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  #16  
Old 09-10-2006, 11:54 PM
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Anytime.

Help is what we do here.

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  #17  
Old 09-15-2006, 02:16 PM
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Unhappy I have the same problem

with a 1984 300TDT w/ 184000 miles

My GP light doesn't always light up when I turn the ignition, and if I try to start the engine, even if I wait like 30 sec. it has a very hard time to start (eventhough temperatures are nice right now still). So basically if my GP light doesn't turn on it sounds to me that the GP are indeed not powered up. Sometimes it would just take a few times turning the ignition on back and forth for the GP light to come on -and then the car would start fine. This morning in spite of many tries, the GP light would not come on, and I did not try to go for a hard start (I could afford to take an other ride, not like I was stuck at work or something which would have required a hard start w/o the GP). I need to mention that I have changed all but 1 GP about 6 months ago (couldn't get to the 5th cylinder and decided to postpone this until I was going to do a valve adjust.); still the old GPs tested fine and must have been pretty new (no history from PO ).
Any lead appreaciated Thanks to all (great forum BTW
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  #18  
Old 10-24-2006, 10:33 PM
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Location: Austin
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Cool Thank you

Recognizing this is a bit of an odd post, I just wanted to let everyone else know how much I appreciate this forum and the GP discussion. It's great, and your time has made me a happier MB owner. I'm learning a lot about my car *really* works.
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  #19  
Old 10-24-2006, 10:38 PM
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Location: Tuscaloosa, Alabama
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9 years and 90,000 miles??????

Quote:
Originally Posted by Rob124 View Post
Recently bought an '87 300D, and suspect a faulty glow plug--or maybe two. Between one and six key turns are required to light up the glow plug lamp. All plugs test good with an ohmmeter, and all pins at the relay have 12 volts. Is the next step a current measurement for each plug? Existing plugs have about 90,000 miles and 9 years on them. The holes were reamed when last replaced. The relay was replaced about a year before that, and there appear to have been problems with the GP light from then. I could also just replace them all and see what happens--maybe it's time?

Rob
The glow plugs should have been changed 60000 miles ago.

BenzDiesel
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  #20  
Old 10-25-2006, 01:08 AM
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BenzDiesel,

Are U saying GP should be changed every 30,000 miles? Thats ridiculous.

Do U change your rlight bulbs before they burn out?

P E H
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  #21  
Old 10-25-2006, 01:15 AM
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300TDT,

The GP light doesn't tell if the GP are working or not. Its mainly a timer. But if it blinks after starting, one or more of the GP are open (burned out).

If the GP test OK with the resistance test, they are probably OK. Did U check the fuse in the GP relay?

Do a search for cabin light and U will find out how to do a quick GP test.

P E H
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  #22  
Old 10-25-2006, 01:59 AM
Craig
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Quote:
Originally Posted by P.E.Haiges View Post
The GP light doesn't tell if the GP are working or not. Its mainly a timer. But if it blinks after starting, one or more of the GP are open (burned out).
That's not true on the (pencil type) 617, the GP light will fail to light if two or more GPs are open. On the 616, the GP light will fail to light if one or more GPs are open. I've never had a 616 or 617 GP light blink to indicate a bad GP.

I agree that the GP resistance test will normally tell you if they are functioning correctly (but there are exceptions).
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  #23  
Old 10-25-2006, 10:03 AM
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Craig,

When I get the 617 back (wife has it) that the GP light always works I will do some experiments by disconnecting some of the GP and checking what the GP light does.

I have had the GP light indicate an open GP by blinking. That's what it is supposed to do so U know its time to replace the open GP.

P E H

Last edited by P.E.Haiges; 10-25-2006 at 01:47 PM.
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  #24  
Old 10-25-2006, 12:54 PM
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I had a very similar problem here... Car will not start at all or starts hard and rough when cold,and GP light comes on about half the time. I suspected the glow plugs, but all voltage and resistance tests seemed fine (although I was only getting about 11 volts at the GP.) I went so far as to pull each plug, leave the wire connected and ground the plug with some jumper cables, so I was getting power directly from the relay and could still see what the plug was doing. Everything looked good, although I didn't get to do this to the fifth plug. It seems like the gp light is an idiot timing light. Watching the glow plugs light up, for the first 10-15 seconds or so only the tip of the plug lights up. After about 15 sec. until you hear the relay click off, the whole glow plug gets wicked hot. Starting might be a timing thing. I think last winter I was waiting a little too long for the glow plugs to warm and was still trying to start the car after the relay shut off. I meant to time the glow plugs heating cycle so I would know exactly when they get REALLY hot for optimal starting. Maybe I'll pull the last one and do that.

