Glow plug weirdness - PeachParts Mercedes-Benz Forum

smitherz · #1 10-28-2011, 07:13 PM

Ok, I've been thru more posts about glow plugs on here than I care to remember. I have not seen my issue and not sure where to really troubleshoot it.

Back in the summer I noticed my glow plug dash lite would not come on anymore but it started fine since it was warm out. Now not so much.

I had gotten a set of plugs back then but was not enthused about changing them. Now that it is getting colder I am having lets say "issues" starting in a timely fashion. The other day I was fiddling with my key and the dash lite came on and seemed to glow almost properly. The time was too short but it lit. After it lites the first time I can cycle the key to off and back to glow repeatably and it will lite, but it still does not stay lite anyway near 30-35 seconds I expect. Each time I head out after it sits I have to fiddle with the ignition key cycling it back and forth a few times then the lite will come on.

Should I be looking at the relay, ignition switch or actually change the plugs? If its the relay where is it? Under the dash or hood?

This is on my '83 300D.

tangofox007 · #2 10-28-2011, 07:20 PM

Quote:

Originally Posted by smitherz

...but it still does not stay lite anyway near 30-35 seconds I expect.

Your expectations need to be trimmed by about 20-25 seconds, assuming you are referring to the indicator light and not the duration of the preglow cycle.

barry123400 · #3 10-29-2011, 09:51 AM

From your description the key switch is suspect. I would temporarily substitute a glow controller just in case the controller is requireing many activations for it's timer to trigger on.

First of all though at least resistance check all the glow plugs and install replacements for any open ones you might find . This eliminates any feedback issues that may be present from them. Highly unlikely but as they say leave no stone unturned.

SD Blue · #4 10-29-2011, 10:22 AM

I'm thinking you have one, or more, glow plugs on their last rounds. Since you already have them in hand, you could eliminate that side of the equation. The light being flaky coming on, with initial key activation, is a pretty good sign of this. When the light cycles, it basically only tells you that all 5 glow plugs have initiated the glow cycle.

As far as the light is concerned, you may be putting too much reliance on it. From what I've seen, it normally only tells you the glow plugs are initially activated and is not on during the entire glow cycle. Unfortunately, I believe this is a short-coming of the circuit design of the controller.

I have also seen a battery going flat cause the glow plugs to fail to initiate their cycle. Checking battery voltage during the glow plug cycle usually points to the problem. If it drops too low, when the glow relay engages, your battery may be barely hanging in there.

tangofox007 · #5 10-29-2011, 10:40 AM

Quote:

Originally Posted by SD Blue

As far as the light is concerned, you may be putting too much reliance on it. From what I've seen, it normally only tells you the glow plugs are initially activated and is not on during the entire glow cycle. Unfortunately, I believe this is a short-coming of the circuit design of the controller.

When operating normally, the duration of the light cycle is a function of ambient temperature. The light being on indicates that the necessary preglow requirement (based on elapsed time) has not been satisfied. Obviously, the "programmed" time requirements are based on optimum performance of the preglow system and do not account for weak plugs, carbon deposits, poor compression, retarded IP timing, etc.

Brian Carlton · #6 10-29-2011, 10:54 AM

Quote:

Originally Posted by smitherz

Should I be looking at the relay, ignition switch or actually change the plugs? If its the relay where is it? Under the dash or hood?

This is on my '83 300D.

You should be performing a test on the plugs.

Do you have an ohmmeter that can read properly down to 1 ohm? If so, you can perform a test to see which plugs (if any) are not performing.

Changing plugs is not difficult if you remove the hard lines. I did two of them on the SD last month. Takes no more than 30 minutes.

If you leave the hard lines in place, it takes two hours...........quite a bit of that time is used to find the 6mm nuts that have fallen beneath the vehicle or in the abyss between the IP and the engine.

smitherz · #7 10-29-2011, 10:57 AM

I guess I'll be giving my wrenches a workout today. Not really what I had planned on doing. I did tell my father when I bought this car that I wanted a car I "could" work on but not so much "had" to work on.

Oh, well off to the garage.

Thanks y'all.

smitherz · #8 10-29-2011, 11:01 AM

Quote:

Originally Posted by Brian Carlton

You should be performing a test on the plugs.

Do you have an ohmmeter that can read properly down to 1 ohm? If so, you can perform a test to see which plugs (if any) are not performing.

Changing plugs is not difficult if you remove the hard lines. I did two of them on the SD last month. Takes no more than 30 minutes.

