67 Bronco electrical issues

[67BroncoBob]

Hello, first time post. I have a 1967 Bronco with an Explorer 5.0 engine and MSD ignition. I've had starting and running problems since I got it. I have a brand new battery, I've tested the wires coming from the MSD box. While driving, the engine can just turn off and voltage drops from 14 to under 10. It will crank but not turn over. After I jump it it can drive for another 2 weekends 20-30 miles and then won't start again. Just last night it was running fine at 14 volts, I pulled the emergency light knob on accident and it cut out. Now when I turn the key it drops down about 2-3 volts every crank until it gets to 5 volts then does nothing. Any ideas moving forward? Thank you.

 

[DirtDonk]

Hey Bob, welcome to classicbroncos! And unfortunately too, the world of classic Broncos 54 years on!

Sounds like you're losing power to the ignition switch. The fact that the voltage drops means that the alternator has stopped charging, which most likely means that the keyed power to the Green w/red wire on the alternator connector has stopped flowing. Which should be coming straight from the ignition switch with no stops or anything, unless you also happen to have a charge lamp on the dash?

The sudden stopping of the engine would not be due to lower voltage from the battery not being charged, but going from 14v (a normal charge reading) to 10v rather than 12.5v of a normally charged battery means perhaps something else is going on. Maybe they're related (which would be a good thing) or maybe it's two separate issues.

What do the battery cables look like? And how is the alternator wired? Is the vehicle's wiring harness the original, or was it totally re-wired at some point?
How old is the battery? It's not often that some internal issue with the battery will cause this kind of thing, but it might not be out of the question either. Just less likely, but not impossible.

Is the engine using a distributor ignition, or coil packs?

Good luck!

Paul

 

[B RON CO]

Hi, as Paul mentioned, I would look at the wiring near the ignition switch first. A common test is to wiggle the wires near the switch when the Bronco is running and see if you can recreate the trouble. The problem could also be the switch itself. Good luck