Bosch CDI unit, testing
 

Is there any way to test the Bosch CDI unit with external power source?

This box has these connectors A B C and two ground contacts. Whats what?

I did some trouble shooting last summer on my 70T's CDS unit. I am going from memmory, so please bare with me! The three wires are blue/yellow, red and black. The b/y is a triggered ~400V signal that goes to the coil, the red is switched 12V and the black is a trigger from the points. The ground is just that, a ground. The points trigger a switch in the cds unit via the black wire. The cds unit then amps the 12V from the red wire up to 400V and sends it to the coil via the b/y wire. To test the cds unit from a bench, you will need to simulate the engine. I don't really see the point in doing that. If your cds unit doesn't 'humm', check for 12V at the red wire. If you get 12V there and still no humm, clean all your ground connections. Reconnect. Still no humm? Unit is no good. If you need additional help, let me know.

Good luck!
BK

The applicable schematic is here:

http://www.pelicanparts.com/911/911_Parts/Electrical/911_electrical_1973_CIS_Part2.jpg

CDI unit is black box #9. A bench variable power supply capable of 13.8 Volts and at least 10 Amperes would be required. Ideally, the proper Bosch 0.221.121.001 coil would be used to test the output, but I suppose a 0.5 Ohm 100 Watt wirewound resistor could be used as a load, monitored by an oscilloscope with an appropriate high Voltage probe. The output pulses should be 350-460 Volts at terminal 'A' ... when ground pulses are applied at terminal C.

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Warren Hall
1973 911S Targa

I just read your post again and I might not have answered your question. Do you have a unit out of a car you want to test? If you attach 12V to the center terminal and ground the box, you should get a humm. If you don't, the box is no good. If you do, the box may or may not be good. If you have access to the right equipment you can simulate the ignition system. The only way to verify the 400V signal to the coil is with a scope. Maybe. I'm not sure if a scope will even pick up the pulse.

Thanks for the tips, I just tested and it gives this pitching sound, 5khz? Well, hopefully thats the sound when its working

//Zen

i've heard them whine and not work. kinda like employees. do you have another car you can attach it to?

Well, no other car until this saturday and then that would be my car. But, gotta test it saturday then.