Testing old style glow plugs, Help Please! - PeachParts Mercedes-Benz Forum

NC240D76 · #1 10-25-2004, 09:30 PM

This is in a 115 body 76 240d with the old pull knob to activate the glow plugs not the system using the key and a relay/timer.

With winter coming on I thought it would be a good idea to make sure the glow plugs are up to par.
I searched here and found you can test them with an ohm meter or remove them and hit them with twelve volts just long enough to see if the element glows. Well, my meter seems to be in hiding along with various other tools so I decided to remove the gp's and test, and just in case Murphy is hanging around I decided to start with the parts car.
No big deal getting the glow plugs loose and out even with the mosquitos helping.
I put the first glow plug in the vise (gently of course) and attached the postive lead on a 6 amp battery charger to the tip and the ground to the body where it would ground to the engine. I could hear the battery charger hum but nothing from the element.
Well, when I was removing the glow plug I had noticed the strange method they were wired and the fact that the jumper to the second gp came from the collar not the body of the gp so I carefully attached the ground there but the element would barely start to get hot before the charger kicked off because it thought it was shorted to ground (which I guess it sort of was).
So I grabbed an old battery that I thought should have some charge left in it and made some jumpers out of some 10 awg, well as soon as I touched the jumpers to the tip and the collar of the gp the tip flamed up and burned off ending up about three feet away from the table the vise is mounted on.

I had sprayed some triflon (teflon based lube spray) on the threads of the glow plugs to help remove them and some could have gotten on the element but I assumed the system ran off 12 volts. Was the gp bad or did I just piss away another ten dollar bill?
I think I would like a little advice before I continue, Help!

What am I missing?

Palangi · #2 10-25-2004, 09:38 PM

Those plugs are in series, so each plug only gets about 3 volts. In fact, on my old 200D there was a dropping resistor to lower the GP voltage even further. Not much to test on those things, kinda like light bulbs. If one goes out, they all go out. At that point, you can test with a voltmeter to see which plug in the string the voltage ends at.

psfred · #3 10-25-2004, 09:44 PM

If the glow inidicator in the dash gets orangy-white, the glow plugs are too, and it will start if you have compression and good injectors.

Peter

NC240D76 · #4 10-25-2004, 10:10 PM

If I wait until the dash indicator gets red hot it seems to take quite a while and it is very hard to see in bright daylight so I was trying to improve my waiting time (this from a person driving a 240d auto) and give the system a break.
This car has unknown miles and seems to start a little slower than I like so I am guessing the compression is lower than it should be so keeping a good battery, good starter, and good glow plugs will help alot.
I am guessing the first gp in the series is the one next to the firewall so I can just check voltage starting with that one and I should have some voltage to the grounding strap.

I seem to remember a post from a year or two ago that mentioned switching this type of glow plug system to a newer type, anybody familiar with this?

psfred · #5 10-25-2004, 10:18 PM

FastLane has the kit, about $100. New keyswitch, relay, and pencil type GP with the larger base of the series type.

Takes about 60 sec to get the indicator up to full temp. You have to sit there and pull, there isn't really any way to speed this up short of putting in pencil type parallel plugs.

If the indicator loop only gets red, not orange, take the wires off the GP and clean them up, you have too much resistance. DO NOT replace the links with plain wire, you will burn the GP out, the system is designed to have 6 resistors and one indicator in it. On four cylinder engines, you have two "zigzag" resistor links (these get red hot) and one straight link. On the five cylinder there is only one resistor link. I suppose on a six (never installed in a passenger car, but I'd bet there are truck engines out there with six cylinders) there would be no resistor links.

Late injection timing will also make it hard to start -- check chain stretch and injection timing (replace the chain if 8 degrees or more stretch before fiddling with the timing) and correct if necesary -- should fire right up.

Peter

Palangi · #6 10-25-2004, 10:45 PM

Using the test you described, you would get about 12 volts going into the first plug, then about 3 volts less going to the next plug, etc. If a plug is open, you would get 12 volts on each plug until you get past the open one, the zero volts after the open plug.