When i start my 87 300 TDT first thing in morning i have noticed that if i turn the key and let the glowplug light go out, turn the key back to off and then repeat the glow, and then start the engine after the double glow, it will run a lot smoother right off the bat, almost perfect immediately, and idle quiets to perfect within a 2-3 seconds.

if i start the car right after the glow plug light goes out the first time, without repeating, the idle is much rougher for about 10 seconds, then it cleans up.

i have not tested the glow plugs yet, but does this empirical evidence mean i need to replace glow plugs?

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punkinfair

Please understand that the light is simply an idiot light. It does not reflect whether the glow plugs are on or off.

When you turn the key to position II, the glow plugs are on for about 40 seconds, or, until you turn the key to position III (start), whichever comes first.

So, if you wait 10 seconds, the plugs will be on for 10 seconds.

If it's very cold and you wait for 30 seconds, the plugs will be on for 30 seconds.

Pay no attention to that light. It's got nothing to do with whether the plugs are on.

For the smoothest idle on the 603, you definitely need to wait 10-15 seconds in warm weather, and longer in colder weather.

The "double glow" procedure is not required.

Got me on need to change plugs out. If glow time on cold engine is within specs then i believe you describe a normal occurance on most diesels. They start and run better with two cycle inputs in general or at least all mine do. Only if I feel two cycles are required do I do this though. Like when it's very cold out. Just to make sure you are not covering up for one bad plug though do the resistance check on each plug through the wiring harness. To be fair though I do not own a 603 engine type.

I don't use a block heater, so I cycle the plugs twice on the first start of the day. Seems to help, even with good plugs. I think the block heater would be nice, especially for you folks farther away from our nice, warm ocean currents! It rarely gets below freezing here in Portland....

Jay.

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On the road, currently:
'83 300D (German spec.) 168k mi. - - Wolfgang (tucked snugly away for the winter!)
'87 300D 375k mi. - finally went over to "The Dark Side"
'87 300TD 225k mi.
Cannondale 66cm CAAD5 w/Campy/Jamis Aurora
Hujsak Custom Fixie/Landshark Track Shark Custom/Ahearne Custom 29'er

Just like Brian said; 10-15 sec's in warm weather, about 20 sec's in 30-40 deg. temps, and about 30 sec's in colder temps for my 603.

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Paul 1987 300 SDL; 2000 ML; '69 MGB; '68 VW Fastback

Brian, I thought I remembered in my OM603 (1986 SDL) that the GPs stay energized with the key turned to position III and remain on during cranking, only shutting off after the key is back to postion II (run) to allow the GPs to continue to assist during cranking.

I also remember the Owner's Manual being specific about starting in one continuous 'cranking' and not using short cycles like a gasser, to allow this continued glow to assist during cranking.

That said, I also remember that on some models the GPs continue to glow for a given time even after the key returns to postion II (after-glow and 'violet wire' subjects) to allow smoother warmup and emissions control.

Of course, I''' bet that there are a lot of model-specific variations...

It was my understanding that the plugs shutdown immediately upon turning to position III. The violet wire gets the signal from the keyswitch and the relay shuts down. If the plugs remained on in position III and shut down in position II, how would the key switch be able to determine this, without some electronics?

The switch would need to remain on in position II on the initial glow, remain on in position III and then shutdown in position II when the key is released. This would be a benefit on start, for sure, but I don't see how the key switch could effect it.

Anybody know what the guage of these wires is? I replaced one (black and blue going to the first plug) with a 12-guage wire, but I'm wondering if that is sufficient. Works okay, so far, but maybe I'm buying time...

Pete.

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Don't know it offhand..........but I would certainly agree that 12 ga. is sufficient.

There is/was a kit to adapt the 90s 'afterglow' system to earlier parallel glow systems. This lets the glow plugs continue to heat the prechamber for the first few seconds the engine is running. It greatly improves engine stability and running after a cold start at the expense of the mileage life of the glow plugs. But if you're double glowing anyway...

Sixto
87 300SDL

The diagram on my CD-ROM, P-107, PREGLOW SYSTEM (as of MY 1986) shows a 'Run-Start Input' (Terminal 1) and 'Cranking Input' (Terminal 2) to the relay.

These are noted as pins on an X2 four-pole connector, with Terminal 3 to the Glow Lamp, and Terminal 4 is a controls ground.

So, the GP relay appears to get separate RUN and Cranking signals. The M-B engineers do not elaborate what is inside the 'black box' but there could well be internal logic that keeps the GPs energized during cranking and drops the relay when cranking stops.

This could be easy to test with a voltmeter during cranking.

The other connections to the relay are:

A Red wire (Terminal 30) supplies power at all times, which is routed through a fuse and relay contact to the individual GP leads via the X1 6-pole connector.

A single white wire supplies a signal from a coolant temperature switch. but this also appears to go to the glow lamp indicator.

There is noted an internal air temperature resistance function.

I always crank mine after the light goes off and she starts right up. On real cold days I will count to 10 after the light goes out. If I didn't plug it in and it was below zero the night before it will run a little rough for a few seconds but that usualy clears out quickly.

I think the glow plugs stay on for 20-30 seconds after the light goes off. An afterglow system would be cool but I really don't see the need for it.

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1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200

brian,

thanks, that is a good, clear reply.

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punkinfair

With the bosch duraterm plugs they come up to max temp within 4-5 seconds, so waiting longer would only be benefit because they will radiate heat all over the prechamber is my guess. When its super cold I usually wait about 30-35 secs, and it starts right up on a couple cranks.

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'13 ML350 Bluetec - 95k - dad's (OC-98k)
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Yes, the relay gets separate run and cranking signals. But, if the plugs are to remain on during cranking, and then shut down during run, there has to be some really sophisticated logic in the relay. When you turn the key to position II (run position), the plugs come on. If you turn the key to position III and the plugs remain on (per your scenario), how would the relay know to shut the plugs down when the key is returned to position II??

I suppose that it's possible that the relay will trigger when it gets the start signal..........then it will shut down after the trigger when it gets the run signal. It would be quite a complex relay to accomplish this.

I think I'll do a little check the next time I start the 603 and see what the scoop is.