History of Bosch CDI ... toubleshooting info, parts list changes, and schematics ...

[Alfasrule]

Took on the challenge of fixing them this Winter. The mechanics is harder the the electronics.
I changed all the caps in the circuit including the Output Cap. The SCR is the ususal culpret. It the device in the lower right hole. Also as a precaution I change the 4 output Diodes, since there cheap. I've repaired 8 already and all work in real applications. All the components are replaceable and I have all the new part numbers.

[BimPorDuc]

I have just installed an overhauled Bosch CDI box (ECUDoctors). The car started and ran normally until and "external" capacitor gave out. The capacitor is wired in parallel with the box and is located between terminals B and C: Power supply from ignition and breaker points. This capacitor is made by Marchal I believe and is NLA. It appears to be used for smoothing out power to the box. Is it still necessary with an overhauled box? My car is a 1973.5 911T which I've owned for 31 years.
Thanks,
Doug

[dicklague]

Should not be connected between the breaker points and 12vdc. Try it without the cap.

B and ground or B and D would be more like it, and the cap should be 10,000 UF 25V electrolytic capacitor with + to B

[BimPorDuc]

It is B and D, my bad. I recall one of the wires is brown and goes to the case on the box. The car runs fine without the cap. Is it really necessary? If it is necessary where would you recommend I get one since they're NLA? The car is currently at a friend's shop so I can't just go look at it. I'll be working on it Saturday. Thx.

[dicklague]

It is not really necessary. Did not come stock with it.

It does smooth things out and and is especially important when you install an aftermarket CDI that puts out more power like the MSD or the Daytona-Sensors CD-1.

It is also good for radio noise suppression.

Here is one source of the cap Daytona Sensors LLC - CD-1 Capacitive Discharge Ignition System for Racing look at bottom of the page.

I run one on my CD-1 setup.

[BimPorDuc]

Many thanks for the link. My car is VERY stock and I'm trying to keep it that way. What does the capacitor look like and how big is it? The existing cap I had was about .625 diam x. 2.5 inches and was an aluminum can. $43 seems exorbitant when Digi-Key sells similar caps for about $2.50 - Unfortunately they don't stock them and minimum buy is 250!

[dicklague]

Cap is about 3.5 inches tall and 1.5 ion in diameter. Comes with a bracket and screw terminals.

I would sho\p around to Mouser, Digikey eBay....etc. It is the type used in high end car audio installs to provide reserve power.

With a stock Bosch CDI it may be overkill.

With a modern high power CDI the there is a constant 12 vdc feed with a heavy gauge wire and a switched feed. The constant on can draw as much as 8 AMPs and it is recommended that you connect to the battery terminal on the starter in the 911, and use the Bosch CDI power as the switched power to turn on the CDI. It is recommended to use the cap in this installation to give some reserve power and avoid spikes. It also can help with radio interference.

[BimPorDuc]

Will an MSD 8830 Capacitor from Autozone work? It's 26,000µF and it's sold by MSD for automotive applications. The cost is similar to what you've proposed?

[dicklague]

that should work fine.

[rick-l]

'd figure out why it was there in the first place. Does it run without it, like the cap was shorted?

[Jesper]

Hi. Does anyone have similar information regarding 8 pins CDI box for a 1976 930?

Best regards
Jesper

[petro206]

Hi to all,
i 'm not the first and i will not the last that would like to fix our CDI.
I need your help to find the correct parts, to avoid to make mistakes.
I haven't the correct devices to check all the component (only a simple FLUKE) so my approach is to change all that i can, like a dummy...

My CDI is 0 227 200 008



Resistors: Is it necessary to replace all to stay safe even if they are not interrupted?

Capacitors: what is the alternative to WIMA FKC3 ? --> i have no idea

Diodes: Is it necessary to replace all to stay safe even if they are not interrupted?

ZD1= 1N3003B --> i found it (Thanks UGO)
ZD2= 8ZY 85 / C6V8 --> i have no idea
ZD3= ZM22 --> i have no idea

T1 = RCA 2N3055 --> i found it
T2=T4018 --> i found it
T3=T4018 --> i found it

SCR OT104 --> is TIC126N the state of art?
Buy Thyristors SCR Thyristor TIC126N-S, 800V 7.5A 20mA, TO-220 3-Pin Bourns TIC126N-S online from RS for next day delivery.

Grazie

[ischmitz]

I would advise not to simply change all the parts. Find out what is damaged instead. Else I don't think the box will ever work again. I repaired several hundreds....

For instance the SCR you mentioned will not work, leave the original one in place unless it is shorted (rare but impossible, can be checked with the Fluke) - trust me.

[jakz]

Is there a guide for fixing these? Kind of cdi for dummies on how to test the components, what are the usual faults to check for, and finally how to rebuild them? Im really interested

Thabks in advance

[petro206]

Thank you ischmitz,
i've tried to measure the SCR (the model is T6N500COBm, and no OT104 as i wrote) and the resistence is very low (i think is shorted).





Is it correct way to measure?

I have another CDI from Alfa Montreal (in very bad condition), and the SCR is Motorola MAC40798 , the resistence is lower than other.






Which is the correct replacement?

[petro206]

I'm not sure that i worked in the correct way, so I try to measure from case to the pins
For T6N500COBm:
from case to short pin the resistence is infinity, and the same is from case to long pin

For MAC40798 the situation is different:
from case to short pin i s 23.8 Mega Ohm
from case to long pin is 23.7 Mega Ohm

Marco

[ischmitz]

Marco,

Remember the housing of the SCR is isolated from the heat sink, its the anode of the SCR. The critical measurement is from the long pin (cathode) to the case (anode). It should be infinite resistance regardless of polarity of your meter. The short pin is the gate. The gate/cathode is a PN junction and acts like a diode. If your meter is set to Ohm you might measure a value when you forward-bias the diode and infinite resistance when you swap the leads. Once you set you meter to the diode setting you need to see open in one direction and some voltage when reversing the leads. This is the forward voltage drop across the PN junction. It should be between 0.4V - 0.8V

Ingo

[petro206]

Hi ischmitz,
Thank you for your help.
For both SCR, using diode setting, i have infinite resistence (0L) from cathode (using red tip of my Fluke) to anode (black).
The measure of PN junction is 0.077 V for the first and 0.031 V for the second, so they are ready the garbage bin

[ischmitz]

Sorry Marco,

I spoke too soon about what to expect for the measurements. And I am surprised I haven't been called out on it. Anyhow, the correct measurements are:

Infinite between Anode and Cathode
You will measure an ohm-like behavior between gate and cathode and so your observations are in line with a healthy SCR.

However, you could still have issues. You might want to follow the instructions in the link here: How to test an SCR - Electronic Circuits and Diagram-Electronics Projects and Design to get a quick functional test.

Then even with these tests passing an SCR can become marginal with age in that it doesn't perform properly once at elevated temperatures. This will only become apart in a functioning CDI box on a test stand. My recommendation is to not change it and correct whatever other issues there are with the box and then operate it. This will be the final test for the SCR.

Regards,
Ingo

[911SauCy]

ischmitz...

You're knowledge of these is evident, I have a friend who is incredible with electronics, but not 100% versed in CDI boxes (mine is a 6 pin). Upon opening we found the large capacitor toasted, completely open, and one of the wires arched to it.

If we can't get our brains around it, are you open to working on these still?

Sauce