CD ignition for '86 on?

[DanHoug]

i've got an old Delta Mark 10C capacitive discharge ignition system i've moved around to various point-based cars... Fiat, Volvo, etc. it worked great, smoothing out the idle and better drivability. anyone tried a CD on the Vanagon?

I'm concerned about damaging the ECU, either through not offering the ECU the expected coil resistance/inductance it was desgined for or somehow back feeding the CD discharge voltage into the ECU, which SHOULDN'T happen but...

i know we have 'electronic ignition' with improved-over-points spark triggering thru the ECU along with a coil that was designed for this. pretty good system but nothing spark-wise like a real CD jolt. besides, i love the high voltage whine of the oscillator circuit saying "Don't Tread on Me".
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-dan
60% of what you find on the internet is wrong, including this post.

'87 Westy & '89 Westy both 2.1 4spd

Past projects can be found at--
www.thefixitworkshop.com

[tencentlife]

Well, Dan, as you already know, the Digifant ECU, like almost all electronic ignitions built since early 80's, incorporates dynamic dwell control, and the ignition driver circuit is of course calibrated to the stock coil primary resistance as well, so you'd have to account for that as far as integrating a CD system with it. Variable dwell might be a good thing to work with the CD, actually, if you can make it match up. I would think you'd need to use a scope to study the output characteristics of the ignition driver circuit first to establish the parameters of what the ECU can handle as far as current, reluctance, etc.

All that said, the stock system is really good. I think the switch to high-current coils that dynamic dwell enabled kind of narrowed the gap, so to speak, with CD systems as far as available spark voltage, but there is something to be said for the rapid discharge and resulting spark intensity of a cap that a coil probably can't match.

But on the other hand, spark duration can actually have advantages in marginal combustion conditions, it's just more time for the right portions of a highly heterogenous charge to swirl past and catch fire. But that problem is best fixed with better combustion design and mixture control.
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Experience is kryptonite to doctrine.

[MarkWard]

I raced an earlier version of the vw ignition system in my Scirocco. The distributor was fixed with hall affect like the digifant distributor, but used an ignition module like the digijet to fire the coil.

Here is what I did. I added an MSD 6AL and MSD coil with wires. Instead of using the ignition module to fire the coil, it sent the signal to the MSD box, which in turn fired the MSD coil. This effectively removed the load off of the VW module. For a timing curve, I had an MSD timing module.

So, it’s possible that the signal that fires the coil from your ECU might see less load just sending the ignition signal to your CD box and it would handle all the load of firing your CD coil.

That said, as Chris is suggesting, you should probably research what’s going on. In the end, probably not worth the risk to the ECU to just try it and see.
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[DanHoug]

hmmmm.... maybe i'll put a 2 channel scope on it to plot voltage and current going into the coil. once EVERYTHING else is fixed, right?

or i could take a sacrificial ECU and just try it, offering its smoke to the gods. not sure how the carbon resistors on the plug end will handle this... always had to put on magnetic suppression wires with the Mark 10C as it would burn thru any carbon based wire. the 10B unit was lower joules and you could keep your existing wires.
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-dan
60% of what you find on the internet is wrong, including this post.

'87 Westy & '89 Westy both 2.1 4spd

Past projects can be found at--
www.thefixitworkshop.com

[RawUmber]

When the Digifant wants to fire the coil, it outputs a ground pulse on pin25 using a pretty strong NPN transistor.

You definitely don't want to back feed a high voltage discharge to the ECU by design, but it does have some components (diodes) to shunt that to protect itself (up to a point).

[DanHoug]

right. the design of the Mark Ten series isolates the trigger input, what was the points opening, from the developed 400VDC jolt to the coil. has a 3/4" long fiberglass bridge with one connector going to the coil that carries the jolt and on the far end of the bridge, the input trigger from the points. so they are isolated physically but i don't know if there is any crosstalk happening within the circuit itself and PCB layout.
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-dan
60% of what you find on the internet is wrong, including this post.

