I'm wondering if my glow plugs are bad. I'm wondering if the glow plug indicator light will com on as long as the glow plug timer relay is working properly, even if the glow plugs are not?

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1984 300D

The light is prevented from illuminating when there is a current imbalance. If no glow plugs are working, there will be no imbalance.

Fantastic. Thank you!

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1984 300D

The glow plug indicator light will work untill two or more glow plugs are bad. If one is bad the indicator light will go out noticably quicker when starting the car. Untill it won't come on at all if two or more plugs are bad.

How the glowlight responds depends on the type of plugs and relay. Loop plug light will not work at all when one plug goes out. Early pencil plug relay responds with a flashing light after the car starts if there is a problem with the plugs (at least my 79 W116 does). I believe later relays respond differently.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

The 617.95 FSM does not entirely support either of those notions. Where do you get your information?

My experience is that a failed #1 GP will cause the light to not light. Also, any other two failed GPs will cause the light to not light. I can usually tell when I have one bad GP because it will miss for a few seconds after a cold start, even if the light is acting normal. I have never experienced the light going out quickly due to a bad GP.

Can any more back this up? I am gong through some glow plug issues right now - light does not light up (not the bulb), opened up and cleaned the relay, all plugs getting power. Tested brand new plugs and replaced all 5 about 5 months ago. What are the chances of two failing? It certainly seems like two or 3 are failing from my awful starts lately...maybe I touched the tips with my fingers and got oil on them? Anything like headlight bulbs??

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TC
Current stable:
- 2004 Mazda RALLYWANKEL
- 2007 Saturn sky redline
- 2004 Explorer...under surgery.

Past: 135i, GTI, 300E, 300SD, 300SD, Stealth

Test the resistance of the GPs from the connector on the relay. When they are cold, the resistance should be about 0.8 ohm to ground. A burned out GP will usually have infinite resistance. Touching them will not hurt them. What brand did you install?

Yeah, 2 bad ones can make a 5 cylinder run very rough till it starts firing on all cylinders.

No they are nothing like headlight bulbs. They get diesel oil on them all the time with no ill effects. In fact, that's the way I used to tell which ones were bad on my VW's. After discovering that the engine won't start even after lots of cranking, i'd let the glow pugs cycle a couple more times with no cranking, then pull them. On a VW, at least, the ones that weren't getting current would look darker and wet from unburned diesel.

No silly OHM meters for me.

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1979 240D, 4spd manual, Power Sunroof, manual windows, 147k miles, Pastel gray/Black MB Tex.
1991 300D 2.5 - Smokes like it's on Crack!

The light does not operate on the basis of "all plugs getting power." It operates on the basis of the the level of current passing through the plugs. There is a relay in the timer which monitors the balance of current through plug #1 versus the remaining plugs (collectively.) If the balance is as "expected," the light illuminates. If not, it doesn't. Because the same timer is used in 4 and 5 cylinder engines, a five cylinder engine may require two plug failures (unless #1 fails) in order to trip the relay.

20% less than the chance on one failing. But a statistical analysis probably won't be very helpful with your situation.

I've found that the #1 plug being bad or two others being bad = glow light not working. This has happened on a couple of our cars.

Mine will miss for a few seconds if a plug is out, its pretty obvious when it happens. My current plugs have been in for over 3 years now....no dead ones yet!

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-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)

FWIW, I was getting a similar miss due to a couple of worn injectors. The cold start symptoms were very similar to having a dead GP, missing followed by some nailing for a few seconds.

I have read thru the thread and have come to these conclusions. If the glow plug light does NOT come on I have at least one bad glow plug. If the light comes one but goes Off quickly i have at least one bad glow plug. My light quit coming on at all. Should i just replace all 6? I have no idea of the age of them.

PS I wait to start until I here the relay click off,

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86 300SDL. 250,xxx on #14 Head. One eye always on temp gauge.. Cruising towards 300K

My glow plug indicator light was working fine but my car was taking 50 cranks to start in freezing weather. I figured my GP's were good since the light was glowing.

I tested the GP's use the directions on dieselgiant
Mercedes Diesel Glow Plug Repair

I had 3 bad GP's. #2,3,5

It only takes a few minutes to test with a multimeter so I would ignore what you think and just test.

Any idea what my indicator light were come on with 3 bad GP's?

Thanks

pete