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  #1  
Old 03-16-2012, 08:56 AM
scottmcphee's Avatar
1987 w124 300D
 
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Location: Edmonton, Canada
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wiring fault causing glow plug relay to stick shut?

I just realized my glow plug relay (GPR) has been getting stuck (or re-pulls) to glow position and has toasted nearly all my GP's. Before I slap another GPR in there, I'd like to be sure it's not a wiring fault on the control harness.

Is there a condition (or combinations of conditions) that can happen on the GPR's small connector group of wires during a drive cycle that would cause the relay to be held shut for way too much time or re-pull during the drive, and thereby burn out the plugs? Like an intermittent fault on a wire, re-triggering the GPR into "thinking" this is a new glow cycle? Or the initial cycle is missing its "finish" signal so it stays on indefinitely?

This is a stock 1987 300D glow plug set up, the GPR has the internal temp sensor. I have a working dash indicator for the glow plugs, that comes on once at the beginning of a start cycle then goes off after 10 or so seconds for the remainder of the trip. So, an auxiliary question is, if I'm getting a re-glow triggered by some input fault wouldn't this show up on the dash indicator as another glow cycle?

The fact the light doesn't come on again seems to me the GPR is staying stuck on as a result of the initial startup glow cycle, rather than re-triggering on its own. So if a wiring fault is causing this, it would be evident and pretty constant near the beginning of a start. In other words, I should be able to test for the faulty condition with the car just sitting there and going through it's first start cycle.

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Last edited by scottmcphee; 03-16-2012 at 09:06 AM.
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  #2  
Old 03-16-2012, 09:15 AM
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Given that you have a good selection of identical vehicles, the easiest approach might be to swap the suspect relay into another car. If the problem remains, you will know that the issue is with the relay unit, not the wiring.
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Old 03-16-2012, 09:49 AM
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Quote:
Originally Posted by tangofox007 View Post
Given that you have a good selection of identical vehicles, the easiest approach might be to swap the suspect relay into another car. If the problem remains, you will know that the issue is with the relay unit, not the wiring.
there is something not right with this statement...

either the problem will move to the car with the relay...
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Old 03-16-2012, 10:32 AM
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1987 w124 300D
 
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I get the idea of swap around, and do this whenever possible but my other 2 cars are non-runners right now. I do have 2 x spare (used) GPR's on hand, so ya I don't have to buy the part...

My pain point is having to change the GP's in my daily driver, being a 603 the manifold has to come off (don't tell me otherwise I've been there). 5 out of 6 GP are dead. Putting in another GPR and having it toast a new set of GP's would not be nice.

Just want to rule out *possible* wiring fault issues in my car. Maybe a fault is causing my existing GPR to stay stuck, and it would do the same for any replacement GPR.

Staring at the insides of the GPR that just came out I see relay contacts have overheated and purpled the copper, and slightly melted some nearby plastic. The fact I now have 5 dead (open circuit) GP's tells me this condition was a result of cooking the GP's until they failed open. There are no fault-to-ground issues on the GP's and a good 80A fuse tells me this was not an over-current situation. The relay was spending too much duty cycle time in the closed position. The circuit board in the GPR looks mighty fine... nothing toasted or loose solder... pretty inert stuff what could fail on that board, capacitors maybe.

Been getting rougher and rougher starts over last few weeks. The strange thing is the GPR dash indicator bulb showed good glows each time... until "yesterday" the indicator stayed dark on startup cycle, and she did not fire up easily at all (because now 5 out of 6 were bad). I am amazed it actually did start after 3 long cranks and huge billows of unburned smelly diesel smoke.

