1976 300D Wont start - Mercedes-Benz Forum

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post #1 of 40 (permalink) Old 05-03-2010, 08:35 PM Thread Starter
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Post 1976 300D Wont start

Hi guys
i know its a needle in a haystack
but cant get the old girl to fire up.
It turns over so starter is good.
Compression test reads + 300 on all cylinders.
Tried new battery. No change
New glow plugs- we still get turny turny but no starty starty.
checked for vacuum leaks around the pipes around the engine. There is a Y piece with 1mm X 2 breather tubes going to it but the third tube is missing off the Y. other than that -everything seems to be all in order.
electrics indide the car seem to be a little hit and miss- no lights/electric windows work/ vacume locks not working.
The Throttle cable which connects over to the round thing on the left body fender looks a bit loose, what is that thing i wonder?
any input here would be great thanks
could it be an electrical short or something?
did these old benzes have ECU ?
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post #2 of 40 (permalink) Old 05-04-2010, 06:28 AM
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Okay, first things first.

Diesel engines fire on compression. The only things required for a diesel engine to run are fuel, compression and air. Compression is #1. It looks like you have the compression part at 300 psi. Next thing is fuel. last is air. Nothing electrical involved. Diesels of this era do not have vacuum controls so missing hoses won't create a "non-start" condition.

Which style of glow plugs do you have? The small "pencil" style or the larger ones with thick metal loops interconnecting? Do you have the "gorilla knob" on the dash for the injectors?

The method for starting your car with the knob is to turn the key to the "run" position, pull the knob out to where the red engine light dims for 30 seconds and THEN pull the knob out to engage the starter.

The object on the fender well is probably the cruise control servo.

Last edited by Mike D; 05-04-2010 at 06:30 AM. Reason: added some more useless advice
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post #3 of 40 (permalink) Old 05-04-2010, 07:57 AM
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Doesn't sound like fuel is getting to the cylinders. Verify fuel is getting through the filters without air leaks and ultimately to the injectors. You can verify this by loosening the injector lines and cranking the motor until fuel drips out and then retighten them. Yes it's a mess but it saves a lot of time and eliminates the fuel delivery variable.

A W115 300D has a key ignition so the timer relay if working would be cycling. Don't burn up your battery on the GP's if you aren't certain you have fuel to the injectors.

1977 - W116 - 6.9
1974 - W115 - 240D /8 - 4(M) OM617.910 + Turbo + 300D FW - Completed 7/8/10
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post #4 of 40 (permalink) Old 05-04-2010, 08:44 AM Thread Starter
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thanks for the replies guys
i'm gonna try that today and see what happens
i really want to see the old lady fire up...

the starter does sound a little lazy when turning over even with jumper cables on.. could i need a new starter?
i read on a nother forum a guy had a similar problem to me and it turned out to be the starter
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post #5 of 40 (permalink) Old 05-05-2010, 11:51 AM Thread Starter
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thanks for all the ideas.
i really want to get this car started
Actually
I pulled out the glow plugs today, and they are supposedly new (according to last owner who was poking around with this car) i put my ohms meter on them and there is absolutel now continuity or even a little resistance on them (between wire shaft and body thread of the plug) i even hooked wires from the battery to them and nothing is heating up so i guess these need replaced.
Also i was looking for the GP heater relay and i cant find it if indeed there is one. I read on another fourm that it is located on one of the fenders but i cant find it. I did find a relay under the dash steering column (pictured the silver thing in the 1st picture)-is this it? if indeed there is one?
i couldnt find the knob to pull out as mentioned in your earlier thread.
also while i had my sherlock homes hat on and i was poking around under the steering column area i found 2 aftermarket switches (1 a red flip switch and the other a push bell type switch)) and a kind of solenoid looking thing which has an access at the top which looks like it would accept a key or thin object some sort. anyone any ideas?<br>
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Last edited by 50 cool 50; 05-05-2010 at 11:55 AM.
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post #6 of 40 (permalink) Old 05-05-2010, 01:55 PM Thread Starter
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the glow plugs are the loop type

incidently, the silver relay box i pictured has 60 amp written on the side of it

does anyone have a picture of a 76 glow plug relay box??

thanks guys

i cant wait to get this baby started!

Last edited by 50 cool 50; 05-05-2010 at 02:19 PM.
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post #7 of 40 (permalink) Old 05-05-2010, 03:02 PM
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timing chain?

