Another glow system problem rears its ugly head - PeachParts Mercedes-Benz Forum

KAdams4458 · #1 01-11-2009, 09:27 PM

These glow plug threads are sort of ubiquitous, aren't they? Well, I searched the forum for an hour and feel as if I have read billion threads, but I can't seem to find an answer to my problem. Funny, I could swear I read about this somewhere. Maybe another forum?

Anyway, here is the problem: My '77 300D has previously had a functioning glow system, but today, something is different. When I turn the key to the run position, the glow plug indicator lamp does not light, and the glow plugs do not heat. Once I turn the key to the start position, the light comes on and the plugs heat.

Uhm. Any ideas? It must be something fairly simple.

Oh, and here's a video of the issue! (I have entered the youtube age!)

http://www.youtube.com/watch?v=C7lY1DjOrlQ

jt20 · #2 01-11-2009, 09:49 PM

there is a large strip fuse in the GP relay box. Check to make sure it is not cracked or melted - this means remove it completely to inspect.

KAdams4458 · #3 01-11-2009, 09:53 PM

Quote:

Originally Posted by jt20

there is a large strip fuse in the GP relay box. Check to make sure it is not cracked or melted - this means remove it completely to inspect.

That can't be it, as the plugs do glow when the starter is cranked. This has been confirmed with a DVM.

I'm mostly baffled at this point, and can only guess. Maybe a bad ignition switch?

kerry · #4 01-11-2009, 09:53 PM

I initially posted on the strip fuse but deleted it once I reread the OP closely. The plugs are working in the start position so the strip fuse must be good. I'm guessing an ignition switch malfunction.

jt20 · #5 01-11-2009, 09:54 PM

sorry, misread that. Thought there was a 'no-heat'

JimmyL · #6 01-11-2009, 10:07 PM

Remember also, this is the early style of glow plugs, where I don't believe there is the same type of glow plug relay, and if one plug burns out they all burn out. This doesn't appear to have anything to do either way with your problem though.
I also would suspect ignition switch issues.....
Possibly somebody more versed in this style of glow system might know of a different issue specific to this model.

KAdams4458 · #7 01-11-2009, 10:08 PM

Quote:

Originally Posted by kerry

I initially posted on the strip fuse but deleted it once I reread the OP closely. The plugs are working in the start position so the strip fuse must be good. I'm guessing an ignition switch malfunction.

Yeah... I got the notification in my inbox.

Most people have no-glow issues, I have a weird glow problem. I can see how people used to dealing with the common issues might jump to a conclusion first, and then realise that something else is up.

I'm just hoping someone else has seen and corrected this problem so that I'm not going in to this blind.

kerry · #8 01-11-2009, 10:16 PM

The only possible sources of the problem I can imagine are the relay or the ignition switch. I suspect that the connections or wiring associated with the glow position are bad. There is additional wiring associated with the start position. This must still be functioning correctly.

KAdams4458 · #9 01-12-2009, 07:39 PM

Okay folks, I need some assistance with this one. Short of removing every component and testing it to find the issue, I do not know what to do. I could locate and repair the problem that way, but it would be time consuming.

I would like to find a copy of the glow circuit wiring schematic for the series glow plug system in my 300D. Whether or not this exists in the manual on CD I purchased, I do not know, because the disc is MIA thanks to our move. I think it was lost, and may need to be replaced.

Can someone either confirm the schematic exists in the manual so that I can order a new one, or post up the schematic for me? I'd rather work smart than hard.

Jeremy5848 · #10 01-12-2009, 08:27 PM

Most Mercedes pre-glow systems are designed to heat in the "run" as well as "start" positions of the ignition switch (as has already been said). There is only one wire to the glow plug relay from this section of the ignition switch. This wire is unfused (since the pre-glow relay has its own fuse). It is possible that this section of the ignition switch has failed (due to wear/corrosion) so that it makes contact in "start" only rather than in both "start" and "run." There are other things that operate from this switch section but the specifics vary from car to car and I don't have a 1977 diagram so I can't advise you as to what to look for.

