Testing the rest

How do I diagnose the other components to the glow plug relay?

Will a simple bad light bulb disable the glow plug relay?

 

This smaller harness is the feed FROM the key switch to the relay. It is what triggers the relay to open for the glowplugs. Some relays open the corresponding switch when they are receiving 12V+. Others open it when they are NOT receiving 12v+. It just depends on the relay design and how it is wired. I don't know for certain in this case, but it would explain why there is power on the output terminals when it is unplugged.

To know for sure if the relay is working correctly, you should test the output on the relay with the switch harness plugged in and having someone cycle the key while you individually test each of the five posts on the relay. They should read 12V+ for 15-20 seconds after the key is cycled to the on position and then go off.

 

I am not sure what you are doing. In you picture you certainly are not going to get 95.0 DC Volts.
Why would you remove both Relay Connectors and then turn the Key On?

The only part of the Relay that should have Battery Voltage going to it is the Fuse and there is a flat Strip of Copper with a Screw Through.
So that part of the Relay always has Voltage available. You need Voltage from the Ingnition switch to close one of the Contacts inside the Relay and to start the Timer. When the Relay closes it taps into the Battery Voltage from the Fuse.

If it was Me what I would like to see is this:
Put all the Connectors back on and the Ignition Key removed from the Ignition.
Set your Meter so you can read 12 Volts. Put the + Probe on one of the Nuts on the Glow Plugs and the negative probe to a good ground.
After that check a few more Glow Plugs the same way. You should not have any Voltage.

If you are showing Voltage like that reach over and pull off the Connector that comes from the Ignition and whoes Wires go through the Fire Wall.
Repeat the test you did before at the Glow Plugs. If you still have Voltage there your Glow Plug Relay is sticking in the on position and the relay is no good. If you don't want to spend for a new one get a used one at the Junk Yard.

If you checked the Glow Plugs and did not get Voltage that is what is supposed to happen as the Key is not in the Ignition and of course not in the Preglow Position.

Turn the Key to the Preglow Position and listen for the Relay click and quickly do the same check to see if you are getting Voltage at the Glow Plugs. If it is working correctly you should get about 11.5-12 Volts at the Glow Plugs.

After 20-30 seconds the Glow Plug Relay internal Timer should turn the Relay off and you should no longer get Voltage at the Glow Plugs.
The above in Blue is normal operation of the Relay.

If you did the above stuff in Blue and you did not get Voltage pull the Connector on the Relay that has wires that go through the Fire Wall.

The #1 Termnal should have a Red Black wire. That may be a Red wire with a Black Stripe. Stick the + Probe into the #1 Socket and ground the Negative Probe and have someone turn the Key to the Preglow Positon. When that happens you should get Battery Voltage. If you don't you have an ignition Switch problem.

Now for the discaimer. I have pieced this together from stuff I have read others do.
Also I have not mentioned the Glow Plug Light because what is important is if the Relay is operating. The light is just an indicator; in fact the Glow Plug Light could be burned out.

Here is the other info on the Connector that has Wires that go to the Fire Wall:
Glow Plug Wire colors on the square Connector and where they go.
Here is a pin out description:

1)rd/bk (Black) ignition switch

2)vi (Violet/Purple) On turbo models goes to a Wire Terminal/Junction block and mates with a White Wire coming up from the Starter Solenoid. When you Crank the Starter + Voltages goes through the White Wire to the Violet one and turns the Glow Plug Relay On.

3)blu/wt (Blue with white strip)glow indicator Light

4)br (Brown)ground

 

I am not sure what you are doing. In you picture you certainly are not going to get 95.0 DC Volts.
Why would you remove both Relay Connectors and then turn the Key On?

The only part of the Relay that should have Battery Voltage going to it is the Fuse and there is a flat Strip of Copper with a Screw Through.
So that part of the Relay always has Voltage available. You need Voltage from the Ingnition switch to close one of the Contacts inside the Relay and to start the Timer. When the Relay closes it taps into the Battery Voltage from the Fuse.

