Testing W115 Glow Plugs - PeachParts Mercedes-Benz Forum

warmblood58 · #1 05-01-2011, 04:22 PM

My GP's are out and I think I just learned that you cannot test them with a battery and jumper cables, true? I know nothing about this 115 car other than it took a lot of work to get it started after sitting some time -anyone have a fast procedure for checking the old style GP's? Thanks!

James

kerry · #2 05-01-2011, 04:55 PM

Test for continuity through the loops. No continuity=bad plug.

tomscat1 · #3 05-01-2011, 05:08 PM

Do you have any of the various service manuals? There is no Haynes Manual for the W115 300D, but there is one for the next series, the W123. The early W123 300Ds use the same basic engine as the W115, with slight variations, so there is some relevant info there.
There are also a couple of Chilton manuals out there, not to mention the Mercedes service manual, and a few manuals on CD.

You can find guidance for testing glow plugs, etc. there, and it can save you a lot of time and.... Aggravation. I tested my GPs years ago, so I can't recall the readings. I know that I tested them in the block, in sequence with a meter. The meter readings basically decline about a volt at each GP... Down to 1 or two volts by #5, if memory serves. If one is out, no power reaches any GP after that in the series. You can also visually see the two little grid connectors turning bright red as they heat up.... So you know if you are getting power there.

You can get the correct meter readings from someone more familiar in here, perhaps, or from the manuals.... Or you may even find it written up online somewhere. Wish I could be of more help, good luck.

kerry · #4 05-01-2011, 05:34 PM

If one glow plug is out, none of them will work, not even the ones earlier in the series because the circuit is not complete. If you put one probe of the ohmeter at the firewall hot wire coming to the first plug and the other on the ground wire at the last plug at the front of the engine, you should have a complete circuit. If that circuit is open, there's either a bad plug or a disconnected wire.

tomscat1 · #5 05-01-2011, 05:41 PM

Quote:

Originally Posted by kerry

If one glow plug is out, none of them will work, not even the ones earlier in the series because the circuit is not complete. If you put one probe of the ohmeter at the firewall hot wire coming to the first plug and the other on the ground wire at the last plug at the front of the engine, you should have a complete circuit. If that circuit is open, there's either a bad plug or a disconnected wire.

Thank you, Kerry. I stand corrected. It has been awhile since I even thought about my GPs. Mea culpa.

kerry · #6 05-01-2011, 05:49 PM

In a pinch a person could put all the good glow plugs in a row and the bad plug(s) at the front of the engine. Then the ground wire could be connected to the last good plug at the front and at least you'd get the good plugs to heat up, although they may be getting too much voltage.

warmblood58 · #7 05-02-2011, 05:21 PM

Could I daisy chain the gp's outside of the car by connecting them all via their loops and attach positive lead from battery to one end gp from and then ground the last in the chain to a metal object? Do I need to individdually ground each gp also at the same time? Will this work? I would like to see how red they glow independent of being mounted in block -

kerry · #8 05-02-2011, 05:58 PM

Quote:

Originally Posted by warmblood58

Could I daisy chain the gp's outside of the car by connecting them all via their loops and attach positive lead from battery to one end gp from and then ground the last in the chain to a metal object? Do I need to individdually ground each gp also at the same time? Will this work? I would like to see how red they glow independent of being mounted in block -

Yes, using the heavy squiggly resistor wires, then ground it to the negative post on the battery and put the positive wire on the other end. No, don't ground them individually, that will short the circuit.

warmblood58 · #9 05-03-2011, 12:00 AM

Quote:

Originally Posted by kerry

Yes, using the heavy squiggly resistor wires, then ground it to the negative post on the battery and put the positive wire on the other end. No, don't ground them individually, that will short the circuit.

Thanks Kerry, this should be interesting -I ordered the fast glow kit today but I am curious to see what happens when I do the above which will tell me much about the quality of glow I was getting on the first start attempt and the general health of the engine which the previous owner knew nothing about -always an adventure in diesel land!

kerry · #10 05-03-2011, 12:07 AM

Did you get a new relay? The 115 relay won't control the pencil plugs I think.