Glow Plug Relay Troubleshooting... - PeachParts Mercedes-Benz Forum

Diseasel300 · #1 01-02-2017, 11:03 PM

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...

sixto · #2 01-03-2017, 12:28 AM

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

Diesel911 · #3 01-03-2017, 09:45 AM

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.

Diseasel300 · #4 01-03-2017, 10:14 AM

Quote:

Originally Posted by Diesel911

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.

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.

Diseasel300 · #5 01-03-2017, 11:46 AM

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?

sixto · #6 01-03-2017, 02:05 PM

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

Diseasel300 · #7 01-03-2017, 05:54 PM

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...

yuke · #8 01-03-2017, 07:50 PM

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.

Diseasel300 · #9 01-03-2017, 09:09 PM

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.

SD Blue · #10 01-04-2017, 06:49 AM

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

oldsinner111 · #11 01-04-2017, 07:13 AM

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

Diseasel300 · #12 01-04-2017, 10:18 AM

Quote:

Originally Posted by oldsinner111

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.

Diseasel300 · #13 01-04-2017, 10:21 AM

Quote:

Originally Posted by SD Blue

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

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)

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.

HauptmanUdo · #14 01-13-2017, 11:55 AM

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!

Diseasel300 · #15 01-13-2017, 11:59 AM

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.