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Old 01-26-2008, 09:01 PM
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Join Date: Jan 2008
Location: Portland, OR
Posts: 4
Glow Plug / Block heater interesting problem / Pin assignment

First time post... Wondering if anyone has had a problem with block heaters and GP operation... Just bought an 80 300td that has been kind of riddled with strange problems. Bought cheap because the previous owner was tired of gremlins. Also wanted to post pin assignments for this vehicle's GP relay as I wasn't able to find them when I was searching.

A week ago no start on the MB. Checked continuity with a mutimeter and noticed that I had two plugs out. It seems from other people that this would still allow me to fire this puppy up, but alas there was nothing.GP light would come on and then I'd wait for 30 sec, crank and crank and crank for one or two minutes but nothing. After cranking the GP light would blink. Figured that was a hint for a newbie so I went online and checked around and found this awesome forum that had tons and tons of info. A month ago I had a mechanic go over the vehicle to get it running and timed etc and he replaced one GP.

So today I pulled the four non-new GPs and tested them with the battery test. Two were completely toast, two were decent but I replaced one of them any for good measure. Why not both? Because I'm broke and $12 is a lot for me.

I also pulled out the wiring harness as I heard a crack when trying to pull plugs without taking the fuel rail off. I wanted to test it an make sure everything was good. In the process I accidentally yanked off one of the block heater connectors. It went into the pit of despair. It's not even magnetic so I can't use the handy picker-uper that I bought because I heard of this horrible pit.

The crack was just the plastic wire casing and all the connections were good. Figure, ok. Time to reassemble. I electrical taped off the broken block heater connection and put everything back together. Not too stressed about the block heater because I live in Portland, OR and it rarely gets below 30 here. Figure half a heater would be fine.

So I get everything back together today standing out in the rain, turn the key and no GP light. Not even flashing. Somehow it seems that I made the problem worse. I just read about the relay click and that sounds good Clicks about 40 sec after I get the key turned. I haven't pumped the fuel system and I'm hoping that plus a jump and I'll be good. just a little nervous about the GP light going out and I was wondering if anyone knew if that was somehow connected to the block heater... Any help would be awesome.

Pin connections for a 300td GP system with block heater:

Cyl1-> X X <-Cyl 2
NoPin - X <-Cyl 3
Block Hea1 X X <-Cyl 4
Block Hea2 X X <-Cyl 5

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Old 01-26-2008, 09:29 PM
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Join Date: Jan 2006
Posts: 172
I wouldn't worry about to gp light too much. Mine rarely works correctly. I'm a little confused by what you mean by block heater though? Did you break a pin off of the gp harness?
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Old 01-26-2008, 09:43 PM
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Join Date: Jan 2008
Location: Portland, OR
Posts: 4
I could be wrong, but I thought that the GP and BH wiring went through the same harness... no pins broken on the harness but the connector on the end of the wire broke off. Thanks for the heads up on the light -- I'm new to the game it it was just so saddening to think that I made the situation worse...
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Old 01-26-2008, 10:55 PM
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Join Date: Jan 2002
Posts: 18,355
Block heater, if you have one, is on the opposite side of the block to the glow plugs. It has a 110volt cord on it that typically exits the front of the car. There is a block temperature sensor on the same side of the head as the glow plugs. This controls when the climate control turns on I believe. On some models there is also a temperature sensor that controls how long the glow plug light stays on.
It's highly unlikely you did anything to the block heater so you need to figure out what you actually did. Connector on the end of what wire broke off?
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13
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Old 01-26-2008, 11:31 PM
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Join Date: Jan 2008
Location: Portland, OR
Posts: 4
oh man... not sure what I broke off then... Here's a diagram of the connector I was talking about previously from the wiki 617 image... I can take a photo tomorrow... Just assumed that they were the block heater connections...
Attached Thumbnails
Glow Plug / Block heater interesting problem / Pin assignment-mb-300tdfront.jpg   Glow Plug / Block heater interesting problem / Pin assignment-mb300tdside.jpg  
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Old 01-26-2008, 11:47 PM
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It looks to me like the arrow is pointing to the temperature gauge sensor.

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Old 01-26-2008, 11:54 PM
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Join Date: Aug 2005
Posts: 192
pdf 15-705

Operation of Pre-Glow System

15-705.pdf - 0.98MB

Here's a pdf file I uploaded to a free hosting site. I couldn't upload it to the Mercedes site because it's about 1 MB (too big). Scroll through the pdf 15-705 and you'll find some schematics.

Use this link to get the pdf 15-705
1985 300D Turbo
"Evolution is God's way of giving upgrades" Francis Collins

Last edited by raslaje; 01-27-2008 at 12:29 AM. Reason: Link I entered not working trying to correct.
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Old 01-27-2008, 05:17 PM
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Join Date: Jan 2008
Location: Portland, OR
Posts: 4
those schematics were great! So here was the deal... There are two connectors. From bumper to firewall they are #1 -> GP temp sensing system #2 -> Temp Gauge sensor. I just took a round crimp-on terminal and bent it over the GP Temp sensor. My GP light is on now and the GP system seems to be running longer due to the colder weather.

This may be a fix for some people who seem to have to change their Glow Plugs every four months -- if the GP system doesn't know what temp the engine is at then it seems to just run for 30 seconds blindly.

Waiting on a jump to get the baby running again but I feel good. Thanks so much everybody!

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