Glow Plug Relay Troubleshooting...
 

On a 1986 300SDL, what is the NORMAL operation of the glow plug relay? My car was built July 1986 if it makes any difference (I know there was some sort of update to some of the electrical systems in "mid 1986").

Since I've had the car, the glow light has never worked properly. It'll literally blink on and right back off when the key is cycled (basically a flash). Up until recently, it would run the glow plugs for ~15 seconds then shut them off (I assume this car doesn't have afterglow). Car would always fire up and run fine since it doesn't get that cold here.

For the last week or so, I've noticed the car running really rough during the day's first startup, so I've been paying attention to the cycle. It seems on the first cycle of the day, I only get about 5 seconds of glow, subsequent cycles get about 3 seconds. I'm having to cycle the key about 4 or 5 times to get enough heat to get a smooth start on the first start of the day now.

Is there something I should be checking? Sensor or something? Or is the relay just kaput?

I've checked all 6 plugs. Bosch plugs, replaced about 6 months ago. All of them read .8 ohms to ground with the harness disconnected from the relay, so I doubt seriously there's a bad plug.

I also doubt fuel system problems. The engine usually catches on the first or second compression hit, even when stone cold. If I cycle the key enough times, I can get a smooth enough start, but up until this past week, it was doing fine without requiring any key cycling at all.

I'm not worried about being stranded somewhere since the car will start even without a glow cycle, but that grouchy start in the morning is getting old...

If it's the original glow relay, it sounds like something's gone bad. There's no external temp sensor in the original glow system. For kicks, open the small connector to the relay and remove the purple wire. That will disable the feature that cuts glowing when the starter is engaged and give the full glow cycle every time, in effect giving afterglow if the engine starts before the glow cycle completes.

Sixto
83 300SD
98 E320 wagon

If you want you can ignore the glow plug light and keep the key in the pre-glow position until the relay timer shuts the glow plug cycle witch means you get approximately 30 seconds or total glow plug time.

When the Starter is cranking the glow plug relay is activated and the glow plugs are on.

It is also possible for the a glow plug to ohm out ok and still be no good.

In the pic is a plug I had that Ohmed out ok but if you notice the tip of the plug that is the only part that sticks out into the prechamber is not the hottest part of the plug as it should be.

The definitive way to check the pencile type glow plugs is to remove them and use a jumper cable to test them with batter voltge.

This is my problem.....leaving the key in the pre-glow position has the relay dropping out after 3-5 seconds, not 30. The light has never worked since I got the car, and I've always ignored it and let it glow for ~10 secs for the first start of the day and never had a lick of trouble.

When I get off work I'll try the purple wire trick and see if there's any sort of change in behavior or if the relay is still dropping out ridiculously early.

Given that this is a 603, I'm not real excited to change the GP's again, it isn't as easy as the 61x since they're buried up under the intake manifold and behind the IP. Since I know the relay is timing out way too early, I'm inclined to start there.

Had a break and quickly did the purple wire mod. Using a stopwatch, the GP's drop out after 8.5 seconds. This at least allowed a relatively smooth start, but it was a long way from the 30+ seconds that were expected.

Pretty sure that means the GP relay is toast. I've attached a photo of it below. Is that the original relay? Is it possible to swap in a later one with afterglow?

The relay looks original. The cheapest afterglow option I found years ago was a Hueco (sp?) brand unit for a 140 SD. It has a strip fuse - OE afterglow relays have a self-resetting breaker - and a fifth small pin for a coolant temp sensor. This noticeably drops glow light time on a hot engine. I forget where else in the car a 6-pin connector is used so the upgrade looks original. Of course you need a temp sensor as well. I used a 300D 2.5 temp sensor that goes in place of the BFS 50*C switch by the thermostat (I had ditched the BFS). You can also use a 3-prong combo temp gauge and afterglow sender from a 3.5, or use one of the other three blank positions in the head with an adapter. It looks like the manifold has to come off for those options. Maybe you don't need the EDS temp sensor by the upper radiator hose fitting if you disable EGR and ARV.

Sixto
83 300SD
98 E320 wagon

So I fixed it.

Decided to take the relay apart and see what was in there. 3 capacitors. I immediately figured they were bad, and of course....they were. For anyone wondering, there are 2 100µF @ 35V and 1 7µF @ 50V.

Replaced all 3 and we're back in business. With the purple wire lifted, it glows for 35 seconds before shutting off. As an added bonus, I now have a working dash light!

Since that worked, now I don't need to worry about replacement parts or afterglow......yet...