Sidebar: I'm pretty disappointed in the usefulness of the dash indicator. Somewhere I read the "rules" of what the indicator is trying to tell when it lights up... but having 5/6 bad for it to not light is pretty lame indicator. Inside the relay I see special attention is paid to current going to GP #1 and GP #6 and somehow 2 through 5 are treated as a group. I'm just trying imagine the order of failures so the indicator finally showed a fault when 2 through 6 are now dead. But no fault yesterday. How many were alive then?
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Last edited by scottmcphee; 03-17-2012 at 09:49 AM.
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  #5  
Old 03-16-2012, 10:57 AM
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Quote:
Originally Posted by vstech View Post
there is something not right with this statement...

either the problem will move to the car with the relay...
The statement is completely correct.

If the problem moves to the car with the relay, then it remained with the relay.
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Old 03-16-2012, 11:43 AM
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right, got the idea ... let's move this discussion forward now with content please
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  #7  
Old 03-16-2012, 11:58 AM
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To paraphrase Yogi Berra, you can observe a lot just by listening. It's easy to tell if a relay is opening or not at the end of its cycle. Perhaps you could install another relay in the problem vehicle and just listen to see if the relay clunks open at the end of the normal glow cycle. If the original relay doesn't open and the replacement does, that would point pretty strongly to a relay issue.
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Old 03-16-2012, 12:19 PM
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One suggestion for diagnosis would be to install a small light in the passenger compartment to tell you when the glow plugs are energized. Any small 12 Volt lamp connected between any one glow plug and ground will illuminate when the glow plugs are energized and go out when the relay clicks off. If the light comes back on while driving then you know the relay has clicked back on, even though it should not.

The relay should not be able to re-energize until the ignition switch has been turned off and then back on. The famous "violet wire" from the ignition switch to the preglow relay tells the relay that the engine start cycle is complete. The preglow relay then turns off. If this wire or the contacts in the ignition switch have a problem, the pre-glow relay could be fooled into thinking it's time to glow.

The "glow light" I describe above will also let you measure how long the relay is staying on, if it is staying on. Early 124 preglow relays without afterglow are supposed to turn off after about 30 seconds. If the light shows you that the relay is turning on and timing out after 30 seconds, then turning on for another cycle a random time later, the problem could be in the violet wire circuit. OTOH, if the preglow relay is turning on and staying on, the problem may actually be a failure in the relay itself.

Jeremy
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  #9  
Old 03-16-2012, 03:19 PM
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Originally Posted by Jeremy5848 View Post

The relay should not be able to re-energize until the ignition switch has been turned off and then back on. The famous "violet wire" from the ignition switch to the preglow relay tells the relay that the engine start cycle is complete. The preglow relay then turns off. If this wire or the contacts in the ignition switch have a problem, the pre-glow relay could be fooled into thinking it's time to glow.
The violet wire has no involvement in cycle initiation. The wire that becomes energized in RUN serves that function. An intermittent connection in that wire would have the same effect as cycling the key switch from RUN to OFF to RUN. Ordinarily, the preglow lamp would illuminate if that occurred.
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  #10  
Old 03-16-2012, 04:35 PM
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This advice is what I was looking for.

Intermittent on pin 1 ("ignition line") could re-trigger. But in this case I would expect to see a glow indication each time it did it. Since I'm not, sounding like a stuck on problem, or lagging timeout.

I like the bulb idea. I will use the existing dash indicator and move its supply at the GPR over to the output of the relay to the GP's.

I showed the relay to my trusted local indy shop owner. He glanced at it, and immediately identified it as a part for 1987 even though the cover was removed and no part number was on it. He saw the purpled copper and darkened contact pad and said "stuck on relay, contact sticking or timer fault but either way, this relay is done." This guy is the absolute guru of MBenz around here and worked for Benz at a dealer for mega years before opening his own shop. Turns out, he's the one that implemented an early afterglow "fix" on several MBenz model years and after consulting with Benz they worked afterglow into the product line model year.. think he said '90?

So I order my 6x Bosch plugs from him, he'll have them this afternoon. And says, you can borrow my reamer for the weekend. I didn't even ask. (He's not working on any Unimog's this weekend because one of his freshly rebuilt OM603's tipped over on his hand yesterday and nearly severed his finger - ouch showed me the bandage and picture of the stitches he took with an iPhone.)
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Old 03-16-2012, 04:47 PM
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My understanding of pin 2 violet wire is that it's the transition from high to low that stops the current glow cycle.