If it was Me what I would like to see is this:
Put all the Connectors back on and the Ignition Key removed from the Ignition.
Set your Meter so you can read 12 Volts. Put the + Probe on one of the Nuts on the Glow Plugs and the negative probe to a good ground.
After that check a few more Glow Plugs the same way. You should not have any Voltage.

If you are showing Voltage like that reach over and pull off the Connector that comes from the Ignition and whoes Wires go through the Fire Wall.
Repeat the test you did before at the Glow Plugs. If you still have Voltage there your Glow Plug Relay is sticking in the on position and the relay is no good. If you don't want to spend for a new one get a used one at the Junk Yard.

If you checked the Glow Plugs and did not get Voltage that is what is supposed to happen as the Key is not in the Ignition and of course not in the Preglow Position.

Turn the Key to the Preglow Position and listen for the Relay click and quickly do the same check to see if you are getting Voltage at the Glow Plugs. If it is working correctly you should get about 11.5-12 Volts at the Glow Plugs.

After 20-30 seconds the Glow Plug Relay internal Timer should turn the Relay off and you should no longer get Voltage at the Glow Plugs.
The above in Blue is normal operation of the Relay.

If you did the above stuff in Blue and you did not get Voltage pull the Connector on the Relay that has wires that go through the Fire Wall.

The #1 Termnal should have a Red Black wire. That may be a Red wire with a Black Stripe. Stick the + Probe into the #1 Socket and ground the Negative Probe and have someone turn the Key to the Preglow Positon. When that happens you should get Battery Voltage. If you don't you have an ignition Switch problem.

Now for the discaimer. I have pieced this together from stuff I have read others do.
Also I have not mentioned the Glow Plug Light because what is important is if the Relay is operating. The light is just an indicator; in fact the Glow Plug Light could be burned out.

Here is the other info on the Connector that has Wires that go to the Fire Wall:
Glow Plug Wire colors on the square Connector and where they go.
Here is a pin out description:

1)rd/bk (Black) ignition switch

2)vi (Violet/Purple) On turbo models goes to a Wire Terminal/Junction block and mates with a White Wire coming up from the Starter Solenoid. When you Crank the Starter + Voltages goes through the White Wire to the Violet one and turns the Glow Plug Relay On.

3)blu/wt (Blue with white strip)glow indicator Light

4)br (Brown)ground

 

I did all the tests in the described link. Mercedes Diesel Glow Plug Repair

Test of Glow Plugs and Glow Plug wire to the Relay 1-5 ... Good
Test of power to the Relay .... Good
Test of power Across the Fuse in Relay ... Good

Test of Power of the terminals That connect to the wires to the glow plugs
when Ignition is first turned on ..... Bad no power

I wanted to know if The other wires that control the relay can be tested.

Could a bad glow plug light bulb cause the relay not to work?

Could a bad connection on 2)vi (Violet/Purple) On turbo models goes to a Wire Terminal/Junction block and mates with a White Wire coming up from the Starter Solenoid. When you Crank the Starter + Voltages goes through the White Wire to the Violet one and turns the Glow Plug Relay On.

Can the (Violet/Purple) be tested? If it's not sending the relay a signal how would the relay know when to turn on?

 

The Violet Wire is there to turn your Glow Plug Relay on so your Glow Plugs are on when the Starter Motor is cranking.

The Voltage comes up from the Solenoid by way of that White Wire to the Violet Wire.

Easy to test the Violet Wire. Clean the tops of the 2 Screws to the reaf of the Termal Block off so you can make good contact with them. Both of those screws have Posative Voltage from the Battery; use your Meter to verify that.

Loosen and remove the Screw that holds the White and Voilet Wire Down. Carefully touch the Voilet Wire to one of the 2 Screws to the rear of the Termanl Block. You should here your Glow Plug Relay Click on.
If you don't your Glow Plug Relay is no good or you have an open circuit in the Viiolet wire.

