Glowplug light works, glowplugs do not. 1985 300D - PeachParts Mercedes-Benz Forum

netboy · #1 02-26-2014, 08:38 AM

1985 300D will not start. Ran fine, shut off one day and would not restart.
Glow plug light comes on and goes off as normal so I assumed that system was good, so I assumed it was compression or fuel problems since those are the 3 things needed to start.
Changed fuel filters and cracked fuel line at top of injector.....getting a squirt of fuel so fuel deliver system is good.
Must be compression right?
So I do a valve adjustment and find only 3 valves that are a little tight.
Keep in mind the glow plug light comes on and goes off as it always has since day one but I pull a glow plug out anyway to check it and find no continuity from tip to tip so its bad....

What I found was every glow plug was bad with the light working perfectly.
I always assumed the relay would indicate bad glow plugs but evidently not.
Hope this helps somebody else...........

.

jake12tech · #2 02-26-2014, 08:52 AM

Nice troubleshooting without jumping to conclusions. Most would have jumped to a silly conclusion of it might be the glow plug relay and throw away money in the toilet. How long was it since you replaced the glow plugs? Most times previous owners don't even record the mileage intervals when new ones were installed or the receipts were thrown in the trash and it makes rather annoying going through the work to figure out if that's really the problem.

Diesel911 · #3 02-26-2014, 09:00 AM

Did you check the Glow Plugs a crossed the Battery Terminals to see if the heated up at the tips?
If they don't heat at the tips first they are also no good.

I thought you checked the continuity from the Electrical Terminal end to the Threaded body of the Glow Plug.

In the DIY Repair Links there is various ways to ream out or clean the Carbon from the Glow Plut Holes.

Zacharias · #4 02-26-2014, 10:09 AM

On an '85 in theory if the first glow plug is bad, the glow plug light shouldn't illuminate. Also it is relatively rare for all the plugs to die at once, or a batch to (which is what would have to have happened, assuming it started okay one day and not the next).

In my experience, the problem is sometimes that over the years the relay is replaced by one that 'works', but isn't the correct part number.

In my case, I had purchased a used relay that turned out to be a post-glow relay (plugs stay engergized for 75 seconds after the light goes out, to assist with warmup). Unfortunately my relay began to stick and remained on all the time. It eventually took out a set of plugs one year old, and killed an already iffy alternator and battery before I figured out what was going on.

Your mass plug failure makes me wonder if possibly you have the same scenario. Look at the part number on your relay and then google the part number along with the relay manufacturer name and you should get returns showing what model it fits.

What brand of plugs were installed, that burned out?

tangofox007 · #5 02-26-2014, 10:23 AM

Quote:

Originally Posted by Zacharias

On an '85 in theory if the first glow plug is bad, the glow plug light shouldn't illuminate.

The light indication is a function of the "balance" of current through the #1 plug and the sum of the current through the remaining plugs. If all plugs are bad, there is no imbalance to be detected.

Quote:

Originally Posted by Zacharias

Your mass plug failure makes me wonder if possibly you have the same scenario.

For sure, the probablility of all five plugs failing simultaneously is off the chart, absent some secondary factor.

tangofox007 · #6 02-26-2014, 10:34 AM

Quote:

Originally Posted by jake12tech

Nice troubleshooting without jumping to conclusions.

What conclusion has been reached?

Random, simultaneous failure of all five glow plugs?

ah-kay · #7 02-26-2014, 11:04 AM

Glow plug light works on the principle of detecting current in all N glow plugs, similar to the brake light warning light. I always measure the GP resistance at the big connector inside the GP relay. Remove the plug and poke the holes to ground. It always works. I do not remove the GP to do a visual or measure it on the bench. It is too much work and not necessary for me, IMO.

It is indeed very rare to have all 5 failing at the same time. $hit happens so it is always a possibilities. Have you replaced all GP and the car started as expected?

btw: I also had a 'slow' GP which comes on/glows with a delay. It will not turn on the GP light in this scenario.

funola · #8 02-26-2014, 11:06 AM

Quote:

Originally Posted by tangofox007

What conclusion has been reached?

Random, simultaneous failure of all five glow plugs?

I think a reasonable conclusion would be the ohm meter was not functioning correctly or something along those lines.

netboy · #9 02-26-2014, 11:29 AM

Well let me preface this by saying I have five of these so I thought I had seen it all but I guess not. I didn't know that the relay compared ohms between plugs. It is possible that that part of my relay that detects resistance has failed or all five failed at once. I did jumper the old GPs across a batt with no heat and found opens in all old GPs from tip to tip, new ones will show continuity. As far as my trouble shooting, its just that I'm too cheap to shotgun parts at my cars. I will admit that I do not own a reamer a need one. I have to record all my maintenance in a garage book because a couple of my odometers don't work so I change oil every 4 months cause my teenagers drive the wheels off these cars.
I will verify the GP relay part# Zach. I know '85s are different in ways but didn't know that about the relays. I have owned this car for about 5 years and that's the last time I did a valve job and R/Rd the GPs.

netboy · #10 02-26-2014, 11:33 AM

My reason for the post was to let the next guy know not to trust the lights, but go ahead and physically pull and test the GPs.

funola · #11 02-26-2014, 11:59 AM

Quote:

Originally Posted by netboy

1985 300D will not start. Ran fine, shut off one day and would not restart. Glow plug light comes on and goes off as normal so I assumed that system was good,..................................................................

The way I interpret this is that your glow plugs were fine one day and then all 5 burned out the next day? If indeed true, I would suggest checking that your alternator is not putting out excessively high voltage or your glow plugs may be on all the time.

lovinthedeez · #12 02-26-2014, 12:30 PM

I bought some junk plugs this winter (autolites

) and all of them burned out in less than 3 weeks. Glow plug light didn't stay on until after the fourth one was burnt. Car got tougher and tougher to start until....nothing. The only one working once I got some duraterms was the old beru plug in the #5 cylinder that I knew was working through my ohmmeter

The bosches were 50 cents more a plug than the autolites.....lesson learned

netboy · #13 02-26-2014, 01:14 PM

The way I interpreted the GP light working as usual and the eng not starting was the GPs were good and so I went elsewhere to find the problem .........I will never assume that again. I will put a meter across the batt today and check GP relay part #

Zacharias · #14 02-26-2014, 03:21 PM

The postglow relay I referred to in my reply was never supplied as original equipment on the w123s. It was sold back in the late 1990s and early 2000s as an upgrade kit from Bosch that also included Duratherm GPs.

Also I should point out for info that the problem I had only occurred below a certain temperature. At that point, the relay just locked itself on. It was early winter, with fluctuating daily temperatures, so is seemed like an elusive problem for while as it came and went and the relation to temperature didn't tell me anything initially.

Also I made it sound, in my reply, that the relay sticking that way was a function of the postglow setup. That was slightly misleading. All the relays stay on longer than the dash light is illuminated, and I don't know if the postglow aspect makes that model of relay more prone to that failure mode than would be the case with other part numbers, or not.

Incidentally, the replacement relay (used) that I put on was a conventional one, and it had the opposite problem: it refused to activate below about zero farenheit unless I opened the hood and pressed down firmly on the small plug. A friend of mine later told me he has the same issue with his SD, only his requires a firm tap on the side.

Over the years I have had these cars I have had very few relay issues, but some of this stuff is simply getting a bit old now.