Testing glow plugs with charger
 

I have 4 old glow plugs from a 300D which may work perfectly. (one is broken) I've seen on youtube a way to test to see if they glow by attaching the positive lead from a battery charger to the threaded shaft, and the ground to the glow plug body.

I've done that will all my plugs, and none even get hot. Does this mean they don't work, or my battery charger is busted? The charger shows 12 Volts.

I have continuity in all the plugs, so is there a reason to replace them considering that they will probably cost me $30-40 each at a dealership?

THanks

Mark

I would check them with a well charged battery.

How many amps is your charger? Is it automatic?

-J

The current has to go somewhere.

Either you are not making good connection or your unit doesn't put out significant amperage.

Power (watts) = [E * E] / R

If it's 12 volts that's 144/R

Let's pretend there is 1 ohm of resistance. Then you are holding ~ 150 watt light bulb in your hand. You'd know if it was on!

With good current, a good glow plug will get red hot quickly.

-J

You don't really want to test 2 volt plugs with a 12 volt charger.

That could get expensive....

I wasn't sure if it was the series or parallel glow plug.

If you have a newer "smart" charger, it will not put out power unless it senses a connection to a battery.

I have an older charger that has no settings. I don't know why they don't glow. I even sanded the contact points. I tested the charger leads with my voltmeter and it read 12 Volts. :confused:

Are these the old loop style plugs? Did you check them with an ohm meter?

-J

I put the dial on ohms, and they read something like 6 or 7. They are the old style.

those are 6 volt plugs. DO NOT ground via body of GP, that wont work.

Apply ground to the threaded stud, and apply positive to the small ring around the threaded shaft which runs into the GP.

Helps if you use a thick buss wire and a ceramic insulator to test them...

I they read 6 or 7 ohms, they are good.

What was wrong with the broken one? Loop was open?

about 9 ohms is normal

More or less.

Depends on the temperature and the meter.

Sorry guys, I tested them again after cleaning off the carbon deposits. Now they all test 0 ohms.

The broken one was the one where the retaining nut was rusted on and broke the threaded shaft.

Well, now you can upgrade to the newer style plugs and enjoy faster, better starts :)

I think these loop plugs are only 1.3 volts with the major part of the voltage drop across the resistance wiggle wires. A battery charger will put out too much voltage if it is capable of delivering the amperage. You were perhaps lucky you did not burn them out.

Ohmeter test is about all the average guy will be able to do plus physical observation of the loop on the end of the plug. Newer type plugs are twelve volts and draw perhaps 20 ampres when cold initially so a car battery for them is best again for the average guy unless you have a really powerful battery charger.

But what about the relay problem? Will the dealership know what to give me?

I am not quite sure what you mean. Will the dealer know. If you are not updating the system then a relay from the auto wreckers would do quite well if needed. They are fairly expensive from the dealer. Anyways get your old system running to see if anything is needed.

Battery charger will both current limit and also try to determine if it is short circuited to self protect. Likely one of these things was indicated due to the glow plug looking different electrically than a battery...

Barry, you suggested I use the pencil type, but I'm told they need a different relay, so I'm hoping the dealer gives me plugs that work with my original relay.

There are no mercedes benzes in the wreckers around here! And certainly no classic W115s. :o

Why can't he just take the output lead from his existing relay, hook up a 5 way split and run the leads to the plugs.

My '75 240D had the hand pull on the dash and the cherry glow indicator.

Does this man really need to buy a new relay?

There have been threads on here in the past regarding the 115 300d relay and pencil plugs. I'm virtually certain that all those threads concluded it won't work. Although in my opinion, it is definitely better to install the pencil plugs and a ford starter relay.

When you tested the ohms on the plugs, where did you run the current? Was it from the outer ring connection to the stud? You can't check the resistance of a loop plug by going from the body of the plug to the threaded stud like it's done with pencil plugs.

I think you could test those plugs by attaching the heavy wires to them and then putting the voltage from the charger thru 4 of them at once.

Something is amiss with the method of measurement. Zero ohms would be pretty much impossible. If measured correctly, it should show in the neighborhood of 5 to 10 ohms.

As stated previously, any plug with a completely intact loop is virtually certain to be good.

Hang in there. You will get this figured out.

not true, could be broken inside.

These are 6 volt plugs btw

Take a look at the wiring diagram in post 23 of the other thread.

Those plugs glow on about 1 volt each. As I recall. they are actually rated at about 1.6 or 1.7 volts or so.

I test the ohms by putting one lead to the body and one to the tip. On the old ones, the readings are from 5 to 50. On the new ones, the readings are 0.

this is not correct. put one lead to the threaded post, and the other to the ring around the threaded post

x2 See post #24

Twas not me that suggested the newer type plugs I believe. Your original problem was termination corrosion it sounds like as your thread progressed.

Buy or get your hands on one good plug of the same type to replace the broken one. . Put the system back together and see if it works. If not troubleshoot from there. Getting every connection point really clean is not an option.

The earliest years of the 123 used the same glow plugs and relay I suspect. Glow plugs for sure. Try your Ontario auto wreckers on the web for donars for the relay if eventually needed. Or on ebay perhaps.

Many years since I have been in Aurora. Last time there was to rebuild a single cylinder enfield motorcycle. To give you some indication of how long ago. A rebore of the cylinder with a new piston and rings was 14.00 or 17.00. Thats for labour and parts on the removed cylinder and there was no sales tax back then either I believe.

With the glow plugs out a compresion test on an engine sitting a long time is not a bad ideal. Especially if you can borrow a diesel compression gauge with an adapter that fits the holes. Engines sitting that long can have stuck piston rings. It is always nice to know what you have compression wise early in the game.

I did as you said, and the reading was still 0 ohms.