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opticaltrigger

Some thoughts on the 205 and it's ignition system

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opticaltrigger

Hi guy's,

Hopefully I've posted this in the rite section. If not my apologies to admin.

 

Over the years I have seen a number of posts relating to 205 ignition problems and issues with the poor old standard set up.

Anyway it kind of got me thinking and I thought that it might be of help to the community in general if I were to try and put down some sort of breakdown of it's function, condensed into a solitary post.

It's by no means intended to be some sort of definitive thesis on what is already a very established and understood study. More like my own ramblings really, based around my own studies of it.

 

Ok, so here goes.

Essentially it's no different in function to an old fashioned set of breaker points. All that has happened is that mechanical breaker points have been replaced with a few basic electronic components.

The primary purpose of this is to remove the problems that were present in the previous generation of mechanical points.

Points actually worked brilliantly, but as with all mechanical systems, wear, contamination, and environment could wreak havoc with it and as time went on manufacturers became embroiled in a relentless battle against these issues. You can see it with lots of what I call “band aid” engineering, where you start to see essentially plastic bags wrapped around connectors and such like.

 

However at some point in the very early eighties really, transistors had become cheap enough and more importantly, reliable enough to enter the mass produced automotive world.

 

Enter the ignition amplifier...............

This little unit warrants some discussion as to many it can remain this unfathomable black box of who knows what....

However it's in essence nothing more than a glorified transistor or if you like, an on / off switch.

I won't go into transistor theory here as that's outside the scope of this ramble of mine but suffice to say it's a switch, just like the points were back when the world was a much simpler place.

This particular switch however needs an electrical pulse to switch rather than a mechanical cam, and this pulse is generated in the distributor.

 

Ok,... lets just stick that bit to one side for a moment and we'll get back to it again in a bit.

Ignition coils are for the most part nothing more complex than a step up transformer.

It's easy to google a circuit for it and it really is a basic device...... When the primary coil is energised with 12 volts a magnetic field is generated, these field lines of magnetic flux move outward, through and across the windings of the secondary coil which has many, many more windings on it than the primary coil does.

If the 12 volt supply to the ignition coils primary is switched off the magnetic field mentioned earlier literally collapses through and across the windings of the secondary coil. This sudden collapse induces a very high voltage in the windings of the secondary coil which makes it's way down the primary HT lead to the distributor cap, and from there across the rotor arm and down to the spark plug.

All this charging and discharging of coils does however take an appreciable amount of time to complete, and on average standard automotive ignition coils take approximately 6 milli seconds to fully charge the primary coil to maximum flux strength and only approximately 6 micro seconds to fully discharge the secondary coil into the spark plug.

Doesn't sound much, but if you are building a very high rpm race engine then 6 milli seconds starts to become very important. Indeed many serious rpm engines that run one coil, and I'm talking here about engines that head north of 8000 rpm, can start to run into poor spark performance due to the primary coil not having sufficient time to fully charge to maximum flux strength before it has to discharge again.

For a while “band aid” engineering popped it's head up above the parapet once more and the capacitive discharge ignition made some improvements to this. However it came with a raft of it's own issues and ultimately didn't stick around to long, as at some point the OEM engineers realised that the real answer laid in coils on the plugs and direct ECU control of them. Much as is seen on most modern applications these days.

They had known about this for years as the F1 guy's were throwing there engines round at 20,000+ rpm, but much like the transistor, I suspect that they just had to wait for the costs to reduce to an amount that was compatible with mass production.

 

Which leads me rite back to the ignition amplifier and distributor again.

As mentioned earlier, the distributor itself contains a mechanism for generating the pulse that the transistor requires to switch and just like the old points system, this pulse is timed to occur when the distributor is in the firing position. Over the years a few different methods of generating the pulse were tried but the two that really lasted were Hall effect triggers and reluctors. In the case of our 205's, Bosch decided to go with the reluctor method. Reluctors generate a rapidly rising and sinking sine wave who's peak voltage and frequency is related to the speed of the distributor shaft.

The four pronged wheel inside the distributor has magnetic tips which pass over a small coil of wire. When this happens, much like in the ignition coil, the magnetic flux present at the tips of the pronged wheel passes over the fixed coil and induces a small voltage in it. The reason that this pulse is an AC sine wave is because as the magnetic tip approaches the coil it's flux effect starts to get bigger and bigger and as it passes over the coil it gets less and less, swinging from one polarity to the opposite.

The transistor in the ignition amplifier is momentarily switched to the off position each time the reluctor sends out a pulse, and as already discussed this causes the primary coil field to collapse and create the high voltage required for the spark.

At full rpm the standard ignition coil will demand about 8 Amps from the electrical system and in turn this 8 Amps must be carried by the transistor in the amplifier. This can equate to something in the order of a 100 Watt load which ultimately becomes a thermal load, which is why the amplifier is mounted on a thick aluminium heat sink and is positioned on heat conductive paste.

