HELP PLEASE - fuse blowing out, rear fuse box for ignition box

[halo]

haven't posted in a while and i haven't had too much time to figure it out but i did do a search on here and didn't come up with much.
1974 911
this is what happened. yesterday i fixed some wiring on my driver's side headlight and got everything working perfectly. i drove around for awhile and thought everything was working fine. i ate and then went for another drive, everything seemed fine until the alternator light came on. i though, great, i have broken a fan belt. so i pulled into a gas station when the light went out. i kept going. i get on the interstate and the light comes back on, but it keeps flickering on and off. so i pull off the interstate and get up to a stop sign, it quits. i coast to a gas station because i was on a hill and then remember that i forgot to tighten the battery cables. so i fix that and it will crank again. it still won't start so i am thinking, what is going on, did i flood it? won't start after 20 or so minutes so i go back and check fuses. the fuse on the rear fuse block is blown and when i replace it, it keeps blowing. i have no idea right now what is wrong. i haven't had that much time to take a look at it but if anyone out there has any idea what might be wrong, from past experience, please let me know where to check. the car has been running perfectly for so long.

[Early_S_Man]

Nik,

Which fuse is blowing, #1, #2, or #3, from the top, down?

Do you have a multimeter and familiarity with Voltage, current, and resistance measurements?

Unless your wiring was previously modified ... the CDI unit isn't fused! It does share a common connection to the fuse box bus, and whatever is blowing the fuse may have caused a burned or broken connection to the CDI Red wire that goes to terminal 'B.'

It sounds as if your alternator or regulator are bad, or getting that way, fast! Wild Voltage excursions can blow the CDI unit, though, so my recommendation is to pull your alternator and have it tested at a shop. Fix the alternator, first!

My second suggestion is to bench test your CDI unit and coil per the instructions in the Bosch bulletin in the following thread. You can trigger the CDI unt by touching a jumper from terminal 'C' repeatedly to grouind -- telegraph key method. A new spark plug connected via a spare spark plug wire to the coil output terminal, and securely grounded is an appropriate load for the bench testing procedure. Don't hold the spark plug or cable in your hands!!!

Perma-tune CD or Bosch CD?

[halo]

it is the first fuse, and yes the wiring has been modified so that I am using a msd unit instead of the old cd ignition box. i looked over the wiring for it and nothing is grounding out or anything like that.
I am proficient at using a voltmeter but I do have one quick question, how would the alternator cause the fuse to blow if it is blowing out only when the ignition is turned on and the engine has not yet been cranked?

I will definitely look into the alternator, though, as I have no idea yet (mostly because I haven't had time to look into it) as to what the problem could be.

thanks for the info.

[tswaney]

What is the first fuse for? If I may jump your thread. My 71 T does not have this fuse installed and maybe I'm missing out on some great accessory!

[Randy Webb]

One tip -- on the schematics - the crazy Germans numbered the fuses from bottom to top - so #1 is actually at the bottom (I think).

[tswaney]

Thanks Randy, So if I may ask, what's the top one (#3)? It's wired, with no fuse.

[Randy Webb]

I can't remember -- and don't know for a '74. I ahve a 73.5 but the car changed a lot in 6 mos. If you see a blue/green & yellow wire at that panel tho it is probably the sportomatic switch or something.

I think you are gonna have to find a wiring diagram if it doesn't say anything in the owner's manual. I know some diagrams are posted on this BBS.

Good luck. If you need the factory manual, I bet somebody can post a few pages for you. I wish I had one.

[john walker's workshop]

it all started with the headlight wiring fix, so maybe go back and double check whatever you did.

[halo]

thanks guys, it turns out was the typical problem, a blown MSD ignition unit. I just didn't want it to be another 300 dollar fix. I may look into changing back to the original ignition soon.

thanks again.