In an effort to save some money I pulled some good looking used glow plugs out of another similar 300D yesterday. Stupid question but 1) if I don't touch them, 2) clean them off, I should be able to give them 12V and a ground and see if they work, right? Do glow plugs either "work" or "not work" or is there an inbetween. I have been having a hell of a time trying to get it to start, but I think my oil is too thick for the cold weather right now, I am using Delo 15W-40 and it is usually about 15-20 in the morning but I was starting to have trouble starting on 10W-30 as well...

There's lots of information on this here on the forum. Try searching on "glow plug testing".

I tested mine by clipping the threaded part (part that screws into the head) to the neg terminal with a jumper cable, and very quickly touching the end of them to the positive terminal of the battery. Good GP's should glow within seconds, dont leave connected for more than 6-7 seconds. Also be sure to be very careful. They should glow orange hot.

Then, when I had the injectors out, to check out the prechambers, I turn the key to glow position to test each one. You will be able to turn the key, run out and check, it will glow orange hot.

Also, If they work, they are fine, there is usually no in between.

I always use an ohmmeter to locate suspects in place. That saves having to remove them to find the non working plugs. Then I clamp them into the vise and ground the vise with one cable and touch the positive cable to the glow plug end. If it is a working plug it will glow right off. I just like to make sure before spending money to replace anything.
You could burn them up there, but they get the same treatment in the car, so might as well find out if they are going to go right off. They do get very hot very fast.

If I'm doing any sort of test like this, I'll always make some effort (as in the post from Old Deis) to NOT make the last connection on top of the battery. If there's going to be any arcin' and sparkin' (and there will be here), there are better places to do it.

Good point to make....Hydrogen Sulfide gas is VERY explosive.

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Proud owner of ....
1971 280SE W108
1979 300SD W116
1983 300D W123
1975 Ironhead Sportster chopper
1987 GMC 3/4 ton 4X4 Diesel
1989 Honda Civic (Heavily modified)
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Section 609 MVAC Certified
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"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche

There is a LITTLE bit of difference between totally dead and totally good with glowplugs. I have had a couple that tested several ohms. Not long after, they went open. If you have a halfway decent ohmmeter, test for resistance and put in a group that is somewhat evenly matched. For best results, put the median one in #1 cylinder. That's because the crazy warning circuit for glow plug failures compares the #1 current flow to the average of the rest.

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'82 300SD - 361K mi - "Blue"

"Good judgement comes from experience. Experience comes from bad judgement."

listen, look, .........and duck.

Well, I'm just thinking about hydrogen gas here, but yes, explosion hazard is what I had in mind.

I remember when I was watching my dad jump start a car once, and I told him that one was supposed to make the last connection somewhere away from the battery. He thought that was just about the craziest thing he'd ever heard of. Mind you, he is not a stupid guy - this was in late middle age, and he'd probably jump started more vehicles in his life than I ever will - but he'd never heard that before, and thought it was just silly.

He later had a small battery explode on his riding lawn mower (while charging, with nobody present at the time). I think that turned him into a believer.

Maybe I've misunderstood something here, but it sounds as though the opening post regards plugs that are used, but are not installed in a car.

Yes, one can check the resistance with the glow plugs installed. Yes, one can check the resistance of the harness. These are both fine things to do with the plugs installed.

If he has plugs just sitting there, though, and he wants to test them (which seems to me to be the gist of his post), then I don't see why he wouldn't take the advice to test them "for real" by glowing them briefly as others have suggested. Personally, I'd still check each for resistance before even bothering to do anything else.

Note that Old Deis isn't advocating removal of the glow plugs willy-nilly anyway. He checks resistance in place, and only removes "suspects" for further testing.