Hello all, It's good to be back, seems my old profile was gone after all these years! I've been battling with this issue for many years, and have yet to find a solution.
For those that don't remember me:
Vehicle: ML320 CDI 2007 W164
Location: South Africa (currently winter 0 to 5'C coldest)

Problem: Difficult to start the car when cold especially the last 2 months.
Since I got the car (2013) the engine would spin and only start if the battery was fully charged. In the last 2 months, trying to start the car normally, the engine will spin and eventually stop without starting. Diesel smoke out the exhaust. If I connect a jump starter to the jump start points in the bonnet I get a 50% start rate. I have changed both of the batteries and tested them 100%, Checked the positive wiring from the battery to the alternator, to the glow plug controller. Checked the earth cable from the chassis to the engine, checked the earthing point for the glow plug controller on top of the engine. I have tried jumper cables directly from the battery to the jump start points. I have replaced the glow plug controller with the correct model from BERU. At one point I replaced 2 glow plugs as per Star diagnostics. I only have a simple ODB2 scanner and get no useful codes from it.

The glow plug light on the dash remains on, as it always has on this car, this was never a problem until the last 2 months. Since I got the car there seems to be an electrical power problem as I do not drive the car daily and I need to keep the car on a trickle charger otherwise the battery drains to around 60% over about a week. This may or may not be related.

I have reduced the problem down to the following: Glow plug/relay/control unit related problem of some sort. I connected an analogue voltmeter to a glow plug and noticed that the glow plug controller does not send power to the glow plugs. By adding a jump starter to the jump start points I get a 50% start rate and I could see the glow plug controller send power to the plugs at the very instant the engine starts. This power is so short that the meter does not even reach the voltage before the voltage is gone. A digital meter is too slow and doesn't even register a voltage. It seems that whatever tells the glow plug controller when to ignite is faulty. I'm not sure how this works, I understand that there is a temperature input, whether this comes from a sensor or the ECU I'm not sure.

Does anyone have any suggestions? I'm going to check the glow plugs for their 0.3 ohm now from the glow plug relay connector and see what I get.
Thank you!

 

Codes must be read in all modules with a scanner.

 

Update! I found 1 faulty glow plug. pulled it out, it looks terrible, is this normal? It measures far too high (12 ohms) compared to the others at around 0.9 ohms. I also discovered that there are only 9 wires connecting to my glow plug relay. 6 Glow plugs (pins 1,2,3,4,7,8) ,a ground wire (6) and 2 more wires (pin 5 and 12). Does anyone know where these go to? I still suspect the glow plug relay is not getting the instruction to fire the glow plugs, causing the problem. This instruction must come from these 2 pins (5,12)

Looks bad to me!

Very corroded connector.

4.4V

 

Try this before starting the engine: switch the ignition on, leave it on for approx 30sec, then ignition off and back on for the glow plugs to do glow cycle and then starup the engine. It might be the pressure loss in the fuel system. If it will start straight away, that what it will be.

 

Thank you for the response, as it turns out I believe I've found another issue, the alternator was the incorrect alternator and after replacing it, the glow plug problem seems to be resolved!

 

Thank you for this thread.