My car has been starting hard so checked the glow plugs at the relay connection using a test light hooked to the battery. 4 of the pins lit up the light brightly, one was dimly lit. I assume that the dim lit one needs replacement..correct? I know its a stupid question..just want to make sure before I start taking things apart.

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'85 300td Frybrid WVO Kit

... are you putting the test light in series with the GP?
I'd think the bright lights are dead plugs, and the dim one is working...

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

Yes, the test light was in series with the GP. Anyone else have any thoughts?

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'85 300td Frybrid WVO Kit

The brightness of your test lamp light would vary inversely with GP resistance. Dim lamp, high resistance, bad GP.

It's the other way around. Think of it this way. A high resitance glow plug (bad one) in series with a light bulb will provide less current to the bulb thus it will be dimmer than a low resistance glow plug (good one), which will provide more current to the bulb making it brighter..

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

I agree. A test light will tell you that a GP is bad if the light doesn't light, but a positive with a test light only indicates that a GP might be good. The best way is to measure current draw. I learned this when I had a Ford with a 7.3 IDI engine. The GP system had to be perfect for cold weather starts. I did find plugs that would light a test light but would barely glow when tested outside the engine. I also learned (the hard way) to use OEM plugs from Ford. I believe they were made by Beru.

Thanks for the Info. I'll replace the one gp that produced the dim light and see how she starts on a cold morning and then go from there.

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'85 300td Frybrid WVO Kit

Glow plugs link thread All diesel models
Glow plugs link thread - PeachParts Mercedes ShopForum



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I understand it after reading the instruction above. I was thinking the light was grounded, and measuring the power to the pins...
in series, of course lower resistance shows brighter. dim or not lit at all, bad.
gotcha. same as measuring ohms resistance.

I like pulling them and watching them glow. or simple ohm test.
basically, if the car has trouble starting... all the glow plugs come out, and new ones go in.

__________________
John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

I've done the ohms check and had them fail.I use
jumper cables to check them.

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1999 w140, quit voting to old, and to old to fight, a god damned veteran

I you measured 99 amps inrush of 5 glow plugs in parallel, I'd say it's ok. If you measured 99 amps with one glow plug, something is grossly wrong with the meter.

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

If the light was grounded, all you're doing is checking for voltage at the glow plugs, it does nothing as far as telling you if a particular glow plug is good or bad. Pulling the glow plugs and powering them up is the best way but not necessary in 99.9 % of the time. I'd pull them as a last resort, and only if, after taking current measurements and all glow plugs measured ok, the car still won't start (and only if the fuel system is known to be good).

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

If one GP has failed, the others wont be far off failing as well. Best remove the lot, do a valve adjustment while they are out & replace all the GP's. That way you wont get stuck mid winter having to replace GP's. Keep any old ones that are still glowing properly as spares if you must.
Just remember its a long time till the warm weather of next summer !!!

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Probably not the situation.

Glow plugs appear to be similar to bearings. The bell curve from the one's that go first to the one's that go last seems to span a factor of five or more.

I keep track of the individual failures. I've changed 1,2,3, and 5 on the SD over the course of eight years. #4 is still operating fine.

The reason for the wide variation in durability is clearly the result of an additional unknown variable, or manufacturing tolerances.

I'd never replace "all of them on principle" unless we're talking about the 603 and the manifold is off. That makes for a different conclusion.

Glow plug is an electrical device and do not behave the same as a mechanical device in the real world. I'd never replace them as a set. I can remove glow plugs 1 through 4 in less than 2 minutes each in winter time so no big deal.

In summer time 1 minute each.

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked