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.

speace · #7 10-25-2004, 10:53 PM

By all means, now that you have blown out one of your plugs and have to replace it anyway, get the conversion kit to the 12v glow plugs that are wired in parallel as you expected them to be.

You will love the improvement. I converted both my 74 and 75 to the pencil type back in the early 80's.

Ok, ok, I don't own a diesel anymore, but I put a kazillion miles on the 3 I had!

dabenz · #8 10-26-2004, 10:07 AM

I test backwards or from the ground at the end of the line, with the system installed and the ground freshly cleaned. You're looking for both opens and shorts (a dash element that glows too fast is an indication of a short). It's pretty easy to tighten the 'hangers' down too hard, crack the insulator and cause a short. A short can occur from coking (unburned fuel creates a path to the prechamber - good practice is to ream each time you remove a plug). Opens occur from blown loops and corrosion at electrical connectors, and results in the dash element not lighting or lighting slowly in the case of heavy corrosion. Checking backwards is easy and checks for shorts and opens at the same time: attach neg. lead of voltmeter to ground then feel your way up the line. The number will drop a bit as things heat up - the hotter a wire/element gets the lower it's resistance gets.

Also check the ground strap at the starter, battery connections, and starter connections. A little corrosion kills a lot of juice.

psfred · #9 10-26-2004, 08:04 PM

Do not short the buss links to ground to check, and do not replace the links with wire -- the reduced resistance in either case will overheat the glowplugs and cause them to fail.

Peter

NC240D76 · #10 10-27-2004, 11:37 PM

Thanks all.

Speace,
How much trouble is it to change over to the pencil type?

NC240D76 · #11 11-01-2004, 01:37 PM

I am thinking of changing over my old style glow plugs on my w115 240d but I am confused, why do I need a relay for this system?
Does it pull too many amps to run through the wire that feeds the old style gp's.
If that is the case then I assume I need to run a hot wire to the relay large enough to carry the amps and activate the relay with the other wire using the same pull knob I am using now.
Any thoughts or explainations will help.

psfred · #12 11-01-2004, 08:02 PM

FastLane has a kit, includes the new keyswitch, relay, GPs and wire. They are now parallel -- all GPs are jumpered together and ground into the block.

Peter

phantoms · #13 11-01-2004, 11:09 PM

Quote:

Originally Posted by psfred

FastLane has a kit, includes the new keyswitch, relay, GPs and wire. They are now parallel -- all GPs are jumpered together and ground into the block.

Peter

Keyswitch?

I just installed this kit on my daughter's 240D. Makes starts much easier. Before, she use to have to glow the engine at least twice to get it started. Now it starts after glow light goes out everytime. If I can get ahold of it during daylight again, I'll take some pics.

Basic install on the 240D is to run a battery wire from the starter to the Relay/controller. I also used fuseable link at the starter to be safe. Run the other terminal on the relay/controller to the firewall glowplug fuse. Leave the other side running to the glow plug as your first link to the new glowplugs and then use the included jumpers to jumper from glowplug to glowplug. Run the included harness in the kit inside the car going through an unused grommet and tie the wires in to the old glowplug relay/controller harness leaving the old relay (under the dash/column) disconnected.

NC240D76 · #14 11-01-2004, 11:22 PM

Sounds good.
I will call Phil tomorrow and see if I have any questions after I get the kit.
Thanks,
Brian

NC240D76 · #15 11-05-2004, 12:54 AM

Okay,
No problem with "disconnect battery, dismantle parts, screw in new glow plugs(I guess this is a good time to ream the passage), mount new timer/controller.
Connect pin 8 to battery w/6mm ( 8 awg aprox?) wire.
Pin 9 to #4 glow plug w/ same size wire.
Connect all the glow plugs and make sure the ground is removed.
Connect wire harness plug to timer/controller.
Replace gp indicator element w/ light bulb and connect w/ green wire/pin 5 and other wire to ground.
Pin 1 brown wire to ground
So far so good!
Pin 3 red wire to terminal 15 fuse box .....my fuse box only has 12 fuses.

Pin 4 black wire to terminal 50 fuse box (or starter).......
Okay these last two have me confused, is there another fuse box I am missing? and yes I know where the starter is but I do not want to have to run a wire across the car if I can keep all wires neat and in one area.
Sorry to bother everyone but the only manual I had was for a w123 and it will not work for my w115.
Any suggestions will be welcome.
Thanks,
Brian