Does anyone know the resistance value the glow plugs are supposed to be???

In my 79 300D, I have the loop style glow plugs and I was wondering what the resistance value out of circuit is supposed to be. All of the ones I have measure .1 ohms accros the element.

If someone knows that would be great!

[The Warden]

Are you talking about the plugs themselves?

If you put one lead on the terminal and the other on the body (the part with the threads), there should be 2 ohms or less. If it's wide open, the plug's bad.

Hope that helps some...good luck! BTW, one morning in January, my van wouldn't start...turned out that 7 of the 8 glow plugs were bad. I put new Beru plug sin and it started without even thinking about it, even from being dead-cold...I was impressed

Do the same values hold true for the pencil type plugs?

I thought that the threads of the MB glow plugs were not connected to the element....therefore you wouldn't measure the resistance there.

[tcane]

I just looked in my M-B engine manual and no resistance value is given. The plugs are tested using a volt meter and an ampmeter looking for current going from one plug to the next and total draw on the electrical system for the glow plugs. When checking plugs in the engine do not ground the plug to the engine because it could damage the preglow timer relay. .9 volts at each plug with 63 amp draw, the wire loop connections are resistors reducing voltage to the plugs, but glow time is very short to measure and access and heat make it hard to use a volt probe.

Test procedure for loop-style plugs (old style): disconnect ground wire on glow plug for #1 cylinder, connect positive probe for volt meter to positive terminal #5 plug (center threaded portion) and negative probe to ground - if you have 12 volts then one of the plugs is bad, if you get 0 volts then either fuse is bad, defective preglow timer relay or activation relay. If you find 12 volts at #5 plug then check the other plugs by connecting the volt meter positive probe to the voltage input of that plug and negative probe to the output of that plug (this varies from plug to plug as the current flows through and you can determine based on how the wire loop resistance wires are connected to that particular plug) until you find one plug that does not show 12 volts - youv'e found the bad plug unless there are other ones since the system works in series and one bad plug will break the circuit for all other plugs.

Check each plug for continuity with an ohm meter from the the silver ring on the plug to the body of the plug - if continuity is found then that plug is defective. Another plug that is bad can cause another plug to fail - so check all plugs even if only one plug is bad.

That said, I do have info about resistance readings with the plugs out of the engine (found it from a non-M-B source), but I'll have to dig the info out since it is buried not having needed it for a few years. I do recall that a plug can be bad with either too little resistance or if resistance is too high. I'll find the info and post later.

Good Luck!
Tom

[tcane]

I dug out my tech info for testing glow plugs with an ohm meter.

.3 - .9 ohms from the theaded stud to the silver ring;
no continuity for either the threaded stud or silver ring to the plug body or block;

sliver ring is the ring surrounding the threaded stud and where the large loop of the wire connectors attaches to the plug


Good Luck!
Tom

[Dropout]

Originally Posted by tcane

Snipped for brevity

Test procedure for loop-style plugs (old style): disconnect ground wire on glow plug for #1 cylinder, connect positive probe for volt meter to positive terminal #5 plug (centre threaded portion) and negative probe to ground - if you have 12 volts then one of the plugs is bad, if you get 0 volts then either fuse is bad, defective preglow timer relay or activation relay.

snipped for brevity

Good Luck!
Tom

End snip

Hi all.

I got this far and have 0 volts to the #4 (1978 240D) GP. The fuse is good. The GP relay under the steering column clicks sometimes. Someone has taken it apart in the past. The internal contacts on the large relay were badly pitted so I cleaned them up as well as I could.

I re-installed the relay and still get no GP light and 0 volts to the fuse and the #4 GP.

What next? Is the relay bad? What and where is the preglow timer relay?

Everything worked fine until during a warm start the GP light flashed on for a second or two and then stopped working altogether.

Dropout

A bunch of you are talking about pin type plugs. These are in parallel and can be resistance checked from the shell to the stud. You can check them even easier by pulling the plug at the relay and checking each pin to ground. It's much easier.

The original thread, however, is on a car with LOOP TYPE plugs. These are a short between them because they are in series and isolated from ground. They will rarely if ever burn out, so a continuity measurement is almost not needed, but for good troubleshooting practice you should do it. Since they are in series, you can check them all at once for continuity by measuring across the whole string at once. Don't worry about the value, just make sure you have a circuit.

