Electrical Help Please

[Ruben911]

Lost juice to the three fuse box to the left of the engine compartment (fuses are still intact). Where is the source? Also, is this fuse box related to distributor electrical juice because the engine is turning over but not firing. I was diagnozing a bouncing tach. Checked dwell and it was right on spec @ 38 degrees. Any quick help is appreciated. Thanks

[Jim Sims]

Year and model please.

[Ruben911]

Sorry, 1976 911 2.7L. Bosch distributor. Thanks

[bigchillcar]

if dwell is 38 degrees, it doesn't sound like points are closing, which will definitely cause an erratic tach. still isn't a bad idea to check your points gap just in case you read the dwell wrong. what about your coil wire? good continuity there?
ryan

[Ruben911]

Just checked points gap. It is at 0.012" which is pretty much spec. How do I check coil wire continuity? Thanks

[Jim Sims]

Same year and model as my car. Dwell is 38 degrees plus or minus 3 degrees; point gap is .3mm (.012"). The red wire that provides power to 3-fuse fuse block also provides power to the CDI unit. If the fuse block has lost power then so likely has the CDI unit hence so spark. Jim

[Jim Sims]

The red wire feeding power to the 3-fuse fuse block has a 2 terminal connector branching off it; the power to the CDI unit goes through one of the terminals of this connector to another red wire to the "B" terminal on the CDI unit. The red wire (from the 3-fuse fuse block) goes forward through the wiring harness in the tunnel (no connectors) to fuse position 11 on the fuse block in the front trunk area. It attaches to the hot side of the fuse so fuse 11 doesn't affect the power. The hot side of fuse position 11 is hard jumpered to the hot side of fuse 12. A red/black wire leaves the hot side of fuse position 12 so fuse 12 doesn't affect the power either. The red/black wire goes through a 6-terminal connector to the ignition switch. The ignition switch gets it's power through two large (4mm) red wires running in parallel though a 6-terminal connector and then to the battery. Do you have a VOM? Jim

[Ruben911]

Thanks Jim. is VOM a voltmeter? I do have one.

[Jim Sims]

VOM=voltage and Ohm meter. Start checking the circuit with the voltmeter for power. The ignition switch has to be in the on position for this circuit to receive power. I would check the easy places first (fuse positions). Jim

[Ruben911]

Electrical gremlin found. It appears I had a loose wire going to the fuse box which I must have moved when I removed the distributor cap earlier in the day to check the dwell angle. I tightened the connection and was able to measure 14 volts at each fuse. Is this correct? I would have expected to see 12-13 volts. Thanks for the suggestion. I found the loose wire while using the VOM to check for juice. Thanks again

[Jim Sims]

Was the engine running while you checking voltages at the fuse box ?

[Ruben911]

Yes, engine was running during voltage checking.

[bigchillcar]

voltage regulator regulates alternator output to 14 volts...12 volt battery is used to energize starter, thus bringing alternator online to run your electrical system.
ryan

[Jim Sims]

Alternator has to put out more voltage than the battery in order to charge the battery so the 14-volt reading makes sense if the alternator is producing power. Jim