If you leave the hard lines in place, it takes two hours...........quite a bit of that time is used to find the 6mm nuts that have fallen beneath the vehicle or in the abyss between the IP and the engine.

Yes, I do have a good meter and electronics background. Remove the hard lines..... hmmmm... guess it will take 2 hours since I'd rather not dribble fuel in my garage today and I can't do it in the driveway. It is sleet and freezing rain right now and 38 degrees. I have the heat in the garage.

tangofox007 · #9 10-29-2011, 11:06 AM

Quote:

Originally Posted by smitherz

Remove the hard lines..... hmmmm... guess it will take 2 hours since I'd rather not dribble fuel in my garage today...

In that case, a fine-tooth ratcheting combination wrench will make things much easier.

Brian Carlton · #10 10-29-2011, 02:32 PM

Quote:

Originally Posted by smitherz

Yes, I do have a good meter and electronics background.

Perfect.

Find the glow plug relay. From memory, it's on the driver's side fenderwell. It's a bit larger than a cigarette pack and has a removable cover.

On the side of the relay is a plug with six sockets. Pull the plug out of the relay. In the plug will be five actual sockets and one empty hole.

Hook up your meter with one side to ground and the other side to each of the five sockets. The meter should be set to resistance and the scale should be one ohm.

Each glow plug should read between .8 and 1.25 ohms.

What you are likely to find is that some plugs will show open. These must be replaced as they are doing nothing.

Removal of the hard lines will spill negligible fuel.

funola · #11 10-29-2011, 03:45 PM

I can change #1 through #4 glow plugs in less than 5 minutes each without removing the hardlines in my 83 300DT. #5 takes longer. The tricks are:

1. Make the wires easy to remove, i.e. without having to remove the 6 mm nuts and drop them in the process

). You can do that by making the "ring" lugs into "fork" lugs with diagonal cutters. Better yet, make them into quick disconnects like I did which takes less than a second to get the wire off.

2. To remove the glow plugs, use a combination of gear wrenches and 1/4" drive socket with various extensions and universals.

Diesel911 · #12 10-29-2011, 04:06 PM

Quote:

Originally Posted by smitherz

I guess I'll be giving my wrenches a workout today. Not really what I had planned on doing. I did tell my father when I bought this car that I wanted a car I "could" work on but not so much "had" to work on.

Oh, well off to the garage.

Thanks y'all.

Test your new Plugs before you install them. It can be done with the Multimeter or by connecting the briefly to your Battery with a Jumper Cable.

Clean the Carbon out of the Glow Plug Holes with a Glow Plug Reamer or one of the alternative Methods in the DIY Repair Threads; it will help make starting easier and your new Plugs will last longer.

Use only Bosch or Beru Glow Plugs.

DIY Repair Links
http://www.peachparts.com/shopforum/forumdisplay.php?f=82
http://www.peachparts.com/Wikka/DoItYourSelf

funola · #13 10-29-2011, 04:14 PM

If you have an electronics background, you will know that most cheapo DVM's will not be able to measure resitance accurately in the 0.1 to 1.2 ohm range repeatably. I have come across glow plugs that measured 0.9 ohms yet would not glow when connected to a battery. It is better to measure the current drawn by a glow plug to check if they are good or not, which I am sure you know how to do.

psaboic · #14 10-29-2011, 04:38 PM

Mercedes Diesel Glow Plug Repair This link should help you out quite a bit

smitherz · #15 10-29-2011, 06:51 PM

Thanks to everyone for all the tips and help. Dash lite lites. The real test will be tomorrow when I actually go to start it.

I got out my handy-dandy Beckman DMM that had not see light in maybe 10+ years and stuck a battery in it. It was not happy and showed all sorts of bad behavior. I was able to get it to mostly work but not nearly as well as it should. I pulled the relay plug and started poking around anyway with it.

I got really lucky... it was the #1 and #2 cylinder glow plugs. The #1 was completely open and #2 was showing very different than all the rest or the new ones so I pulled it as well. It sure was nice of MB to label the socket with the plug numbers.

I did check the wires before proceeding anyway.

I do know that I am going to buy an 8mm and 12mm ratcheting box wrench next time I am in town. I was able to fish a passthru socket on some universals and get them out.

Without this forum I am not sure I would be able to own one of these cars. I'm an electronics and computer geek so this is a bit different even though I grew up with a piston aircraft mechanic father.