One last point of note on these amplifiers is that the transistor gate is quite sensitive to erroneous EMF signals which can emanate from spark discharges at the rotor arm and at the spark plugs. The EMF can be picked up on wiring much like as if it were an antenna. This is why the lead from the distributor is a screened line. As rpm goes up the magnitude and frequency of this EMF interference also goes up. Bad screening may well not cause issue at low rpm's.

 

So to bring this keyboard happy ramble to a close.

This system works tremendously well but over what can now be some thirty years of service they do and are getting tired. The problems that can arise are many, but amongst the more common yet subtle to be found are things like reluctor tips loosing there magnetic flux strength over time and the shielded lead from the distributor loosing the integrity of the screening. The thermal paste is very important to dissipating the heat generated and without it premature amplifier failure can be expected, especially on cheap ones where the internal transistor is already of a low thermal rating.

 

Anyway guy's I hope this rant of mine finds itself to be of use for someone at some point and helps to shed a little more light on the inner workings of these great cars and there ignition systems.

 

All the very best

O.T.

Edited by opticaltrigger
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jackherer

It's generally regarded as acceptable for owners of older classic cars to replace their points with a discreet electronic ignition module to make their vehicles more reliable and usable and I think it should be similarly acceptable for owners of cars like 205 GTIs to fit modern engine management systems as the original systems wear out.

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opticaltrigger
22 hours ago, jackherer said:

It's generally regarded as acceptable for owners of older classic cars to replace their points with a discreet electronic ignition module to make their vehicles more reliable and usable and I think it should be similarly acceptable for owners of cars like 205 GTIs to fit modern engine management systems as the original systems wear out.

Totally agree with you K. I think you put it perfectly.

As these cars get older and older I think that it's perfectly acceptable and in fact a really good idea to upgrade it to a more modern ignition / fuel system.

Of course that's not to detract from the purists out there, I get that to.

But for me , I think it's a great move with lots of benefits.

 

All the best guy's

O.T.

 

Edited by opticaltrigger

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DamirGTI

Very interesting and informative info O.T. ! didn't know there's so much detail about distributor specifics , thanks for sharing this with us and taking your time writing this .

 

I seem to remember you did some modification on the distributor few years ago on turbocharged 1.9 ? 

Myself , i did some experiments too with various distributors .. on a 205 dizzy i tried changing springs and tensioning the stop pegs , and found to be really beneficial "upgrading"/tensioning the initial spring , after which the engine picked up some "lost" power/torque and more important - the unstable idle was cured almost completely after upping the tension on the initial spring , interesting the engine idles steady afterwards just like late Motronic equipped 205 .

 

But also , agreed with Kieran , it's a shame that all 205's didn't came with factory fit at least Motronic 1.3 management , instead of mechanical ignition/distributor .

 

D

 

 

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ALEX

So in brief .... if you're new to all this and all that above was a bit hard to swallow and you need a Lamens term explanation just to get your 205 running better.

I think what Opticaltrigger is saying is.

 

Replace the cable between your Distributor and ignition module! ;)

(And while your at it change the grease behind the ignition amplifier and heat-sink.)

 

I think the problem though for many is the availability of the cable. Something I'm not sure about.

 

 

 

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DamirGTI

The signal cable can easily be repaired on a DIY basis , even if you want to make yourself an new one knock sensor cable can be used as an alternative for cable/connector "donor" (i think the CAS sensor cable can be used too) - just cut the sensor off and wring along with the 3pin connector is the same as the distributor signal cable , just solder on the ends two single spade connectors for the pick up coil and job done ..

 

Did a few of them trough the years for myself and others - no problems what so ever .

 

 

D

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Puggybish

Hey guys I'm looking to change my ignition system to an msd 6al2 with msd blaster coil from what I can see it is a simple job and only requires minimal wiring. 

I'm running twin 45 Webber's and I believe that I need a magnetic pick up can this be taken from the distributer? Or maybe the crank sensor? Any advice on doing this would be great. 

 

1030293.jpg

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welshpug

why do that if the standard stuff is cheap, and works?

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petert

I've used MSD ignitions gear a few times and it has never failed to impress, even on an Mi16. If you can drive it from the magnetic reluctor, all the better. Otherwise connect the white wire to the output  of the standard ignitor. The benefit of the 6AL2 is that it's programmable. Thus it would be easy to dial in a better curve for higher compression engines.

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Puggybish

I want to use this as I brought it about 8 years ago and the wiring loom in my car is a total mess this would get rid of most of it.  

I am assuming that this would also take the ignition amp out of the equation. 

If you have any details on connecting it up to the standard 1.9 engine that would be great

Like I say I'm running twin 45s and will have a high lift cam in it so the program option I believe will come in quite handy. 

Thanks for your help guys it's been off road for 15 years so your help will finally have it back on the road soon I hope. 