The BIG problem with Loop plugs is that they can get enough crud packed in that they leak current to ground. Remove the plugs and scrape out the crud with a hardwood stick or use the fancy reamer that you can get from Fastlane. If this crud is tight enough it bleeds the power off to ground rather than dividing it among the plugs equally.

Also there is a bar fuse in a holder on the firewall behind the engine. Check that with your ohmmeter. Don't rely on a visual inspection to determine if that fuse is good. It often cracks at the edge and looks good but is not.

Good luck,

[dieseldiehard]

Originally Posted by brandoncrone

In my 79 300D, I have the loop style glow plugs and I was wondering what the resistance value out of circuit is supposed to be. All of the ones I have measure .1 ohms accros the element.

If someone knows that would be great!

The fuse is good huh? Always suspect it first.
Loop style GPs are less than half an Ohm in resistance, just a little more than the resistance of the leads on an Ohmmeter. hard to get a valid reading on the typical Ohmeter, DVM's excluded.
Pencil style GPs are around 2.2 Ohms and are easily read on a decent Ohmeter like a Simpsoon 260. Forget it if its a Radio Shack $3.95 meter.
To test loop GP's you can connect a heavy wire (jumper cable of necessary but be careful!) from the + terminal on the battery to the GP nearest the firewall ( the one in the front is grounded) there should be a spark as current flows through the series connected plugs. I have used this to start a car when necessary (after the timer failed) yes the points take a beating in the timer box and can be burnished back to give some more life. Is the relay clicking? If not then its an indication the timer might be kaput. Or the timer relay may be receiving an "engine hot" signal, later models use a thermal switch to drop out the long ON hold time
addendum:
The wire should be at least as large as 8 Ga and can be connected to the battery + or to the three terminal connector block that the starter motor is connected to (on the right side of the engine).

[Dropout]

Thanks for the responses.

I checked the fuse (and everything else) with a DVM.

The relay does click but stopped doing after my fooling with it for a bit. I assumed that there was some kind of protection built in so some idiot (me) would not be able to overheat the GPs by constantly turning the key on and off.

The fact that there are 4 used GPs in the drivers door is also a hint that there is something not quite right, but I don't know enough about these cars yet to take the hint. I suspect that the GPs in the engine are relatively new.

What gauge wire for the jumper? Battery cables

Dropout

[Krueger]

Drop out -

Is the GP Relay on your model attached right below the steering column? It would be just the basic silver relay (no fuse attached). I have a '77 300D with the same set-up and I'm looking for the timer relay (which everyone keeps telling me is a screw down with an 80 amp fuse inside) however I see nothing like it under the dash.

[Krueger]

Oh, and an 8-gauge wire will do fine to jump the plugs. I would attach the wire at the fuse block however - just to be safe.

[Dropout]

Originally Posted by Krueger

Drop out -

Is the GP Relay on your model attached right below the steering column? It would be just the basic silver relay (no fuse attached). I have a '77 300D with the same set-up and I'm looking for the timer relay (which everyone keeps telling me is a screw down with an 80 amp fuse inside) however I see nothing like it under the dash.

Yes that's right. There are two relays inside so it's possible one could be the timer relay.

Even if the circuit feeding the plugs is okay and it has new plugs does not mean that everything is good. As I tried to explain earlier, crud packs in around the plugs and offers a path to ground. You have to remove the plugs and use the special reamer to clean the holes out or use a hardwood stick and lots of time. Don't get the bright idea to use a steel brush because a broken bristle could work its way in a cylinder and cause damage.

Good luck,

[Dropout]

Originally Posted by LarryBible

Even if the circuit feeding the plugs is okay and it has new plugs does not mean that everything is good. As I tried to explain earlier, crud packs in around the plugs and offers a path to ground. You have to remove the plugs and use the special reamer to clean the holes out or use a hardwood stick and lots of time. Don't get the bright idea to use a steel brush because a broken bristle could work its way in a cylinder and cause damage.

Good luck,

And that would cause 0 volts at the fuse? If so, does it make sense that I could check the relay etc by removing the wire from the fuse to the #4 GP and see if I get 12V?