Testing 240d glow plugs - PeachParts Mercedes-Benz Forum

ToddG · #1 02-21-2009, 03:48 PM

Question for you all about testing glow plugs. I have a 76 240d, just bought new glow plugs. Had to stick with the loop style because the shop couldn't get the upgrades. Put in plugs 4, 3, 2, but not 1. Connecting all with the 12v and grounded the thread of plug 1 to the neg of the battery. Had my friend pull the start switch to warm up position. Got 12v on all 4 plugs, but plug 1 didnt heat up with a darn. Shouldn't it have glowed?? Didn't even bother trying to start the car, since it seems like none of the plugs are heating up.

Any ideas as to how a brand new plug can get 12v but no heat up?

Thanks!

toomany MBZ · #2 02-21-2009, 04:02 PM

Maybe the wiring has a problem? Use a set of jumper cables on that non glowing plug, ala Diesel Giant.

#3 02-21-2009, 04:14 PM

Connecting loop plugs individually to 12V will destroy them. Properly functioning, each glowplug should get about 1.5volts since they are wired in series with resistor wires between them. So, if the #1 plug isn't working none of them will.

kerry · #4 02-21-2009, 04:19 PM

Are your insulators all in the right positions? If the wires coming in and out of he plugs are touching, you could get 12volts thru the circuit without any voltage going thru the loops of the plugs.

Palangi · #5 02-21-2009, 04:53 PM

If you have 12 volts at all points of a series circuit, that indicates that you have an open circuit (in this case, it means there is no path to ground). The possible causes are:

A. A bad plug (probably the # 1).

B. Miswire in one or more locations.

C. Missing / disconnected / miswired ground wire on plug # 1.

If you have voltage on all 4 plugs, the problem is around plug # 1 somewhere.

Stevo · #6 02-21-2009, 05:05 PM

The new style plugs for your car are available. I believe they are 18mm the same replacement GPs used in the later 616 engines. They are a worthwhile investment which will give quicker and better starting and you can get rid of those resistor wires and the ground. Are the ones you got Autolite, Champian or Monarch?, if so I would RUN back and return them.

ToddG · #7 02-21-2009, 07:21 PM

thanks for all the replys!

Let's see, I swear the 240d resistor wires touched each other initially. From plug 4 (cyl 4) to 3, to 2, to 1. So I can see how power was just jumping across.

But I used to have a 76 300d and still had the spacers that sat between the hard resistor wire. So I put those in, and starting from the block on plug 4 with the plug screwed in, it went: 21mm nut, heating element, 12v from starting switch, spacer, resistor wire to plug 3, 8mm nut, end of screw

plug 3 went 21mm nut, heating element, resistor wire from plug 4, spacer, resistor wire to plug 2, 8mm nut, end of screw. and so on to plug 2 and plug 1

with plug 1 not in the block, I grounded it to the - of the battery, had a friend pull the switch to "warm", and I still got 12v on the tip of the plug, yet no glow, again.

however, on a whim, I pulled the ground off of the threads of plug 1, pulled the switch, and STILL got 12v on tip.

So, I'm lost. Same results on plug 1 with or without a ground. Still no glow.

Oh, and the shop gave me some Bosch loop plugs. Which is wierd, because in the past they sold me Mann filters b/c Bosch's quality has really dropped.

Is there a way to bench test a plug just to see if it works?

kerry · #8 02-21-2009, 07:31 PM

What do you mean by 'heater element' when you list the order of things at the end of the plug?

It's been a long time since I worked on loop plugs so I can't recall the exact correct sequence.

Looking at an old loop plug I have, it looks to me as if on the one nearest the firewall, 12v would come last on the threaded screw portion. There should be an insulator separating that 12v from the heavy gauge resister wire which will be around the larger portion of the inner end of the threaded portion, not touching the threaded portion and taking the 12-volts over to the next plug.
Not absolutely certain about it but it's clear that the pathway is thru the threaded portion and out the larger segment near the hex head for screwing the plug in. Does that make sense?

ToddG · #9 02-21-2009, 07:42 PM

well, you've got the threads in the block, then the 21mm nut, those are the ground. then there is a shaft (the same shaft that the loop is connected to) that the power wire sits up against.

i just called it a heating element, since it is connected to the loop at the end of the plug. is that right?

kerry · #10 02-21-2009, 07:45 PM

Quote:

Originally Posted by ToddG

well, you've got the threads in the block, then the 21mm nut, those are the ground. then there is a shaft (the same shaft that the loop is connected to) that the power wire sits up against.

i just called it a heating element, since it is connected to the loop at the end of the plug. is that right?

Each plug is not grounded. The only ground the system gets is from the ground wire at #1 plug at the front of the block. The loop is insulated from the body of the plug and the 12v in and out are also insulated from each other.
I added more to my prior post to try to explain the system as I recall it.

ToddG · #11 02-21-2009, 08:08 PM

Kerry, I think we are getting somewhere here! So, power goes from the threaded screw (on which the 8mm nut screws on) down to the loop, then back to the larger bit that sits right next to the 21mm hex, and that is where the power (resistor wire) runs to the next plug? Damn, my wiring is all wrong. This really helps.

One more question, you said there should be a ground wire that comes off of plug 1. Should that wire sit on the larger bit next to the 21mm hex, or sit on the threaded screw? And I guess it can go to the - of the battery, since it's right there? Oh wait, that will just send Volts to the - of the battery, wouldn't it?

kerry · #12 02-21-2009, 08:18 PM

That's the route. I'm just not sure where the hot wire initially attaches. If it only fits the threaded portion then it has to go into the plug nearest the firewall on that threaded portion. I'm guessing that it also leaves the #1 plug at the front via the ground which attaches to the threaded portion. I think which is the in/out is simply determined by the diameters of the ends of the resistor wires and where they can fit on each plug.

I was confused myself on all this when I had loop plugs on my 77 and worked on them about 8 yrs ago.

ToddG · #13 02-22-2009, 10:50 AM

Was looking at the car this morning. I remember know, reading in the shop manual, from plug 1, the power is sent to the thermo time switch (which is grounded) then the to dash glow plug indicator, then back to plug 4. That's how it's a complete circuit. Now I just have to jam my hands back in there and redo the resisitor wires. Will let you know how it goes.

pizzachef · #14 02-22-2009, 11:04 AM

Just install the #1 plug correctly in the block. You can't ground the threads of the plug, you have to use the ground cable on the threaded portion on the end of the plug. So install the plug in the block, connect the ground wire, and go to the drivers seat and pull the knob. If your dash indicator glows, you're got a completed circuit with electricity flowing. If you have an incorrectly wired plug, or a bad plug, the indicator will not glow and you'll have to troubleshoot some more.

You can buy the pencil type plugs from fastlane. There are threads in the 'vintage' section here about switching to the new plugs. I did it and I think it was worth it.

ToddG · #15 02-22-2009, 07:50 PM

Weird, posted a thank you this morning and it didn't post. So, thanks for all the tips guys. Got the sucker glowing red hot this morning. Appreciate it!