I would first try running a separate wire from the battery to the pre-glow relay and see if the relay operates normally in this "manual" mode. If it does, you can be pretty sure that the pre-glow relay and glow plugs are good. The next step would be to remove the ignition switch and test the connections with an ohmmeter, see if you can reproduce the failure on a workbench. If the failure can be proven to be the switch, it can then be repaired or replaced, depending on your desire and ability.

Jeremy

KAdams4458 · #11 01-12-2009, 10:19 PM

Quote:

Originally Posted by Jeremy5848

Most Mercedes pre-glow systems are designed to heat in the "run" as well as "start" positions of the ignition switch (as has already been said). There is only one wire to the glow plug relay from this section of the ignition switch. This wire is unfused (since the pre-glow relay has its own fuse). It is possible that this section of the ignition switch has failed (due to wear/corrosion) so that it makes contact in "start" only rather than in both "start" and "run." There are other things that operate from this switch section but the specifics vary from car to car and I don't have a 1977 diagram so I can't advise you as to what to look for.

I would first try running a separate wire from the battery to the pre-glow relay and see if the relay operates normally in this "manual" mode. If it does, you can be pretty sure that the pre-glow relay and glow plugs are good. The next step would be to remove the ignition switch and test the connections with an ohmmeter, see if you can reproduce the failure on a workbench. If the failure can be proven to be the switch, it can then be repaired or replaced, depending on your desire and ability.

Jeremy

Clever. So there is only one wire from the ignition switch to the glow plug relay for power? That does seem like a failure of a detent in the ignition switch.

I will try a jumper wire from the battery to the relay and check it for reliable operation. I'll have to figure out which wire it is. I think it is a red/black wire? that is the only other one that runs to the relay besides the purple wire, I think.

Given this car's prior history of a damp interior, I would not be shocked to find corrosion in the ignition switch. The original gauge lamp dimmer in this car was functional after the excessive corrosion was removed - same thing for the window switches and the rear defroster switch. It is interesting to note that when I purchased it, there was a momentary contact switch hidden under the dash that spliced in to the starter motor, so maybe there have been ignition switch issues in the past.

It's getting late, but I'm going to go head out to the shop and see what I can figure out. Thank you for the help! I shall report my findings!

kerry · #12 01-12-2009, 10:51 PM

Splice that momentary contact switch into your glow plug relay and off you go.

KAdams4458 · #13 01-12-2009, 11:46 PM

Quote:

Originally Posted by kerry

Splice that momentary contact switch into your glow plug relay and off you go.

Meh, I removed it because it was a poorly execute modification - wasn't a fuse or relay anywhere. It looked more like a good way to burn the car to the ground than anything else.

I have time to do it right. The car isn't a driver at this point, and to be honest, I no longer have a lot of money to throw at it. I've just been fiddling with it more often as a form of stress relief, since the economy seems to be making it impossible to generate income these days. Low to no budget repairs like cleaning and lubricating stuff is where it's at for me at this point. Making the various parts of this car perform properly is my only real way of obtaining a sense of achievement at this time.

Anyway, I'll get back to work. I just stripped down a scrap chassis harness from a dismantled car for the jumper wire i need, and am about to solder a couple of alligator clips to it.

raslaje · #14 01-13-2009, 03:15 AM

Haynes has a wiring schematic for a 77 300D 123.130 U.S. pp234-5.

jt20 · #15 01-13-2009, 03:51 AM

http://www.peachparts.com/shopforum/showthread.php?t=82198&highlight=filament

if time is all you have, searching is an option.

here are some keywords: w115 filament relay gp series glowplug

here is a motivational thread as well. From what i have briefly read on your system, faulty gp's can effect the light. Read about how to test them accordingly.

Your gp relay may be in the dash