I plan on bench testing the relay simulating a start sequence to see what it does. I have a hunch with no load on the relay output it will cycle normally. But if I were to put 60 A load on there it might stick. We'll see.

Anyway, I'll feel much more comfortable having the dash indicator showing the actual glow ON period.
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Old 03-16-2012, 11:35 PM
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OK, I bench tested the bad GPR, and it functions perfectly. The timer on cycle is right around 30 seconds, and the crank signal releases the relay earlier than the 30s if voltage drops on that pin. All normal operation.

Closer examination of the relay contact surfaces show there is the problem! The contacts had pitting, metal transfer, oxidation / contaminants. The extra resistance from poor contact caused heating at the contacts, which turned the copper purple and started to melt plastic in the area.

My best guess is that on occasion the contacts stuck closed too long due to temporarily welding. And this took its toll on the GP's. If contacts are stuck closed, GPs would continue to get heat even with key out of the switch.

Interesting relay design: there are two sets of contacts in parallel sharing the same armature so all close in tandem, each set of contacts carrying some of the current to power the GP's. One pair of contacts is made of a softer metal (silver?) and the other set is larger diameter and a VERY hard metal (who knows what?). The softer metal set had much metal transfer (from the armature side to the other contact). The harder set was more "pitted" than anything and had dark oxidation.

I'd like to hear a relay expert comment on the use of the different metals on multiple pairs of contacts in the same relay - it must be for some reason..

In any event I filed, sanded and burnished all relay contact surfaces till they gleamed again, and adjusted the armature leaf spring by bending to make sure all contacts meet and close at nearly the same instant. (Cleaning the softer set of contacts really increased that gap quite a bit, so adjustment was necessary.)

I don't know if you're supposed to ever clean relay contacts like this, but we'll see if it helps.

Like I say, I'm changing over the dash indicator so I can SEE a stuck-on fault if it happens again.
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Last edited by scottmcphee; 03-17-2012 at 12:22 AM.
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  #13  
Old 03-16-2012, 11:42 PM
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Quote:
Originally Posted by scottmcphee View Post

. . . stitches he took with an iPhone.)
Stitches with an iPhone? Wow, guess they can do anything!

Jeremy
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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970
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  #14  
Old 03-17-2012, 05:09 AM
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I just went through the same problem and did the same contact filing and adjusting.
I took the board out of mine to an commercial electronics sales company and was told the two contacts and different material was to control arcing on contact opening. The black contact opens a millisecond after the softer contact. He showed me a similar relay and said with a very fine measurement I would see the different space between contacts and their base. I have ordered a new GPR from Pelican Parts.
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  #15  
Old 03-17-2012, 10:13 AM
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1987 w124 300D
 
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Quote:
Originally Posted by The Gears View Post
I just went through the same problem and did the same contact filing and adjusting.
I took the board out of mine to an commercial electronics sales company and was told the two contacts and different material was to control arcing on contact opening. The black contact opens a millisecond after the softer contact. He showed me a similar relay and said with a very fine measurement I would see the different space between contacts and their base. I have ordered a new GPR from Pelican Parts.
This is awesome information, thank you! I had a hunch one of these contact pairs was "sacrificial" for taking the arc, while the other handles the "main job" of passing current in closed position. You've just answered it. The softer silver set must be better at handling arc.

I will adjust the contacts so the larger hard set touches first while closing followed by smaller soft set. And the release would be the opposite, smaller set opening first followed by the larger set.

I think I'll use pieces of paper to set the gap. One sheet held "snug" between the large set of contacts (using finger pressure to hold the relay closed) should allow one sheet to slip around between the smaller set, and maybe two sheets between the small set would become snug. Then test by pulling the relay coil with 12 volts, and check that both sets of contacts are firmly gripping one sheet of paper each.

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