'87 Westy & '89 Westy both 2.1 4spd

Past projects can be found at--
www.thefixitworkshop.com

[tencentlife]

[RawUmber]

[tencentlife]

Yeah my question was rhetorical, and yes you're exactly right, it does work much like a points set, the difference being the ECU can vary the duration to provide variable dwell, both to to limit coil heating, and to maximise coil saturation, which is very sensitive to the voltage available.

Since the CD system Dan's talking about has isolation between the rpm signal and the power-handling, I'll bet it could work, probably pretty simple. I'll be interested to see what he comes up with.
_________________
Shop for unique Vanagon accessories at the Vanistan shop:
https://intrepidoverland.com/vanistan/

Please don't PM here, I will not reply.

Experience is kryptonite to doctrine.

[DanHoug]

well, that was interesting in a nothing way. continuing on with this experiment, i rebuilt an ECU by re-capping and hitting a few solder points. an interesting find on this fairly early digifant unit was there was no heatsink compound between the bank of triac/scrs (whatever the heck they are) and the big aluminum heat sink. so added some compound there. sorta interesting that this ECU from Dave was removed because it would become flaky after 20 minutes or so as i recall. that could certainly contribute. anyway.

installed the vintage Delta Mark 10 C capacitive discharge unit with the sacrificial ECU in place. wen to fire it up and completely dead. oh yeah, put the coil wire back on. fired right up. the 10B and 10C have a bypass switch the takes the CD out of the circuit so i started with it in bypass, went round to the back and engaged the unit. no difference in running. pulled a plug wire and put a spare plug on to check spark. bypassed, i had a really excellent spark, which surprised me because i sorta remembered my spark being reddish and wimpy. this was snappy and blue/white. engaged the CD and the spark was a bit more robust, nothing to write home about.

went for a test drive with it engaged. tach is now dead, an expected problem with fairly simple work arounds. couldn't say i noticed any difference at all. my van runs great anyway and so i didn't have a weak ignition system to start with. one MAYBE difference was when i was backing up hill into the shop it idled up the ramp pretty confidently. maybe the ignition under load was better but i think this was me more wanting an effect.

so. had i checked my spark before doing this experiment and seen how nice it was, i probably would have left well enough alone. but i did end up with a rebuilt and functioning Delta Mark 10 C (caps, power transistors, and the SCR), a rebuilt ECU, and a bit more knowledge. i'll run with this a while since its in and check mileage to see if it makes a difference there. otherwise i'll probably remove it to reduce complexity and save it for the old Fiat or Volvo project in my future. onward!
_________________
-dan
60% of what you find on the internet is wrong, including this post.

'87 Westy & '89 Westy both 2.1 4spd

Past projects can be found at--
www.thefixitworkshop.com

[DanHoug]

aaaand the final installment.. ECU died with the CD in place.

had a long crank episode 2 days ago that gave me pause. it started tho and ran fine. went to start it this AM and no start. took a different car. tonight i removed the Delta unit and brought the ignition back to stock. still no start. swapped the sacrificial ECU out and a refurb in. immediate start. soooo.... the reasonable conclusion is the CD unit fried the ECU.

took it apart and there is no obvious circuit damage but it is done dead. Dave, your ECU took one for the team, all good in the pursuit of knowledge, right?
_________________
-dan
60% of what you find on the internet is wrong, including this post.

'87 Westy & '89 Westy both 2.1 4spd

Past projects can be found at--
www.thefixitworkshop.com

[crazyvwvanman]

Use your ohm meter to trace which big transistor is connected to the ECU connector pin that fires the coil.
Replace that transistor.

Mark

[DanHoug]

i'll give that a try, Mark. pin 25, correct? this older ECU is a bit easier to work on as i think the heat sink tension clips are screwed vs riveted on.
_________________
-dan
60% of what you find on the internet is wrong, including this post.

'87 Westy & '89 Westy both 2.1 4spd

Past projects can be found at--
www.thefixitworkshop.com

[crazyvwvanman]

[DanHoug]