 

It will work with a non-functioning lamp. The purple wire that goes to the starter solenoid essentially tells the relay to turn off once an attempt has been made to start the engine. That way the glow plugs do not continue to burn after you've started the car if the 30 second time limit is not reached. When you have the small connector off see if you have 12V between the red/black wire and ground when the ignition is on. If you do then the relay is likely bad. If you don't then you need to track down that open circuit.

 

Here is the Breakdown of the smaller socket wires and pins from the relay
Please verify accuracy.

Terminal 50) Cranking input. vi (Violet/Purple) On turbo models goes to a Wire Terminal/Junction block and mates with a White Wire coming up from the Starter Solenoid. When you Crank the Starter + Voltages goes through the White Wire to the Violet one and turns the Glow Plug Relay Off.
Terminal 50 input is to interrupt the glow cycle following engine start.

Terminal 15) Run start input. rd/bk (Black) ignition switch from the starter.

Terminal 31) Ground. br (Brown)ground

Terminal LA) I Pre glow lamp indicator yellow light. blu/wt (Blue with white strip)glow indicator Light


So what is important Is to test the rd/bk wire that hooks to terminal 15 and see if there is power when I turn the Ignition switch to the glow position. I tested and got nothing... So the relay is not getting the Turn on signal... I double check and verify tommorow .... It's to dark outside.

Thanks, Ross

 

So what does the ignition wire tell the Relay? To turn off when the key returns to its resting spot after starting? So the violet wire should have 12 volts when the glow position is on before starting...

sorry for being such a newb

Ross

 

Awsome between you and 300dman this has been so much good information. I'll keep you up to date on what actually fixes my system...

 

Nice job on the video -- thanks for posting it.

 

Repaired !

Replaced glow plugs good
Checked ohm to the plugs good
Checked the voltage to the Glow plug relay. 12v good

Diagnosis Bad Relay...

Got a used one for less than 50 bugs on ebay.
Changed it out in less that 15 min.
Turned the key and the glow plug system was working.
Waited untill it got really cold and BINGO it started up really nicely.

Project done. Ty guys !!!!

Ross

 

Got me so motivated, after replacing nozzles, I looked for why startup was so rough ... Found number 2 GP was bad ...getting GPs without pulling intake manifold painful, but doable on my 91 SDL ...WAY easier startup and between new nozzles and replacing the one GP, WAy less smoke!

 

BTW, my GP light was not coming on, but apparently most of the plugs were getting hot since the engine fired right up - rough, but did fire quickly. Prior to repair, the glow light stayed off after startup for about 30 seconds and then came on for about 30 seconds ...

After replacing #2 GP the glow light comes on in correct switch position and then goes out after startup.

 

I have an 85 300D, and live in Southern California. The temp. rarely gets below 50 degrees. I noticed my glow plug indicator light is not working, and the car struggles slightly to turn over. Are there any long term effects on the engine if you continue to run the car w/o warming the glow plugs/engine prior to starting the car?

 

The issues that I would be concerned about would be the battery and starter.These cars are old, and it would add a little strain on the starter every time. And, the battery is stressed a bit more on every start.

The other issue I would be concerned about is the "you never know" factor. There is always a slight chance that you would have an unseasonably cold day and the car won't start. It would make me nervous.

It may just be that a couple of the glow plugs have failed. It is an easy fix.

Merry Christmas!

 

Hello almost a decade later
This was helpful to me, thanks.
I managed to get hold of an earlier engine compartment-based relay (dated 1979 on the back) and wanted to test it on the bench. It has the strip fuse lying along the length rather than crosswise, and a black plastic housing with three anchor points. No part number -- probably on the cover which is lost.

I made up 5 14V lightbulbs to connect from the glow pins (G1-G5) to ground.


1. I ran all 5 "glow plugs" in case it figured out that some of my "plugs" were blown,
2. I found that I needed to connect a 4.8V lightbulb between ground and the temp sensor pin (NTC) before the glow light functioned properly (5V registered between the sensor and ground)

There is a "B" terminal which I couldn't figure out, one among the six smaller pins and one among the 7 fatter ones (is it for the oil temp sensor?)

Then everything went as it should.
When I powered up 15, on came the glow light and the "plugs", glow light turned off telling me to take the next step
I touched another +ve lead to 50 and off went all the "plugs"

When I left 50 alone, it powered off after a while.

Nice to have a working spare.

 

Hello almost a decade later
This was helpful to me, thanks.
I managed to get hold of an earlier engine compartment-based relay (dated 1979 on the back) and wanted to test it on the bench. It has the strip fuse lying along the length rather than crosswise, and a black plastic housing with three anchor points. No part number -- probably on the cover which is lost.

I made up 5 14V lightbulbs to connect from the glow pins (G1-G5) to ground.


1. I ran all 5 "glow plugs" in case it figured out that some of my "plugs" were blown,
2. I found that I needed to connect a 4.8V lightbulb between ground and the temp sensor pin (NTC) before the glow light functioned properly (5V registered between the sensor and ground)

There is a "B" terminal which I couldn't figure out, one among the six smaller pins and one among the 7 fatter ones (is it for the oil temp sensor?)

Then everything went as it should.
When I powered up 15, on came the glow light and the "plugs", glow light turned off telling me to take the next step
I touched another +ve lead to 50 and off went all the "plugs"

When I left 50 alone, it powered off after a while.

Nice to have a working spare.

(edit LOL- just realized it was you 300Dman who made the above post. you must truly be "(D)ah man")

I know ..... old thread...... question here is ......... many say in cold weather to cut/disconnect the purple wire so it does NOT turn off the glow plug relay when key is released from "start/crank positon" allowing plugs to stay lit (compete there time cycle if any left) while engine is running..... but that would seem to mean you also loose GPlugs being on during cranking..... sooooooo ?????????

I have no idea about disconnecting or cutting the purple wire. Once the engine starts, I am happy, and the burning fuel is going to do the heating.

Maybe someday I will work up enough curiosity to test that.

The later than 1984 models that had glow plug relays that stayed on depending on the coolant temp (the afterglow feature) after the engine started have different glow plugs. Never looked into the details on that.

 

somewhere I read the burning fuel .... particularly when engine cold produces some heat yes.... but the glow plugs a good bit more. Something like 400f vs 1900f in the pre-chamber

 

I have no idea about disconnecting or cutting the purple wire. Once the engine starts, I am happy, and the burning fuel is going to do the heating.

Maybe someday I will work up enough curiosity to test that.

somewhere I read the burning fuel .... particularly when engine cold produces some heat yes.... but the glow plugs a good bit more. Something like 400f vs 1900f in the pre-chamber

Not sure if you are joking or not.

The green arrow is the injector and the direction of injection of the fuel. Inside of the pre-combustion chamber the red arrow points to the ball pin. The ball pin is made of a heat-resistant metal and the injector fuel stream in part sprays on it. When your engine is warmed up that ball pin retains the heat of combustion for your next injection and if the heat of compression does not ignite the fuel for sure the hot ball pin will.

So, after you start your engine, it takes a small bit of time for the ball pin to heat up fully.

The key to things on Mercedes is to understand how the individual components work. For all of the faults in it the Mercedes Factory Service manual usually tells you thar.

 

Thanks for that. previously understood this info and the drawing gives even more clarity.

what I am not clear on is what part of post you quoted you think I may be joking / having a huge misunderstanding about.

Info you post and the drawing seem to support the idea that the GP just after initial start up can heat the prechamber ball (red arrow) quicker "initially"in that first minute or two of engine running than the combustion process can. Which also would explain why MB on the last of the OM617 models added the "after glow" systems into the Glow Plug Relay/Regulator

 

The glow plugs don't heat the prechamber ball.

The glow plugs heat the air in the prechamber and some minimal heating of the metal in the prechamber. However. When you start cranking (with the correct wiring) the glow plugs are back on and the glow plugs are close enough to the fuel spray to ignites the atomized fuel.

The burning fuel heats the prechamber ball/ball pin.