Newbie's question

[Rana1]

Hello everyone!

I'm the happy owner of a '78 911 SC. As lots of us I dreamed of that car since I was a child and now, more than 2 years after I bought it, I still feel passion for that car. For the moment my car is still trouble free (important things I mean), that helped

You may have noticed my written english is not the best, I'll do my best to make myself understandable.

My question regards the cdi module. I've got one with a missing transformer and I would like to make it work (in case mine breaks down). The problem is that the little electronics I've learned is forgotten most of it. I've been looking like an idiot to the diagram without really undertanding how it works. The dc-dc inverter I understand (kind of..) as well as the discharge circuit, not so clear for the trigger. What are the expected values coming from the hall "sensor" at 31d and 7?





And most important for me, w1 w2 and w3? What are their values? They are simple coils with a determined thickness of the wire and nē of turns, no? What kind of ferrite goes inside?

Thanks for your help

[flatsixjunky]

Hola Rana,

I'm also "novatillo" Sorry I can't help you with this, anything else feel free to ask.

[boyt911sc]

Rana,

There are some guys in this board that do know about these CDI. One name that come to mind is 'ischmitz'. I think his first name is Ingo or something like it. Good luck.

Tony

[otto in norway]

Hi there!
I have never seen this diagram before, but I am a electronics technician, so I get some of it...
The W;1,2, and 3, is one transformer, but different coils within that same transformer. So if you are missing this, it won't work..!

As to the "31d": this is 0V. (Same as chassis)
But the "7", I guess this is the trigger for the ignition...?
The layout of the circuit strikes me as a high voltage transformer, with very unusual values. But this is my opinion. My field is audio...

[Rana1]

Thank you guys

Yes 31d and 7 are the connections to the green coax, but I've got no idea what's the voltage input at 7.

I found some posts about the cdi... lots of good info on this forum, I'm really impressed

6-pin SC/Turbo CDI unit repair documentation

6 pin CDI DC DC Converter Talk

I've sent an pm to hkz bob already. Hope he can help me beacause he has already changed the transformer, must know about.

[ischmitz]

Transformer on Bosch 3 pin CDI defect. read here. Looks like Bob found a source for the transformer core. Still, I don't know where to get the turn ratio's from. Maybe he can chime in if he has unwound one and counted.

As I said earlier, these components seem to be proprietary Bosch parts that were never sold on the open market. Bosch buys stuff like that in bulk to their specifications rather than picking common parts.

The trigger input stage looks for a negative transient. A simply AAA battery with its (+) connected to GND and its (-) tapping to (7) will trigger the box.

Good luck,
Ingo

[Rana1]

Hi there!

I'm still waiting for the answer of hkz bob, he must be absent

I realised there is something else I should know.. wich sense should the spires go in each coil?
I think I read in a post that in the diagram there's a mistake in W3 ("a" and "e" should be "e" and "a" respectively), is it correct?

[livi]

Hola!

Always great to see yet another European on the forum! I have spent most of my Christmas holidays in Puerto Banus, Marbella the last 30 years. Great place!

[Rana1]

Hola livi!

We are a few europeans already. Great forum!

[Rana1]

Hello everyone.

I finally found out and the CDI works fine.

The only difficult thing was to find the corretc ferrite core. I couldn't find it so I made it.. and it works just fine!! 460 Volts exactly!

I would prefer to buy directly the appropiate core instead of modifying another but the problem is that no one seems to have it stocked and I should buy an entire production lot (1320 units min.).

During this time I found out something interesting I ignored about those cores. If overvoltage happens (faulty regulator) the core can easlily deteriorate and never work as expected again. I tried with a standard E42/21/15 N87 ferrite core with different input voltages. For low voltage the relation Vin & Vout is almost linear, increasing Vin results in a curve as we approach saturation of the core not allowing Vout to grow much further. Even worse, at a certain point Vout drops down drastically (core damaged, not to use anymore).
At Vin = 14 volts that core was over saturated and Vout did not better than 330 volts. E core must be a gapped one in order to avoid that, saturation happens much later as the gap increases, and the linear relation between Vin & Vout is maintained in much higher voltages. For low Vin Vout is lower than with ungapped core but it can grow further for higher Vin.
Trying with different gaps (grinding one of the E cores with a mini dremel..) I found the same response as the stock core, and output at 14 Vin was exactly 460 volts when hot (445 when cold).

Another issue with that CDI was that C8 was in shortcut, I found a replacement but it's value is 1'5uF 600V instead of 1'4uF 600V. Works fine but one thing I'm not sure about is temperature. Stock C8 is metallic and screwed to the box, so able to dissipate heat. Mine is isolated from the box (plastic) and said resistant just over 100ēC.. How hot can it get in there?