3 Pin CDI box question

cgarr · cgarr's Garage

Working with a 76 3 pin box, pins A,B and C I know A and C are coil and points but where does pin B pick up its power? and is it 12v? We are having some problems getting spark to the plug wires, we did the test using the coil wire and a spark plug along with tripping the points and get a spark there but I am thinking when we crank the starter something is dropping out.

jjrowe · 12-10-2009, 08:37 AM

Yes, I believe that B should have 12v. Here is a good post from early s man on troubleshooting that system.

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

cgarr · cgarr's Garage

Thanks, The car this is going in has hacked wiring, everything seems to work up to the point of cranking the engine then it seems to loose power so we will check voltage as we crank and see what happens.

jjrowe · 12-10-2009, 09:13 AM

One other thought, you can hook up a test lamp to a 12v source in the engine compartment and then attach the other end to your points output on the dizzy. Turn the motor by hand and the light should turn off and on as the points open and close (providing ground). If it doesn't then you need to adjust the points. It sounds like you already did the telegraph test so you probably have an issue somewhere in the distributor (or like you say a very strange wiring issue).

T77911S · 12-10-2009, 10:31 AM

nice looking car. looks like it will be fun

76911 · 12-10-2009, 01:21 PM

OK, got it, What was happing , I had 12v to the B wire but when cranked the 12v went away. So there needs to be another 12 v supply only with the key turned to the start position. I found out the the one wire came of the fuel pump rely which its suppose to do. That one loses voltage when cranked This wiring harness is a mess, if you could just see it you would pull your hair out. Put 12v to the B wire and it fired up!!! so everything is good

Thanks
Need some dry roads now

T77911S · 12-11-2009, 04:03 AM

nothing worse than fixing butchered wiring. thats why i hate to see people "make things work" instead of fix them right. these cars are too nice and getting more and more rare to do hack jobs.
mine was done the same way. i have 2 wiring harnesses in the engine. the orginal and one someone spliced in. when i put in the 3.0 i will straighten it out.

76911 · 12-11-2009, 02:20 PM

After getting the ignition fixed here is how it sounds very happy!!!

YouTube - 74 911 widebody first run

HarryD · 12-11-2009, 04:08 PM

Power goes into "B". The CDI provides power continuously from "C" to the points so when they close they complete a circuit and cause a pulse to be sent out of "A" to the Coil. Note that the Coil and CDI are Grounded to the same point to eliminate any unwanted potential differences.