Glad to hear you figured it out 300. I too have a problem it seems that the glow light on dash and cycle knocks off to soon under 10 sec. it seems the minute the temps. drop below 30 and the car has been sitting for 12 hrs. I have a bear of a time starting my 617 took me 20 min. other day with repeated shots of WD in the intake have been thinking about going with a manual glow of which some members have good write ups on only thing holding me back is I like the light on the dash as a somewhat indicator if all plugs are working or not. I do not think light on dash would work if I went to manual glow system. What I currently am doing is plugging car in any time temps. drop below 30 it even kicks right off at 3 below then. I do think these M.B. engines are just a bit more harder starting then say a cummings in cold weather call it the nature of the beast. If you do go to a manual glow a benfit would be only cycling it when needed for as long as needed thus lengthing the life of glow plug a real plus in your engine with them being so much harder to change then a 617.

Mine was to the point where it was just a quick flash when you'd turn the key on. The engine is healthy enough (despite burning some oil) that it's been starting, even without proper glow, just rough for those first 5-10 seconds.

It's worth it to replace the capacitors in your relay if you have the skills. Took less than 5 minutes to solder in the new caps and cost less than 50¢ in parts. Now I have a functional glow plug relay.

Of the 3 capacitors, one of the 100's measured .08µF, the other was 0.00. The 7µF measured .18µF. They were definitely toast. Seems to be a common occurrence with electronics on 80s Benzes.

Did you happen to take pics of the capacitors and internals of the glow plug controller?

relays are meant to be replaced,my new one in my sd stays on long after I start.

You sure about that? Afterglow didn't show up on MB diesels until the 90s. The upgraded controller for afterglow requires some wiring modifications to tie into the non-afterglow wiring harnesses.

Unfortunately I tend to work faster than I think. By the time I got the thing all put back together, the thought occurred that someone might want a picture.

Fortunately the internet can help. My GP relay is the style on the right (non-afterglow)

http://www.w124performance.com/image...low_relays.jpg

Note that the 2 100µF caps are the large blue cans. The 7µF cap is the red/yellow can at the end of the long black IC chip on the right hand side.

Hey Mercedes Diesel fans: I need help! The glow plug light in my 2005 E320CDI comes on and then goes out very quickly but the "Check Engine" light stays on after the car is running. Had the fault code read at Auto Zone and they showed me that the alleged fault is the "Glow Plug Relay". Have looked it up and know what it is supposed to look like. However: I can NOT find anything that looks like the picture anywhere in the engine compartment. Can anyone tell me where the relay is located for sure? I can clear the check engine light and it will stay off for one or two days but then comes back on so apparently I DO need to replace the relay. HELP!

The CDI will have a different glow plug system from the earlier cars. Your glow plug relay will likely be very different and located in a different place to the W126 car being discussed here. You may get better responses by creating a new thread with your post title highlighting the vehicle in question. Sorry I'm not more helpful, I'm not familiar with the CDI engines.

Diesel 300 where did you purchase your 3 capacitors from. Currently I am experiencing the light in my dash sometimes does not go on but if I turn the key on and off sometimes 3-4 times then the light goes on I wait couple of seconds and then start and having it plugged in couple hrs. before it kicks off pretty quickly Im not sure if it would in less then 20 degree temps. W/O plugging it in and do not want to find out and make the engine struggle in these very cold temps. we are currently having in chic. area. Just had a situation which was my fault that my fuel gelled up on me had to truck the car home and warm the snout out of it in a garage to get it started not fun. I think I want to go to a manual glow system not sure if you can rig it that the light on your dash still works to let you know if all glow plugs are working as if they are not these are hard starting engines in less then 30 degree weather any thing under 30 I plug it in had no trouble this morning starting at about 17 degrees.

The values of capacitor used in the glow plug relay are ones we keep in stock at my workplace, so I just coughed up the 35¢ and soldered them in.

You might try Radio Shack if you have one around, or being in the Chicago area, you should have an electronics supply house or two. If all else fails, try Mouser or Digi-Key, you'll wind up paying more for shipping than the parts tho.

7µF or 4.7µF, I cant find a 7 and I haven't opened up either of my relays to check. I'm going to attempt to source come caps locally and hopefully solder them in tomorrow.

I can only speak for the relay for the 603, but it was a 7µF. I used a 4.7 paralleled with a 2.2 (comes out to 6.9).

Opened mine up today and replaced the caps, I'll install it later to see if it works. Found the caps locally used a 100uf 35v and a 6.8uf 50v

Tested the old caps, both of the 100uf caps were dead and the 7uf tested at 1uf

That's pretty well how mine was too. GP relay has been working fine since the caps were replaced. Let us know if yours works.

