Well my 240D glow plugs failed this morning - Page 3 - PeachParts Mercedes-Benz Forum

toomany MBZ · #31 02-23-2015, 08:49 AM

Using the plug itself is fine, just more cumbersome.

To make sure they are operating correctly, remove the plug and apply 12 volts, only the tip should light up. Pulling the injectors seems to be much more work than necessary.

funola · #32 02-23-2015, 10:35 AM

Quote:

Originally Posted by pj67coll

Ah I see. I was actually putting the red tip on the plugs themselves. I'll try using your method and see what happens.

- Peter.

What plug are you talking about? Glow plug or the glow relay plug?

Did you unplug the glow relay plug when doing the ohms measurements? That's important!

funola · #33 02-23-2015, 10:53 AM

Quote:

Originally Posted by BillGrissom

.......................
Re the GP relay box, I was going to swap the one out in my 1985 today, but I couldn't find the spare box I had set aside. It is getting tricky to get the GP lamp, and usually (always?) when I don't get the lamp it truly is hard to start. It could be the key switch is going since seems I must get it in just the right spot to get the lamp, but swapping the GP box is much easier than the key switch (3 dang screws are hard to get at).

Over about a year I tried to sort out similar intermittent issues on the glow relay of my 85 and in the end, determined it was a flaky glow relay, gave up and converted it to manual and couldn't be happier. Should have done that from the start and not waste the time spent. Internal schematic of the glow plug relay is not available and I was not able to trouble shoot and repair it (note: I have a degree in EE and electronics was a hobby).

pj67coll · #34 02-23-2015, 07:00 PM

Quote:

Originally Posted by funola

What plug are you talking about? Glow plug or the glow relay plug?

Did you unplug the glow relay plug when doing the ohms measurements? That's important!

I meant I'd placed the red probe on the glowplugs themselves. I had not unplugged the glowplug relay. I'll try again doing it this way and seeing what happens.

- Peter.

funola · #35 02-23-2015, 07:27 PM

Quote:

Originally Posted by pj67coll

I meant I'd placed the red probe on the glowplugs themselves. I had not unplugged the glowplug relay. I'll try again doing it this way and seeing what happens.

- Peter.

If you did not unplug the glow plug relay, then you were reading all glow plugs connected together and would not be able isolate which one is bad (if one or more is bad). You said you are getting 2 to 4 ohms? That's a strange reading with glow relay connected. I would have expected around 1 ohm if at least one glow plug is good and infinity (open circuit) if they were all burned out.. Glow pugs must not have power to them when taking resistance measurements.

pj67coll · #36 02-23-2015, 08:14 PM

Ok. So trying to troubleshoot everything systematically. I first tried reading the voltage at each glowplug by the method. Black probe to ground (by fuel filter), Red probe to the glowplug itself with the key in position II. ie the glow cycle should be operating. No sign of a glowplug light in the dash but my readings, starting from front of the engine towards the back assuming the front is #1.
11,43V 11,36V, 11,37V and 11,36V.

Each time I held the probes in position till the relay clicked off and the voltage on the meter went to 0V.

I wanted to re-check these results before moving on to testing Ohms etc but before I could do that the damn black cable on the meter good hooked on something and pulled out of the probe.

I printed out another coupon, started the car - started immediately but real rough so increased Idle knob to max for about 30 sec's. Went to HF and got another free tester. Second round of testing Voltage at the glow plugs themselves with the new tester.

11,73V 11,6V 11,6V and 11,5V.

Again meter went to 0V at each plug when the relay klicked off.

Wifes' just come back with the dinner so I gotta go. I'll try to do the Ohm testing later this evening if I can get to it. But from these figures it seems that I'm getting good voltage at the plugs am I not?

- Peter.

pj67coll · #37 02-23-2015, 10:43 PM

Ok, so I did some ohm testing here. Never done this sort of stuff before so I hope I've read the posts here correctly.

First I tested the internal resistance of the multimeter across the two probes by touching their ends. It was 1,2 Ohms. I read that you need to do this and subtract it from the actual readings to get the correct result. Is this correct?

I used the same methodology as for testing the voltages. Using the ground by the fuel filter and the red probe on the tip of each glow plug. Readings from front to rear as follows. First reading is direct from the meter, second in parentheses is with the 1,2 ohms subtracted.

1,5 (0,3) 1,3 (0,1) 1,3 (0,1) 1,6 (0,3)

I then pulled the smaller plug, the pigtail that goes back to the ignition and measured the resistance inside the four plugs therein. They have numbers on them. Not sure what they mean but #50 measured 1,7. #31 1,3. #15 12,4 and #L6 18,2.

Finally I pulled the large plug, that goes to the glow plugs themselves and found the following resistance. #1 = 67. #2 = 1,7. #3 = 1,8 and #4 = 1,7. I tried several times again on #1 but still got a reading that fluctuated between 65 and 69.

So. I don't really know how all this ties together. Seems to me that the plugs should be glowing but the rough start and lack of dash light would indicate otherwise. Might it be best to start by replacing the relay?

- Peter.

funola · #38 02-24-2015, 12:02 AM

The 67 ohm reading on #1 is not right. Snug up the 8 mm nut (do not over tighten) and measure again. Could also be corrosion on the wire crimp or solder joint on the large connector which can be inspected by removing the plastic cover.

pj67coll · #39 02-24-2015, 07:53 AM

Quote:

Originally Posted by funola

The 67 ohm reading on #1 is not right. Snug up the 8 mm nut (do not over tighten) and measure again. Could also be corrosion on the wire crimp or solder joint on the large connector which can be inspected by removing the plastic cover.

To which 8mm nut are you referring?

- Peter.

Partscarver · #40 02-24-2015, 08:02 AM

(which 8mm nut?) the ones on the end of each glowplug.

pj67coll · #41 02-24-2015, 08:29 AM

Oh I see. Ok thanks. I'll try that after work this afternoon then.

- Peter.

pj67coll · #42 02-24-2015, 09:16 PM

Ok. So. I snugged the 8mm nuts on all the glow plugs. Removed the fuse and reseated it. Removed the power lead to the relay to see if it looked like there was a problem with it. There doesn't seem to be. The soldering and the clamp seem tight and the wire is still good. Plus I'm getting 12V thru that power lead. I checked the Ohms' in plug number 1 which was giving me 67 yesterday. It was giving me 58 this time but when I tried the resistance at the actual glowplug itself it also gave me 58 Ohms. So is it a bad glowplug? or a problem in the plug harness itself? Or should I just spring for a new relay and see what happens?

- Peter.

funola · #43 02-24-2015, 10:50 PM

Glow plug #1 is bad. How did you get 1.5 (0.3) ohm at glow plug #1 yesterday?

pj67coll · #44 02-25-2015, 08:01 AM

Quote:

Originally Posted by funola

Glow plug #1 is bad. How did you get 1.5 (0.3) ohm at glow plug #1 yesterday?

By grounding the black probe on the fuel filter block and the red one on the tip of the plug itself. I don't know why I'd now get such a different reading again using the same method unless it is the plug.

Why though does the glow light itself not light up at all. I thought that with the pencil style plugs it would still light even if one was bad although flashing to indicated a problem. Is my understanding of the light's operation incorrect?

- Peter.

funola · #45 02-25-2015, 09:18 AM

Quote:

Originally Posted by pj67coll

By grounding the black probe on the fuel filter block and the red one on the tip of the plug itself. I don't know why I'd now get such a different reading again using the same method unless it is the plug.

Why though does the glow light itself not light up at all. I thought that with the pencil style plugs it would still light even if one was bad although flashing to indicated a problem. Is my understanding of the light's operation incorrect?

- Peter.

Glow light in dash not indicates one or more burned out glow plug (read your owners manual). Replace #1 and go from there.

Maybe you had the glow plug connector connected when yous measured 1.5 ohms on #1? To proof to yourself why it's important to disconnect the connector is measure the 4 pins on the glow relay, you'll see they are all connected together. That means (if the connector is plugged in) when you take resistance measurements, all you need to 1 good glow plug to get good readings on the rest, i.e. the readings are not to be trusted.