Another starting issue on my 4.5 - Mercedes-Benz Forum

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post #1 of 12 (permalink) Old 01-24-2016, 08:19 PM Thread Starter
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Another starting issue on my 4.5

Hi All
A few weeks ago I fitted the pertronix ignition module 1885. It fired up lovely and ran nicely for about a week. I went to start the other day and it just cranked over. It did fire at first, but ran very lumpy and very low power. I could barley move it. Now it just cranks, and every second or two try's to fire.
What I have checked so far.

!. I checked with stroboscope all plug leads were getting power... Pass.
2. Removed all plugs (noticed they were all wet and smelled of gas) cleaned and refitted after checking I did have spark to the plugs.... pass
3. I also removed the return line at the tank and cranked it over and got a good flow back....pass.
4. changed out an inline fuel filter I fitted after tank, but before pump....pass
5. Removed cold start valve and tested that it was activating with a 12 volt power supply....pass
Any other suggestions would be great.
I did notice that the two wires going into the cold start valve were worn through in a couple of places and appeared quite brittle. (Possibly these have touched and caused a short?) If so, is there a fuse? Would this completely stop the car from starting?
Also should I check the points that control the injectors in the distributor? I did not go that route yet because I do seem to be getting fuel at the injectors indicated by wet plugs.
I am sure I have fuel delivery and a good fat spark.
What am I missing?
Thanks Guys.
John.
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post #2 of 12 (permalink) Old 01-24-2016, 09:01 PM
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Frustrating I'm sure. With spark and fuel present the following can be eliminated: fuel and master relay, fuel pump (maybe), ignition coil, plugs and wires. The pertronix should be okay since it worked after you installed it. I'd double check connections on the pertronix as well as checking to see if the pertronix module isn't overheating. Also remove distributor cap to make sure nothing is amiss.

Beyond that it could be bad electrical connections at ECU and/or at injectors, clogged injector(s), incorrect ignition timing or faulty ECU.
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post #3 of 12 (permalink) Old 01-24-2016, 09:55 PM Thread Starter
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What would indicate the petronix overheating? I don't think its that since I'm getting a nice fat spark. I suppose to check the injectors I can attach a noid light to check they are getting the signal? The car does seem to behave like it's out of gas? Trying very hard to fire but not quite getting there.
In the next few days I will be able to get out there again to give it another whirl Weather permitting.
Cheers John.
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post #4 of 12 (permalink) Old 01-25-2016, 07:53 AM
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One quick check would be to pour a small amount of fuel through the throttle body while trying to start. If it starts and runs a second or two, fuel. If not, probably spark.

Did you remove the entire old MB ignition system, or did you leave the old MB switchgear in the loop? I've seen it done both ways. Just asking as the failure of my old stock ignition switchgear was gradual over the course of 2 weeks...just kept getting weaker spark and less power each day.
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post #5 of 12 (permalink) Old 01-25-2016, 08:16 AM
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Quote:
Originally Posted by TZ_280SEL View Post
One quick check would be to pour a small amount of fuel through the throttle body while trying to start. If it starts and runs a second or two, fuel. If not, probably spark.



Did you remove the entire old MB ignition system, or did you leave the old MB switchgear in the loop? I've seen it done both ways. Just asking as the failure of my old stock ignition switchgear was gradual over the course of 2 weeks...just kept getting weaker spark and less power each day.

+1

Also, did you replace the cool with a flamethrower or something? I think you can use the stock coil, but I believe people have reported much better results with something like a flamethrower coil.

The 107 forum has lots of great information on using pertronix 1885. I've never done it myself (and my only d-jet at this point has an Allison/crane unit in it already, running with no issues).

Maybe I will try and point the 107 d-jet / pertronix gurus over here.

Personally, I am thinking spark is the issue of the plugs are wet. Sometimes plugs will look like they are sparking well, but under the fuel and compression they will end up far weaker inside the cylinders.

I had a w114 that looked like it was sparking ok when I pulled the plugs out and grounded them and took a video of the spark. It looked ok, but it turned out that it was actually a VERY weak spark and I didn't know it. A new (used) Bosch coil solved my problem. Might you have a spare coil you could swap in for a test?

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post #6 of 12 (permalink) Old 01-25-2016, 01:07 PM
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check your rotor to see if it got 'cooked'... my 75 450sl 's rotor got burnt on the one edge...the leading edge... because my timing was WAY out after the install...was firing out on the tip of the rotor...install a new rotor and then check the timing...
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post #7 of 12 (permalink) Old 01-26-2016, 04:43 PM Thread Starter
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Thanks guys for all the suggestions.
I raced home today to recheck everything.
!. I cranked it over again and removed the plugs. They are wet with fuel and smell of gas. So I'm defiantly getting Petrol to the injectors.
2. Tz... what you said seems to be what is happening here. I rechecked spark with a spark tester and they are sparking, but seem to be much weaker than when I first fitted the 1885 petronix. I ran the car for about 10 days to 2 weeks before this started.
3. The rotor and cap etc seem to be in good condition. no burning etc.
4. I used the original coil not the flamethrower. I guess I could pull the coil from my 280 to check.
Thanks Guys I'm going back out to stand next to the car and look bewildered
Cheers John.
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post #8 of 12 (permalink) Old 01-26-2016, 05:37 PM Thread Starter
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Ok..I think I've found the culprit. As suggested by a few of you I checked the coil. I tested the primary side on my 280se which is running fine and I got a reading of 1.8ohms. The one on my 4.5 I read only 0.3 ohms? So I figured it was shot. My question is. By fitting the petronix did I just magnify an already failing coil? Or did the petronix damage the old one in some way? I'm a little unsure here. Also should I fit a flamethrower as suggested or the stock blue bosch coil?
Thanks everyone for all your help.
Cheers John.
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post #9 of 12 (permalink) Old 01-26-2016, 05:40 PM
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Still not sure how you installed wired the Pertronix.

If you bypassed the switchgear, a Flamethrower coil would be best choice.

If you still have switchgear in system, then original coil is retained.

The wiring for the two alternatives are in the link below. Which one do you have?

http://www.benzworld.org/forums/r-c1...witchgear.html

1972 350SL 4.5L - MBGraham, near Kingston, Ontario
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post #10 of 12 (permalink) Old 01-26-2016, 06:15 PM Thread Starter
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Hi Mb Graham
I have it wired like the second diagram. The thing is that the instructions that came with the 1885 had me connect the black wire to the negative side of the coil? Which is different to the second diagram you posted.
Did I just screw something up here? It did run for a while. As I said the coil now is showing very little on the primary side 0.3 as opposed to my other car.
Thanks John.
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