W140 afterglow and voltages - PeachParts Mercedes-Benz Forum

JHZR2 · #1 01-16-2022, 12:19 AM

I have a new to me 300SD with the 3.5L OM603. It starts strong when fully cold in low teens weather. I have a few observations I’d like some info on:

1) the glow plug light doesn’t illuminate when preglowing. Instead, it comes on for about 15s after the engine starts… sometimes it does this. Not all the time…. Some reading indicates that this may mean that at least one plug is burned out. Anyone else experience this? I’ll check the plug resistances tomorrow.

2) I know we had a recent afterglow discussion regarding the 617. I know the 603 can do afterglow. When I start the car, the voltage stays at 11.9 for a few minutes. Even if the temperature in the water goes above 40, the voltage will stay down at 11.9, then all of a sudden when driving, when going fast, say over 2000-2500 RPM it will pop up to 14.1V and be perfect for a long time until the next cold soak. Does this seem right? Or is my afterglow going/staying on?

Just want to shake out a few loose ends with all this.. thanks!

psaboic · #2 01-16-2022, 10:54 AM

Sounds like you may have a small issue. For Question #1 I had that issue a few years back on my W123. The glow light would not come on until a few seconds after starting and then intermittently after that. My problem turned out to be the glow plug relay, but in your case definitely check out the glow plugs first. Question #2 It does sound like you have an afterglow relay. Mine does the same thing whether I am driving or sitting at idle, and yours should too. My suggestion is since you know it goes to full voltage after a few minutes of driving/hitting higher RPM, now see if it does it while at idle. Again, my voltage will be about 11.9ish for 3-5 minutes then pop up to about 14.1 whether I am driving or just letting it idle.

dieselbenz1 · #3 01-16-2022, 10:59 AM

I believe glow light not coming on during a glow then on afterwords indicates a burnt out plug. Your after glow should remain on for 90 seconds when it's cold out.

JHZR2 · #4 01-16-2022, 11:33 AM

Quote:

Originally Posted by psaboic

Sounds like you may have a small issue. For Question #1 I had that issue a few years back on my W123. The glow light would not come on until a few seconds after starting and then intermittently after that. My problem turned out to be the glow plug relay, but in your case definitely check out the glow plugs first. Question #2 It does sound like you have an afterglow relay. Mine does the same thing whether I am driving or sitting at idle, and yours should too. My suggestion is since you know it goes to full voltage after a few minutes of driving/hitting higher RPM, now see if it does it while at idle. Again, my voltage will be about 11.9ish for 3-5 minutes then pop up to about 14.1 whether I am driving or just letting it idle.

The 603 has afterglow that ties in with a coolant sensor.

What I observed was that with a warm-ish start in cold weather, with sustained idling, where the temperature got over 40C, the alternator voltage never picked up. I need to put a current sensor on the GP leads to see if it’s that. What I know is that giving more rpm on the engine to excite the alternator did not help. But as soon as I drove and took the car over 2000-2500 rpm (and probably some more heat but not much, it happened right away), the alternator perked to 14.1V rapidly.

psaboic · #5 01-16-2022, 01:02 PM

Ok, based on the new info, after checking the glow plugs, I would suspect (in order) either the coolant sensor you mentioned, the glow plug relay, or a wonky alternator.......it is obvious to me that the glow plugs are staying on until you reach a certain temperature or RPM as they are the only thing I know of that would cause that much of a voltage drop when on.

To see if it may be the alternator needing higher RPM to deliver proper voltage, disconnect the power lead to the glow plug relay, start the car, (might need a warmish engine to do this easily but you said it still acts weird with a warm engine) and read the voltage at the battery. If the voltage is around 14 volts at idle the alternator is ok.....if it still takes high RPM to get the proper voltage then I would suspect the alternator as you have taken the glow plug system out of the equation.

JHZR2 · #6 01-16-2022, 11:59 PM

Quote:

Originally Posted by psaboic

Ok, based on the new info, after checking the glow plugs, I would suspect (in order) either the coolant sensor you mentioned, the glow plug relay, or a wonky alternator.......it is obvious to me that the glow plugs are staying on until you reach a certain temperature or RPM as they are the only thing I know of that would cause that much of a voltage drop when on.

To see if it may be the alternator needing higher RPM to deliver proper voltage, disconnect the power lead to the glow plug relay, start the car, (might need a warmish engine to do this easily but you said it still acts weird with a warm engine) and read the voltage at the battery. If the voltage is around 14 volts at idle the alternator is ok.....if it still takes high RPM to get the proper voltage then I would suspect the alternator as you have taken the glow plug system out of the equation.

So I kind of did this test today. Was working on the car, started it up, saw low voltage, let it idle for a while, and watched. No change.

Granted, the engine was warmed up, but I took the engine up to about 2500 rpm and then the voltage started to rise and it went to the correct voltage. Battery is just days old.

psaboic · #7 01-17-2022, 09:54 AM

Sounds to me like the alternator is not charging properly until higher RPM, Can you do the voltage check again with the glow plug relay unplugged just to se if it is not staying on for some weird reason? Outside of that, I suggest you check the alternator belt for proper tightness and if it is ok and the issue still exists, I would pull the alternator and get it tested. Hope I was able to offer some help, electrical issues can be challenging some times....