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opticaltrigger

Hi David,

I just had a quick look through the .pdf  https://documents.holley.com/6421.pdf  for your MSD, and it seems to me that it's requesting a trigger input on the white wire from an amplifier as opposed to a direct VR input from the distributor. Page 10 of the .pdf shows that the white wire can be connected directly to the original coil negative wire, which is also the trigger line from the amplifier. 

Double check it though, please don't just take my word for it. But that's how I read it anyway....

 

Raw VR signals, usually would require some form of internal conditioning before being handed to the controller, however your MSD should see the connection to an amplifier as no different to that of a set of breaker points,,, It's just a dump to ground.

Hope it helps,

 

All the best,

O.T.

 

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petert

Start by connecting the output of the standard ignitor (pin 1) to the white cable.

Disconnect any existing leads to your coil. Connect only the orange to coil +ve and black to coil -ve, as per the instructions. You don't need to use the MSD coil initially. Your standard coil will be fine.

Connect the power and ground.

 

Your tacho would have been connected to coil -ve. It can no longer be and must to connected to the tacho output of the MSD. However, it's unlikely to work as the 205 tacho is looking for a high voltage spike from the coil, not a 12V 25% square wave. It's well documented in the ECU section on how to fix this for aftermarket ECU's.

 

If all goes well, then consider the direct reluctor challenge.

 

To eliminate interference, the coil wires should be twisted. It doesn't say that in the instructions. Ditto for the +12V supply and GND if they're a long run. Worry about that once you've got it sorted and ready to do final installation.

 

edit: I deleted all that stuff about ignition curves. Sorry, I had it mixed up with another model.

 

Edited by petert

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Puggybish

Hi I'm now looking into fitting the msd but my ignition amp wiring seems to be different to everything I've see no some help would be great! DSC_0344.thumb.JPG.468431982086eea3a3584428a0ec0d28.JPG

DSC_0345.JPG

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Puggybish

Sorry it's a late 1993 cat car originally don't know if they changed it later I want to know what pin to contact the trigger wire to thanks in advance. 

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welshpug

so being a late 93 car car it'll be running a motronic ecu, ecu controlled ignition advance, the dizzy is a dumb item, all it does is distribute spark rather than having weights and vacuum reference like the jetronic engines.

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Puggybish

Ok makes sense why I have a distributor with no fly lead in my spares box. 

So I will need to make up a new harness to my ignition am so I can use an old distributer or can I use the crank sensor on the green and violet wire from the msd to get my trigger? This must be the original dizzy. It's been so long out of the car in not sure what was on it and what wasn't. 

DSC_0346.JPG

DSC_0347.JPG

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welshpug

the motronic is the best you can get for a 205 short of swapping for a coilpack ecu from a bx/zx.

 

you can't use the crank sensor as the ecu uses that for the injection timing.

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Puggybish

But I am no longer using the ECU as I'm running carbs, I need a magnetic pickup to trigger the msd ignition system. Is the cas magnetic and if not is there one I could fit that would straight swap? 

Sorry guys I am a complete novice with this stuff. 

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petert

Pin 1, as I said above.

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petert

The fundamental issue of converting a Motronic 205 to carbs, is that the ECU calculates ignition curve from a combination of RPM and air flow meter position. As you no longer have an AFM, I would expect you'll be getting a very compromised ignition curve. At the very least, there will be no light load advance, resulting in poor fuel economy. You should verify with a timing light, every 500 rpm to check you at least have a typical advance curve.

 

The easiest solution is to fit a Jetronic distributor, which has a functioning mechanical and vacuum advance mechanisms, if fuel economy and driveability is a concern. If it's a race car, just check it works as described above.

Edited by petert

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opticaltrigger
7 hours ago, petert said:

The easiest solution is to fit a Jetronic distributor, which has a functioning mechanical and vacuum advance mechanisms, if fuel economy and driveability is a concern. If it's a race car, just check it works as described above.

Hi David, sorry I hadn't re read the updated posts again before my last one.

 

Pete couldn't have put it better. Just follow what he put above and you will be there.

 

O.T.

 

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Puggybish

Ok that's great I have a working jetronic dizzy but I'm still unclear on where to take my trigger from as I have the newer wiring and my pin one just goes to the coil the only one that returns through the bulkhead is the small green wire sorry I can't remember the number. 

And no wiring diagrams I can find have the number cables I have. 

Thank you all for your help. 

I obviously have the 60-2 flywheel and crank sensor used for the metronic system can I use that?

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petert

This will take a while. Bear with me.

 

Here is what you currently have.

Screen Shot 2020-05-11 at 7.56.47 pm.png

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petert

Forgetting about the Jetronic dizzy for the moment, hook it up like this. Once it's going and you're familiar with what's going on, then go the next step of installing the Jetronic dizzy.

IGN and MSD.jpg

Screen Shot 2020-05-11 at 8.29.10 pm.png

Edited by petert

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