My strange glow plugs... - PeachParts Mercedes-Benz Forum

DieselHead · #1 03-05-2003, 08:33 PM

The glow plug problem continues. I cleaned the contacts on the relay and changed the fuse and the glow plugs worked just great. This afternoon they refused to work BUT they worked in the morning. I noticed that they will always work when the car is still warm, and I noticed that I didn't have any problems for the past week or so when the weather was dry. I just went out to use the car a few minutes ago and the plugs didn't work. Car was cold and it is damp out from the rain today. Any connections you guys can make? Oh, my mechanic tested the plugs themselves and said they were fine. This only leaves the relay or the wiring, which I think is also fine. Oh, and I have a voltimeter wired into my electrical system so I know the plugs aren't working because there isn't the usual 2V drop.

Also, I was planning on taking this car skiing friday for a day trip and don't really want to get caught if my plugs don't work. How would I go about connecting the plugs directly to the battery as some have done? What are the proper connection points? I don't want to screw it up, fry the plugs and destroy the battery.

Thanks so much! Sorry to bother you with another GP issue.

Alex

JimSmith · #2 03-05-2003, 09:07 PM

Alex,

When they don't work, do you try a few times and they still don't work, or just once or twice? Are there any other electrically operated anomalies?

The whole process starts in the fuse box and goes to the light switch and then to the key, I believe. I would start by cleaning off the fuse box connections as they are spring loaded and as they get crudded up, cooler, damper weather will make the connection there most susceptible to intermittent operation.

Since the light switch and starter switch are pretty much inside the car and protected, you might check some of those outside connections again. On my old 220D, when it was pretty new (< 2 years old) we took it to Aspen, Colorado on a skiing trip. The car became impossible to start unless we pushed it and I was getting no glow plug resonse. Turns out the wire to the plugs broke inside the insulating sleeve to the eye type connector and while it looked ok, it was not ok. I found it by yanking on the various electrical wires. The bad one broke off in the cold, I fixed it and everything was fine for about 20 years.

Your next bet is the light switch and next the starter switch. I believe the power to the starting switch, operated by the key, comes from the light switch. Over time, these connections can get worn and unreliable and the wires themselves can fatigue from becoming embrittled (usually due to getting pretty hot from use) and the typical Diesel vibration. In colder weather they can make poor or no contact, become more susceptible to fatigue, and moisture just adds to the problem (fatigue cracks become corrosion sites). Either switch can be at fault, as can the wires themselves, especially where they enter the connectors.

Good luck and I hope this helps. Jim

DieselHead · #3 03-05-2003, 09:23 PM

Jim,
Ok, I'll go look at the main fuse box now. I thought the glow plugs and the relay were complete separate from those small plastic fuses. No other electrical problems at all. JUST the glow plugs. I'll it out and report back in an hour or two.

Alex

JimSmith · #4 03-05-2003, 10:17 PM

Alex,

After reading your response, I went to consult my manuals. You may be correct in the belief that there is no fuse in the power supply from the battery to the glow plugs, and the relay operating circuit.

I only have detailed schematics for older 220D types, and they had that actual glow plug on the dash above the steering wheel, which did not involve a relay. But power from the battery gets to the light switch and from there to the key operated start/run system, without passing through a fuse of any kind.

I have a crude schematic for the early W123 models from a manual for the 1974-76 240D with a supplemental section on the W123. This schematic shows the power to the glow plugs themselves coming from the plus side of the battery and going through the light switch (terminal 30) to the glow plug relay, while the power to activate the relay comes from the light switch (also terminal 30) and goes to the key operated starter/preglow switch. This implies there is no fuse in the supply line to the light switch on these models as well. I am having a hard time believing MB would provide a hot wire to the interior of the car without a fuse but it appears they have been doing it for a while.

If you have a later model some of the details of the relay change, but I am fairly certain the power for the plugs and the control of the relay runs the same route. If you need it I can scan and post the schematic.

There is a fuse (50 A) for the glow plugs themselves, and it is mounted on the firewall on the early W123s (on the front left - driver's - side inner fender on later ones).

Sorry if my first discussion was bum dope. Good luck, Jim

fz500sel · #5 03-05-2003, 10:32 PM

I was just wondering (excuse my ignorance) but if you have a diesel with an automatic transmission and the glow plugs are having a problem, can you still get it started by pushing the car and then popping the clutch?

JimSmith · #6 03-05-2003, 10:44 PM

FrankieZ,

Well, you cannot "pop" the clutch because there isn't one to "pop" but you can tow the an automatic in neutral up to about 35 mph, then, with the key in the run position, put it in drive and it will start as if it had a clutch to pop. I believe the specifics are in the opeator's/owner's manual. Jim

DieselHead · #7 03-05-2003, 11:04 PM

Jim,
Thanks for the detailed response, and thanks for taking the time to consult your manuals. So I think that what you're saying is that I should check the ignition and headlight switches. Nuts. Sounds like work.

Alex

fz500sel · #8 03-06-2003, 12:07 AM

Jim,

What an idiot I must've seemed like. I was talking about a manual tranny but my fingers typed automatic tranny.

So I assume with a manual tranny you can "pop" the clutch and it will crank!

JimSmith · #9 03-06-2003, 01:12 AM

Alex,

If the connections under the hood to the glow plugs and relay, which you have already checked once recently are still good, then the only ones left are the light switch and the key start/glow switch. I would look at the stuff under the hood carefully and make sure there are no cracked or damaged wires that make intermittent contact. Stuff inside the insulation at or near the crimps to the connectors is where I would focus my attention for the moment.

FrankieZ,

Yes, the most reliable way to start one of these in dead of Winter is to pop the clutch. My daughter learned to park hers facing down hill when the starter was shot. She could start the car in less than a car length on a warm day. Good luck, Jim

240 Ed · #10 03-06-2003, 04:30 AM

I have found more success with using second gear, rather than first. Sometimes in first, the wheels just skid rather than turning the engine.
Oh, and don't forget to put in the clutch when the engine starts, and rev it a little.
Oh, and don't forget to turn on the key to ignition first either!

84300DT · #11 03-06-2003, 08:01 AM

alex - i assume you took off the cover of the relay and checked that big fuse inside- correct?
if you need a relay i have a spare.

DieselHead · #12 03-06-2003, 08:31 AM

I might take you up on that relay offer. Where are you located in CT? I'm in lower westchester now and through sunday I think. Thank. ALso, how much are you looking to get for it.

Alex

EDIT: Oh, and yes, I have replaced the large fuse. After I replaced that fuse it worked great for a whole week. No it's back to only working 4 out of 5 times.

84300DT · #13 03-06-2003, 08:47 AM

alex - i am in very south western part of ct. i was thinking you could try out my relay to see if yours is bad cause they cost about
140 bucks or so from the dealer

if i remember correctly.

that way you would know if you need one or not. no sense in paying for a new one just to test it.
what do you think? we are not that far away from each other..
bob in ct

edit: just saw your edit - looks very much like the relay itself is bad. lemme know what you want to do.

DieselHead · #14 03-06-2003, 08:52 AM

The problem with trying before buying is that my problem doesn't always occur. The measure of success would be going over a week without incident I think. I don't know if it would be fair to hold on to your relay for a week without paying for it. Is there a way I could come by today late afternoon? I don't know if I mentioned that I was planning on going skiing for the day tomorrow and even if I can't install it today, it'd be good to have in the trunk with the tools that I would need to install it in an emergency. Thanks!

Alex

84300DT · #15 03-06-2003, 08:57 AM

yes fully agree you are welcome to 'borrow' it for a while to see if that indeed is the problem- they are a piece of cake to install - take you 5 minutes tops.
i am here all day - but its snowing like mad out...