With a cold engine on a warm day, how long should the glow plugs light up before fire-off? On my '85, the light light sup for maybe a second or two, and the car starts right up, no smoke or anything. Just seems a bit quick for me...is that normal for warm weather, or could a plug be dead or dying?

Again, just wondering...thanks!

Also, what's a good way to test the plugs? On my Ford, the way I learned was to use an ohmmeter and put one lead on the plug prong and the other on the body (specifically the part with the threads, that would normally gound the GP out to the block). More than 2 or 3 ohms and the plug's bad. Will this test work with these plugs? I'm assuming so, but figured I'd ask. Again, I'm not complaining about how it starts now (well, how it did before the battery bit the dust); just some preventative maintenance so I don't kill batteries/starters in the winter, if something is wrong.

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2001 VW Jetta TDI, 5 speed, daily driver
1991 Ford F-350, work in progress
1984 Ford F-250 4x4, 6.9l turbo diesel, 5 speed manual
Previous oilburners: 1980 IH Scout, 1984 E-350, 1985 M-B 300D, 1979 M-B 300SD, 1983 M-B 300D
Spark-free since 1999

warden,

On the MB diesel the light goes off in the cabin when the car can be started. The glow plugs continue to glow after the indicator light stops from 15 to 40 seconds depending on the temp. I used to get rough starts even in warm weather until I learned this fact. I thought that the glow plugs turned off when the cabin light turned off so I would turn it over once it would shut off. Now I just give it a good ten count even in the summer and it starts up great, no smoke or rough idle.

To check the glow plugs I am not sure. I guess that you could ohm test them but I would suggest just replacing them if you think that you are having any problems or live in a climate that can get cold. When ever I get a used diesel I always change out the glow plugs, They are pretty cheap.


Good luck

bob c

The GP are for one thing only: easy starts. So if your car starts easily after a short GP warm up and runs smoothly on all cylinders, that is the best test of GP you can get.

My one 300SD always started a little hard. Then I got another 300SD and I noticed the GP warm up time (indicator light on) took much longer but that car started easily. So the solution to getting the first car mentioned above to start easily is to wait about 10 seconds after the indiator light goes out to start the engine. Then it starts on the first crank or 2.

The GP system has a built in tester for the GP. If any of them go open, the indicator light will blink after the engine starts. If this dosesn't happen and your engine starts easily, forget about your GPs. They are not a maintenance item like spark plugs.

P E H

Okay, thanks...again, I was just wondering.

On my Ford, the GLOW PLUG light lights up whenever the plugs are activated (there's a controller that senses coolant temp; depending on how cold the engine is, it will cycle the plugs on and off for up to a minute after the truck fires off, to help curtail white smoke in the morning), so I had assumed the Benz was the same way (i.e. the light lights up whenever the plugs are on). That's what I get for assuming, eh? Speaking of which, I assume the plugs are 12 volt...? Especially with a duty cycle like that. Ford Mickey-Moused the plug setup to allow for faster starts by taking 6 volt plugs and applying 12 volts to 'em. If they're on longer than 8 seconds without a 10 or so second rest, they'll burn themselves out...sometimes breaking the tip off and forcing you to pull the cylinder head to get it out.

Again, I'm not new to diesels Just new to diesels as refined as these...I rememebr how amazed I was when I found out that these engines aren't governed somewhere in the 3500 RPM range. Due to the governor and low gearing, my pickup can't go faster than 75, and it's really complaining at 70...

My 84 Chevy pickup truck was the same way: 6 volt plugs and a controller. I think this was a way to get the GP hot quicker. The previous owner was too cheap to replace a bad controller and wired in a manual switch to replace the controller. I had trouble starting it one day and I burned out all the GP. I found that 12 volt plugs are available so I replaced the bad 6V GP with new 12V GP and haven't had any trouble since. I push the manual switch and wait 10 seconds and it starts almost instantly. I don't need GP at all when engine is warm.

On the older model MB, GP are turned off as soon as the starter is energized. I think some of the newer MB leave them on like your truck to make the engine run better when cold.

Yes, the new pin style GP are 12 volt plugs. The older loop style were 1.2 volts and were in series with themselves and resistors to drop the voltage. Its almost as if the plugs were made for a 6volt electrical system. I could never figure why MB didn't go to higher voltage GP because half the energy was lost in the resistors.

P E H

The duration the glowplug light is on dependant on engine temp. Now, even though the light goes out and you have started the engine, they are still on for up to about 1 min, again based on temp.
Checking the GPs is real easy. Get a basic meter, place the pos. lead on the GP it'self or, even easier, pull the GP connection off at the GP relay. There are 5 "sockets" in this connector. Place the positive lead in one and the neg. lead on a good ground, the battery is best. Good plugs should read no more than .5 ohms. Anything more and the glowplugs are on their way out. Obviously an open means the wire inside the GP tip has broken all together. The resistence gets higher as the GPs wear, until it becomes an open. GPs are very much like light bulbs, there is a "wire" inside that touches the metal tip, and like a light bulb, when it "burns out" like a light bulb's filament, it creates an open and no longer works. You could touch each GP tip to check resistence, but thery are harder to get at than using the connector in the GP relay.

__________________
1999 MB SL500 (110,000 mi)
2004 Volvo V70 2.5T (220,000 mi)
2014 Tesla Model S 85 (136,000 mi)
MBCA member

When I first bought my '85 300SD, it was hard to start so I suspected a bad GP. Here's my cheap and dirty test method:

Wait until the engine is cold, turn on the ignition and wait to hear the GP timer relay go "clunk", then do this again. After the second cycle, feel around the head by each GP, the one that feels cold is the bad one.

ICTAMOTS,

How do you turn on the "ignition" on a Diesel? Do you mean turn on the Key Switch?

That's a novel way to find the bad GP. Sometimes you can find a misfiring cylinder by feeling the exhaust manifold ports to see if one is cold when other ones are hot. This only works where there is separate exhaust port for each cylinder.

P E H

I suspected bad plugs or a bad relay when the light wouldn't come on when I would put it in the preglow position. I would have to go from off to preglow several times for it to come on, and when it would go out the car would start VERY hard and I had to almost always floor it when cranking in the winter. It would run like it had dead cylinders (which it did because I had 2 or 3 dead plugs, I had all of the replaced). Now it starts like a champ everytime. I am waiting for winter I want to see how these new plugs heat her up in the cold!

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1984 190D 2.2 Auto 220k
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Sorry for my colloquialism, but I think you get the idea. Also, I usuall say "I've got to stop and get some gas".