Quote:
When I get the 617 back (wife has it) that the GP light always works I will do some experiments by disconnecting some of the GP and checking what the GP light does.
I'd like to see those results. Maybe for some reason there's a plug that tests okay, but doesn't always light up.
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  #25  
Old 10-25-2006, 01:12 PM
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Location: Willamette Valley, OR
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Quote:
Originally Posted by P.E.Haiges View Post
300TDT,

The GP light doesn't tell if the GP are working or not. Its mainly a timer. But if it blinks after starting, one or more of the GP are open (burned out).

If the GP test OK with the resistance test, they are probably OK. Did U check the fuse in the GP relay?

Do a search for cabin light and U will find out how to do a quick GP test.

P E H
I second the suggestion about the fuse. My car is pretty much like yours in terms of the GP light, 300TDT. If the light comes on it starts up fine as if all plugs are working. If the light doesn't come on, it's a hard start as if none of them are working. Usually the light will come on if I wait a minute and try again, but yesterday I was having no luck with that and used another vehicle.

I tested the system later in the day yesterday (what do you know, it was working again when I tested it) and everything tests out good. Plugs at 8 ohms, voltage to the relay 12.6 volts, just like the battery, and all the pins giving more than 12 volts to each plug.

At this point I suspect the fuse has some tiny cracks that are, at times, preventing the electricity from getting to the plugs. I hope I'm right as that's the easiest fix.

As a side note, my car may know things that I don't know, because on my way home yesterday I was rear-ended by a Ford Expedition. The damage to the pickup I was driving seems to be very minor. If I was in the lower-sitting Mercedes, though, I might be on the phone yelling at insurance people right now. There was no damage to me, either, which is good.

Edit: By the way, 300TDT, if you don't know all the ins and outs of testing the GP system, check out diesel giant's tutorial on the subject: http://www.dieselgiant.com/glowplugrepair.htm
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1985 300D Turbo, CA model
248,650 miles and counting...

Last edited by Maroon 300D; 10-25-2006 at 01:35 PM.
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  #26  
Old 10-25-2006, 01:20 PM
Craig
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Quote:
Originally Posted by P.E.Haiges View Post
Craig,

When I get the 617 back (wife has it) that the GP light always works I will do some experiments by disconnecting some of the GP and checking what the GP light does.

I have had the GP light indicate an open GP by blinking. That's what it is supposed to do so U know it time to replace tha open GP.

P E H
I have no reason to doubt you, but mine have never done that. Every time I've had a single GP fail on the 616, the GP light would not light on startup. On the 617, I have no indication of a single bad GP but if two GPs fail the light does not come on. I've never seen mine blink. I did have a bad relay once that would cause the light to randomly come on while driving, but I fixed that with a new relay. Maybe there are some different relay designs out there.
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  #27  
Old 10-25-2006, 01:44 PM
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Jkid,

The GP relay is supposed to stay on and energize the GP for about 40 seconds no matter what the GP light does. Watch your cabin light, it will dim slightly when the GP are energized and U can tell by watching the light if the GP are heating.

U can tell how long the GP relay stays on because U can hear it click on and off. U can also time the GP relay by watching the cabin light.

11 volts may be OK at the GP. There is IR loss in the wires caused by the ~70 amperes the GP draw.

There have been posts here that the fuse in the GP relay can get a hairline, almost invisable, crack that will prevent the GP from working, sometimes intermittantly.
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  #28  
Old 10-25-2006, 01:59 PM
Craig
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Quote:
Originally Posted by P.E.Haiges View Post
11 volts may be OK at the GP. There is IR loss in the wires caused by the ~70 amperes the GP draw.
I agree, 11V is correct at the GPs. As I recall, the Bosch GPs actually have 11V stamped on them. When it's really cold, I just give mine an extra 15 or 20 seconds after the light goes out. Leaving them on is better than cycling them.
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  #29  
Old 10-25-2006, 06:09 PM
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On my 617 I had all 5 plugs test OK with a meter. But it still started really hard. Put in a new set and problem solved started right up, hmmm. Took the old ones, I think three of them when hooked to a battery only got a little warm. The others turned red like they should.


So from what I have seen even if they test OK they may not be. These looked like original plugs. So in the future when I get a new diesel if the plugs look old I'll just change them and not have to mess with it.
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  #30  
Old 10-25-2006, 08:30 PM
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Quote:
Originally Posted by Hatterasguy View Post
On my 617 I had all 5 plugs test OK with a meter. But it still started really hard. Put in a new set and problem solved started right up, hmmm. Took the old ones, I think three of them when hooked to a battery only got a little warm. The others turned red like they should.


So from what I have seen even if they test OK they may not be. These looked like original plugs. So in the future when I get a new diesel if the plugs look old I'll just change them and not have to mess with it.
Please describe exactly what tests you performed with this meter and what the results were.

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