7 uf. ?? Never heard of that one.
Standard value would be .47uf or 75uf.
Does the print on the capacitor appear to be faded, etc..?

There's some weird stuff out there. It was VERY clearly marked as 7µF, no fading or damaged text. Definitely a weird value, that's why I subbed the 4.7 + 2.2. Repair hasn't missed a beat, so it seems to be the right value.

Ill take a closer look at the ones I removed today, I didnt even bother to look when I was replacing them. So far so good with the 4 starts I've made since replacing them. I'll find out in the morning when its 20 outside. I also made a video of replacing the capacitors if anyone was curious.

Now I have no output from the relay to the dash light, I'm half tempted to cut the trace on the board and solder it to the output for the glow plugs but I'm unsure of the indicator light output voltage on a properly working relay. I do not see any voltage regulators on the board so I assume it's not a 5v output to the light.

It's a 12V bulb. On my '87 300D, I wired an additional idiot light to glow when the #1 glow plug was energized since I had an afterglow relay.

Sixto
83 300SD
98 E320 wagon

Well **** I'm just going to wire it straight to the glow plug output

Did you get the capacitors in the correct polarity when you replaced them? There is a right and a wrong way.

Yes I did, marked the board before removing the original ones

After reading this thread I realized the GP light in my 87 was only staying on for 2-3 sec no matter how cold it got. I tested the relay with the purple wire pulled and it stayed on for 38 seconds. I swapped a spare 87 relay and sure enough the light was on much longer. After taking the bad relay apart I found a couple of bad capacitors. I'll replace those, clean the contact on the relay and report back. Its good to have a spare.

Specifications of 3 capacitors?
 

First, this post-thread is closely related to this thread:
http://www.peachparts.com/shopforum/diesel-discussion/397328-35-cold-start-toronto-1985-mercedes-300d-original-engine-2.html

Second, this concerns my 1985 300CDT - W123 OM617.952 (California).

Third, I'm looking for help identifying the technical specifications for the 3 capacitors that are situated on the glow plug relay's circuit board.

From the outside of the glow plug relay unit's housing:
001-545-98-32
89 91 80
12V/5-Zyl.

I have extracted the unit and attached are three pictures.

I've used as strong a light and magnifying glass as I have to read the details on the capacitors but as I've notated on picture #1, I am having difficulty reading the details and deciphering them.

If anyone is able to help clarify the details of the capacitors, that would be great. Otherwise, baring not enough information about the capacitors, I'll reassemble the unit.

Thanks for any help with this.

The difficulty is there were different versions with different values used. Also, it appears someone has been inside this unit previously due to the green laquer. Perhaps removal of the laquer may help with identification. MEK (methyl ethyl ketone) will clean it easily but VENTILATION is paramount.

There is a thread about repair of the Glow Plug Relay. It should be easily found with a search. I'm currently on a tablet and unable to use that feature. If you would like, I'll see if I can locate it later using my laptop.

All three are electrolytic capacitors, which are used for bypass filtering, almost never for timing purposes. ELectrolytics, unlike other types of capacitors go bad with time, just replace them if they're old. One of them is identified as 47 uf, which is its capacitance value. Just replace it with any electrolytic cap that's around 47 uf rated 24 V or above and you will be fine. The numbers on the other 2 does not look like capacitance values. Take closeup pics of the 2 caps and the markings, maybe I can tell.

A free source of electrolytic caps are old cell phone car chargers, LCD TV's etc

I've taken a bunch of pictures.
Here's the link to them:
https://imgur.com/a/MM3VAjw

> The "47" was easy to clearly photograph.
> The large blue one with the green lacquer was tougher.
> The really small black one on the corner next to the Texas Instruments chip is next to impossible to get clearly.

Anyway, best I could do for the moment.

Thanks again-

With the values missing off the capacitors, you're kind of SOL unless someone else has an identical relay with legible numbers you can copy from.

If the unit is working fine, leave it alone. If it is not, then try replacing the electrolytics and see if it helps. The big one with rubbed off numbers is about the same size as the 47 uf, so replace it with a 47 uf. The small one with rubbed off numbers, replace it with a 10 uf. The exact capacitance values for those 3 electrolytics are not critical IMO, as I said earlier, they are not for timing, i.e. control the duration of glow on/ off times.

Beautiful.
Sincere thanks for sharing your knowledge. Very helpful and new learning for me.
:cool: