Glow plug resistance differences? - PeachParts Mercedes-Benz Forum

JHZR2 · #1 06-21-2022, 10:55 PM

On my 91 300D, the glow plug light was giving signs of a bad GP. I found that #2 was different than the rest. Older. Higher resistance by like 40 Ohm.

I pulled it, reamed it, cleaned the junk out of the bore and then installed a new GP.

I torqued it to 20nm, seemed a bit much threading into aluminum.

The other GPs all meter out to 0.9 Ohm through the harness. I think they’re all Bosch. #2 looked older than the other five.

The new one I put in was a Beru, and after installation, it metered to 0.7 Ohm through the harness.

Not that it’s that difficult to remove and redo, but It’s a pain to remove the intake. And takes time.

So… if you were me, would you dump $40 more to install a full matched set of Beru plugs when four of the five are metering 0.8-0.9 Ohm… or just leave the rest and do some more when the next one goes?

Do Bosch vs Beru plugs have a slightly different resistance?

I’d like to know because changing the GPs when they work is kind of wasteful. But pulling the intake again to do the other four is also wasteful later on.

Is there a real difference between 0.7 and 0.9 Ohm? How much of a difference is necessary to trigger the GP light after startup?

Thanks!

ykobayashi · #2 06-21-2022, 11:41 PM

Not only is that a negligible difference, depending on the meter you’re using the number is suspect. Handheld meters don’t work well at those low values. You need a four wire meter with a Kelvin connection. Like an HP 34401A.

Also the resistance changes dramatically when heated. So what you’re measuring at room temperature is irrelevant except for diagnosing an open (burned) glow plug.

So don’t worry. Enjoy the car.

dieseldiehard · #3 06-23-2022, 10:38 PM

as was stated, the slight difference in resistance doesn't matter at all.
I am of the philosophy that if one failed the others are not far behind, if they are all of the same type and age. I'd replace them all while you are in there.
Also good time to clean any carbon deposits which cause GP's to get stuck, especially the longer ones. OM603 not so much but OM606 is particularly known for carbon build up and stuck GP's.
I was advised that a routine replacement of GP's on engines with aluminum heads at something like 150K miles is recommended.

JHZR2 · #4 06-23-2022, 11:28 PM

Quote:

Originally Posted by dieseldiehard

as was stated, the slight difference in resistance doesn't matter at all.
I am of the philosophy that if one failed the others are not far behind, if they are all of the same type and age. I'd replace them all while you are in there.
Also good time to clean any carbon deposits which cause GP's to get stuck, especially the longer ones. OM603 not so much but OM606 is particularly known for carbon build up and stuck GP's.
I was advised that a routine replacement of GP's on engines with aluminum heads at something like 150K miles is recommended.

I went in prepared to swap all of them, but all except #2 were clearly much newer judging by the finish brightness on the GP body.

I think based upon that, the other four are the same age, and when one of them goes I’ll change all five. That seems like a a logical approach.

t walgamuth · #5 06-28-2022, 10:07 AM

When I was younger I'd change only the dead ones, and then sometimes with a good used one. Now if one fails I do them all. That was on a 240D which was pretty easy to access the gp.