Yellow 300D (The Mustard Toad)
1980 Blue 240D (The Iron Toad)
1989 Grey Mitsu.4WD Mighty Max Pickup (Needs a Diesel transplant bad)

(Open the pod bay doors HAL)