Former MB: "Miss Juliana" - 1972 280SEL 4.5
Current MB: 2013 C250
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post #8 of 40 (permalink) Old 05-05-2010, 03:48 PM
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Quote:
Originally Posted by 50 cool 50 View Post
thanks for all the ideas.
i really want to get this car started
Actually
I pulled out the glow plugs today, and they are supposedly new (according to last owner who was poking around with this car) i put my ohms meter on them and there is absolutel now continuity or even a little resistance on them (between wire shaft and body thread of the plug) i even hooked wires from the battery to them and nothing is heating up so i guess these need replaced.
Also i was looking for the GP heater relay and i cant find it if indeed there is one. I read on another fourm that it is located on one of the fenders but i cant find it. I did find a relay under the dash steering column (pictured the silver thing in the 1st picture)-is this it? if indeed there is one?
i couldnt find the knob to pull out as mentioned in your earlier thread.
also while i had my sherlock homes hat on and i was poking around under the steering column area i found 2 aftermarket switches (1 a red flip switch and the other a push bell type switch)) and a kind of solenoid looking thing which has an access at the top which looks like it would accept a key or thin object some sort. anyone any ideas?<br>
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The silver thing is indeed the glow plug relay. Do you have the parts number of the box?

The old loop-style glow plugs must not be directly connected to a power source, such as a battery. They don't like that very much.

The resistance of an individual glower should be close to zero, if there is an infinity reading, the glower is not good. And one bad glower shuts down the pre-glowing all together. The entire procedure for checking the glowers and relay can be found here:
15 Electrical System Engine (Preglow) - OM617
and there select 15-510, and look at section A.

I cannot help you with the aftermarket switches. The first one looks like an alarm. Sometimes owners bypass an original system because they cannot make the original system work.

A good starter is essential, certainly on a 300D. You might consider taking the starter apart:



and check the brushes:

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post #9 of 40 (permalink) Old 05-05-2010, 05:23 PM Thread Starter
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Quote:
Originally Posted by Govert70227 View Post
The silver thing is indeed the glow plug relay. Do you have the parts number of the box?

The old loop-style glow plugs must not be directly connected to a power source, such as a battery. They don't like that very much.

The resistance of an individual glower should be close to zero, if there is an infinity reading, the glower is not good. And one bad glower shuts down the pre-glowing all together. The entire procedure for checking the glowers and relay can be found here:
15 Electrical System Engine (Preglow) - OM617
and there select 15-510, and look at section A.

I cannot help you with the aftermarket switches. The first one looks like an alarm. Sometimes owners bypass an original system because they cannot make the original system work.

A good starter is essential, certainly on a 300D. You might consider taking the starter apart:



and check the brushes:

thanks for the info about the glow plugs and starter... where would i get new brushes if i needed them???. i really am going to get to the bottom of this non starting problem if i have to rebuild the whole damn car.

why cant the glowers be connected to a direct power source? does the relay increase the resistance or change the voltage or amperes before it goes to the glow plug apart from adding a time to the pre heat process?

just asking because if my relay is shot i would hate to spend a ton of money on a new relay and i could probably design a circuit myself with modern day components that would work as a relay

i will let you know the part number of the relay in a few...

p.s. turns out the battery on the car was goosed as well
so the new battery i got should help although i would imagine this is due to the last guy troubleshooting the problem and leaving the car sitting up and ruining the battery .....

Last edited by 50 cool 50; 05-05-2010 at 05:26 PM.
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post #10 of 40 (permalink) Old 05-05-2010, 07:13 PM Thread Starter
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mike would the pull out start stop knob mentioned earlier be located in the middle console to the right of the ashtray ?



Uploaded with ImageShack.us
i got a pull knob there that lights up when i pull it as seen in the pic bottom right... its strange that the engine turns with the key though like a conventional car does... i never started a 70's benz before and as i got the car non running i am feeling my way here... would i be right in thinking maybe you pull this to preheat the glowers, then pull it all the wa out and hold it while turning the ke or something??

the glowers relay is part# 0015451832


Uploaded with ImageShack.us

i found another 3 pin relay lay abondoned in the glove box..n ot sure what thats for...


Uploaded with ImageShack.us

the after market switches - the thing i thought was a soleniod turned out to be a rotary switch for the blowers with 3 speeds! so i'm guessing the red flip switch next to it is a.c
i traced the door bell thing wires back and they are not connected to anything...

got a new set of glow plugs coming tomorrow and i'm gonna clean the glow plugs relay with circuit board cleaner. maybe it will work.

Last edited by 50 cool 50; 05-05-2010 at 07:21 PM.
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