Glow plug questions - PeachParts Mercedes-Benz Forum

can-do · #1 10-24-2002, 08:08 PM

Greetings All,

I know it's probably been hammered in the archives many times over but searching through 2000 + post could get lively and time consuming. Could someone please go over the proper procedure for testing glow plugs in a '84 300SD. Please include what ohm reading should be, and what setting you have your digital meter set at, as well as where you took the test from, harness or straight off the glow plug. I think I have at least one dead plug, but hard to tell for sure with all the posted test procedures, and none being actually the same. Just testing them from the harness to ground with the meter set to 2K I got a zero reading on all, if that helps.

Thanks,

Charles

BJ300SDL · #2 10-24-2002, 08:26 PM

I think the best way to test glow plugs is to check the amperage draw of each glow plug. Each one should pull about 8-16 amps. If you don't have a meter that will measure amperage that high I would recommend pulling them and testing them with a battery.

psfred · #3 10-24-2002, 09:12 PM

Resitance should be less than 1 ohm. You won't see a thing at 2K ohm on the meter! Dead plug will be open (infinite resistance).

Verify that you have voltage to the plugs, too! If not, check the fuse (80A flat, between two screws) for cracks. Quite often broken, but you don't see it until you poke it to separate the crack. Makes for very hard starts.

If you still have dead ones, replace them all, since they were probably all done at once the last time -- the "good" ones are going, too!

Peter

rickjordan · #4 10-24-2002, 11:57 PM

Ckecking the glowplugs is one of the easiest things to do. Pull the glowplug harness connector off. You'll see 5 "sockets", they are numbered. Take any basic ohm meter, put the pos. led in each "socket", and the neg. lead to a good ground. You shouldn't read anything higher than .5-1 ohm. An open (infinite) reading is obviously a bad plug. It is possible to have a reading higher than 1 ohm, this would be a glowplug on it's way out. The resistance gets higher before the wire inside the glowplug physically breaks.

Ken300D · #5 10-25-2002, 07:39 AM

In my opinion, testing glow plugs can take several strategy paths, depending on how thoroughly you want them tested and how much effort you want to expend - and the equipment you have available.

1) Easy Test - check GP resistance as described above.

2) Test with better equipment - Check Amperage the individual GPs draw as described above. This gives a better picture of what the GP is doing under the stress of heating up. You need a multimeter capable of measuring / handling 20 Amps DC.

3) Dangerous but Effective - Remove all the GPs, use a nice wire and car battery and apply power. Watch how GP heats up. Tip should quickly get nice and red hot. Warning: This is very dangerous - you could burn yourself badly or set something on fire.

#1 will catch totally bad (open) GPs. It may or may not show marginal ones.

#2 will start to catch GPs that have a portion of their heating element burned out or shorted, but not totally open circuit. Such plugs will show resistance in the normal range but will not draw normal current and will not heat properly.

#3 is the acid test AFTER you have verified current draw in test #2. If the plug draws the right current and quickly (like three seconds) glows red on the tip, its good.

--------------------------

I'm sure in the vast majority of times, in-the-car tests #1 and #2 will find a bad GP that can be individually replaced.

Ken300D

#6 10-25-2002, 12:04 PM

Ken 300D

Well stated, but it does seem that in checking the ohmage on these plugs one can get fooled. The #3 method is the most accurate and reliable way of checking them. It always works. I clamp them in a vise, attach a ground to vise and touch the end of the plug with a positive lead. Amazing how many times it does'nt heat when the ohms read about right.
You are correct though. Whenever using this method one can burn the gargage down. Always be carefull.

Stevo · #7 10-25-2002, 08:08 PM

Good info. I might just add one thing and that is don't do #3 test on the old loop style as it will toast them

can-do · #8 11-03-2002, 07:51 PM

Greetings All,

Thanks, for all the responses and suggestions on my glow-plug issue. The weather has been so nasty since I asked for help, not to mention I am trying to get some outside projects done that were weather sensitive. Concrete pouring isn't fun in the rain.
At any rate I did go out today in the rain none the less, no choice it seems and recheck the glow-plugs with an ohm meter set to 200 Ohm and found all but two plugs reading .8 The 2nd one came up reading a 2.4, not good and the 4th came up with a .9 To play it safe I think I will replace both as one seems to be definitely out of the game, and the other probably not far behind. I am wondering if this could contribute to a somewhat rough idle not only at start-up, which is obvious, but also after the engine has warmed up? I remember someone mentioning that if the tip of the glow-plug actually burned through then the compression in that cylinder would actually go down somewhat. As always, your thoughts and experience is appreciated.

Charles

kerry · #9 11-03-2002, 08:13 PM

If a newbie is reading this thread looking for troubleshooting info, remember these instructions are for the parallel glow plugs used after 1981 (I think) and don't apply to earlier series glow plugs.
You tell the difference in this way:
The parallel plugs have an independent wire going to each separate plug.
The series plugs have a hot wire attached to number 5 or 4 at the firewall and a solid heavy wire joining each glow plug to the next and finally a ground strap from plug #1 to the block.

can-do · #10 11-03-2002, 08:29 PM

Greetings Kerry,

Not all glow plugs prior to '81 were series. I owned an '80 300TD that used the same parallel set-up I have on this '84 300SD but the timers are different. For the newbie's I'd say for them to look to see if there is an interconnecting solid wire between the glow-plugs to distinguish them as a series verses parallel system that has none. Parallel systems contain a seperate wire from the timer for each glow-plug.

Thanks,

Charles

McRoth · #11 11-04-2002, 10:29 AM

Since this thread is about glow plugs, I thought I'd ask a couple of questions about them. I just changed mine this weekend. The ones that came out were Bosch plugs & the tips were worn off by at least a third of their original diameter. I'm just concerned about what happens to the part that corrodes off? Can it damage the engine? When I pulled them there were some flakes on each one that came off in my hand.
Also, what is the thing between the two middle plugs that is screwed into the block? It has a wire that comes along with the glow plug wires attached to it. It looks like a sensor, but I'm not sure.

I put new Bosch plugs back in, are these good ones to use or is there something better?

Thanks!

Ken300D · #12 11-04-2002, 11:23 AM

Oh, another thing I would recommend for glow plug replacements is a glow plug reamer. Costs around $40 for the later parallel type GPs. This reams out the carbon that was packed in around the old GPs and helps in two ways - the new GPs go in easily, and they heat better because they are not touching any carbon deposits.

Ken300D

P.E.Haiges · #13 11-04-2002, 11:40 AM

McRoth,

The "thing" in the middle is a temperature sensor. It sends a signal to the GP relay that determines how long the dash GP light stays on.

P E H