Mercedes 500 SL 1990

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Mercedes 500 SL 1990

Postby mikeevans » Sat Jan 07, 2017 11:40 pm

WE have a Mercedes 500 sl V8, 1990 which refuses to start. We have all the ingredients present but will not attempt to start. It's a LHD on German plates and not checked it's build date yet.
Engine cranks normally.
Good fuel pressure.
New Spark plugs.
Crank sensor resistance normal.
Good spark from coil lead into cap.
Good spark at plugs.
Good cyl compressions.
Valve timing ok
Ignition timing ok
No air inlet restriction
Removed exhaust incase blocked.
Tried pouring fuel into inlet when cranking
Injection seems ok, cold start injector ok.
Everything seems to be there but it will not start/fire/splutter-- nothing.
Any advice will be appreciated.
mikeevans
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Re: Mercedes 500 SL 1990

Postby STC » Sun Jan 08, 2017 12:33 am

Hi Mike

W107 or W129 Chassis (Bobby Ewing or Princess Diana) ?

If you do have Compression, fuel and spark then is everything happening at the right time ?

If that was presented to me I would have the scope hooked up to a Pressure Transducer in Cylinder 1 (fuel pump paralysed), #1 HT lead, Crank Sensor & A Current Clamp on Starter Current.
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Re: Mercedes 500 SL 1990

Postby mikeevans » Mon Jan 09, 2017 10:51 pm

Thanks for your reply. I don't have a pressure transducer but still have a timing light gathering dust on the shelf. Checked with this and Ignition timing showed up fine so I was reasonably confident that the spark was somewhere near.
It is now fixed, turned out to be a faulty ignition coil. I had checked this previously-
primary resistance.6 ohm
secondary resistance 12000ohm
Spark appeared to be good would jump a good 1 cm
As we were running out of ideas we opted to fit a new coil which we had in stock. It started instantly and ran perfectly.
Is there a reliable coil test to perform with picoscope or do we have to fit a new part to eliminate that item?
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Re: Mercedes 500 SL 1990

Postby STC » Tue Jan 10, 2017 5:39 am

Hi Mike.
Good that you got your fix.

Just to answer a couple of your questions, observations.

turned out to be a faulty ignition coil. I had checked this previously-
primary resistance.6 ohm
secondary resistance 12000ohm
Spark appeared to be good would jump a good 1 cm


A spark that can be seen to jump 1cm at atmospheric pressure is no indication of what it will do in the cylinder under 100psi plus. For that reason the resistance and spark tests are more or less redundant nowadays. Much better to throw the Secondary Probe around a HT lead and look at Peak KV, burn time ....... ScannerDanner is big on this and covers it well in his videos.

I don't have a pressure transducer but still have a timing light gathering dust on the shelf. Checked with this and Ignition timing showed up fine so I was reasonably confident that the spark was somewhere near.


In the absence of a pressure transducer, an amp clamp on starter current would have sufficed. Although nothing wrong with your Timing light method.

If you don't have the time to attend training courses (something I would always argue with - perhaps another day, another place) then maybe get a few more accessories to go with the Pico to save time and headaches.

My suggestion on hooking up the 4 channels earlier in the thread would / should have nailed that in 20 minutes, a fraction of the time it took you to do a conventional compression test.
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Re: Mercedes 500 SL 1990

Postby STC » Wed Feb 22, 2017 11:36 pm

This thread has woken up the "Test Everything on the Car Non Intrusively Obsessive Compulsive Disorder Syndrome" Gene in me.

The ONLY thing missing when you tested the spark was Compression Pressure at the business end of the plug, inhibiting the spark, which inevitably hindered it on its natural / designated path with the consequence being a non start engine

Watch this space, I have an idea that can test, eliminate or confirm that Spark Issue in Situ. Just need to test it and prove my method is Viable, Reliable and Accurate beyond doubt !
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