I don't have much time on the web, or spare time, these days and decided to get to the point after a mild forum and internet search. I know the subject has been covered.

1) Replaced all 5 plugs 1 week ago.
2) Plug light (plugs) worked when it was cold outside. After running a little no light, but "easy" start as the engine was warm. After driving to work and sitting for 6 hours the light would not come on.
3) 1 week after new plugs the light only comes on after sitting over night and leaving for work.
4) If the glow plug relay is good, should I be getting voltage to the plugs even when the glow plug light is on?
5) I cleaned the 4 contacts in the relay but it made no difference.
6) 80A fuse in the relay is fine.

I am trying to isolate relay versus failed glow plugs. Again, not much time to do anything these days.

The Glow Plug Light Bulb in mine must have burned out but I always start OK so I just don't worry about it.

You did not give your Year and Model.

On 300Ds the older models have a Temp Sensor in the Cylinder Head and a wire to the Glow Plug Relay.
On the new ones up to 1985 there is a temp sensor built inside of the Glow Plug Relay so there is no need for on on the Cylinder Head and the Extra Wire.

In both cases as far as I know the Temp Sensor only controls the Glow Plug Light and has some troubleshooting ability by having the Glow Plug Light acting abnormally to idicate there is some issue with the Glow Plugs or the Relay.

My best guess is you have an issue with the Temp Sensor itself (or that circuit) or you really have some issue with the Glow Plugs or the Realy that has not gotten bad enough to effect your starts.

__________________
84 300D, 82 Volvo 244Gl Diesel

Does the cart start good in the cold?

__________________
What Would Rudolph Do?
1975 300D, 1975 240D, 1985 300SD, 1997 300D, 2005 E320 , 2006 Toyota Prius

84 300D with an 85 engine.

It starts and runs in the cold. I cycle starting x3. I mean, I let it crank for about 5 second, stop, crank for another five seconds, stop, do that a third time, and it starts every time.

When the light works, I cycle the plugs 2 times before trying to start. The, when I crank to start, it starts instantly. This when hot or cold.

I suspect your issue is either the relay or the 80 amp fuse.

Which brand glow plugs did you install? What is the resistance of each plug? Did you ream the carbon from the holes prior to installing new plugs?

Your starting method sounds very odd to me, unless we have a confusion of terms. "Crank" means engage the starter to make the engine turn over in my mind (key turned all the way to last position, which is #3). Do you have your owner's manual? Recommend you read the starting procedure and the cold starting procedure on page 48.

__________________
Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC

When the GP Light does not come on you might want to check and see if when you turn the Key you are getting Voltage to the#1 (Red Wire with White Stripe on my year and mode), Run Start Input.
That is coming from the Ignition Switch. If the GP Light is not on and you check and there is no Voltage at #1 I would take that as an indication that there is an issue with the Ignition Switch.

Of the next time the light does not go you can use a Test Lignt or a Voltmeter and determe if the Glow Plugs are getting Voltage. If they are it means like on mine despite the GP Light not working My Glow Plugs are actually working. (Note this is a rough test and having voltage does not tell the condition of the Glow Plugs.)

__________________
84 300D, 82 Volvo 244Gl Diesel

Didn't ream with plug install. It was quite tidy in there.

Cheap plugs out of Virginia.

100% of the time when the glow plug light is not on, I do not get voltage to the plugs. My guess is relay, but I don't want to spend time setting up the manual plug system with a starter solenoid if it is crappy plugs. I don't have time to waste.

Yesterday and as of today, the plugs have been working as they ought. Instant start with 1 cycle.

Yes, crank means engaging the start. Cycle, is to on position before crank.

I suspect you have a failed glow plug or two. I also think you should have reamed the carbon from the glow plug holes, excess carbon may have already shortened the life of the cheap glow plugs.

I'd start by replacing all the cheap plugs with the correct size Bosch plugs, and ream the holes prior to installing them.

__________________
Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC

Cheap plugs from anywhere is bad news.

__________________
83 SD

84 CD

I don't think anyone on the Forum wants to lead you down a path that will not solve your problem.

It is just that many parts of vehicles operate as a system and it is sometimes hard to tell which particular part or parts of the system are causing the issue without doing some systematic trouble shooting.

Trouble shooting is time consuming; especially if you have never done it before and don’t have the Tools or Devices that would commonly be used to do that.

Occasionally dumb luck comes along and the first thing you check or you simply observe something that is amiss but you can't count on that.

I was having intermittent Glow Plugs on mine. I looked during an extremely bright Sun light day and did not see the crack on My Strip Fuse on the Glow Plug Relay.

I saw the crack a couple of days later in the somewhat dark early morning when I had My Wife do the Preglowing while I watched and I saw a tiny spark from the Strip Fuse and took a really close look with a Flashlight. and saw it was cracked.

So I had bad dumb luck on the bright sunny day and good dumb luck on the dark morning.

So I wasted my time at first but the point is I was actually investing time and trouble shooting.

__________________
84 300D, 82 Volvo 244Gl Diesel

Thanks for the input fellas.

I did check the fuse, even removed it when I cleaned the contacts on the relay. I had it indoors, in my hand, up against light and it appears(ed) good.

In the end, I from here on out, not that there is anything left to diagnose in my car, anywhere, I will post a thread about my concern, even if I know what it is and how to fix it. Why?

Because the old boy must have found out I was talking about it. Since the first post, I haven't had 1 instance of glow plug system failure.

However, temperatures are up by 20 degrees. If the cold is messing with something, and does again, I'll replace the fuse just because.