Only runs one way

1993 FORD F-150
156,000 MILES • 5.8L • V8 • 2WD • AUTOMATIC
Avatar
ERIC ROPER
  • MEMBER
  • 4 POSTS
The entire ignition system has been changed, key switch and brain box included. Distribute rotor button, cap, plugs, wires, and control module capacitor, everything. Yes, even the coil. And voltage regulator. Last thing was the alternator. It will only crank and run with the shunt out.
Jun 12, 2021 at 7:58 AM
Repair Safety Notice: This information is for general instructional purposes only. Vehicle repair can be dangerous. Verify all information, follow manufacturer service procedures, use proper tools and safety equipment, and consult a qualified repair shop when needed.
Advertisement
Avatar
ERIC ROPER
  • MEMBER
  • 4 POSTS
So I'm not the only o e puzzled I see.
Jun 13, 2021 at 10:54 AM
Avatar
ERIC ROPER
  • MEMBER
  • 4 POSTS
Only one puzzled.
Jun 13, 2021 at 10:55 AM
Avatar
KASEKENNY
  • AUTOMOTIVE REPAIR CONTRIBUTOR
  • 18,907 POSTS
Sorry for the delay but these questions are put in que from oldest to newest according to the last update. So unfortunately when you updated it, it put it back at the bottom of the que.

Just to be sure we are talking about the same thing. When you say the only way it will run is with the shunt out, you mean the wire that is used to display the charge state of the vehicle?

I attached a description below if you can review that and let us know. Then if you don't mind getting a video of this starting and running with it out that would help us figure out what else this could be.

Thanks
Image (Click to enlarge)
Attached Media
Jun 13, 2021 at 7:32 PM
Advertisement
Avatar
ERIC ROPER
  • MEMBER
  • 4 POSTS
Yes the ammeter shunt.
Jun 14, 2021 at 9:51 AM
Avatar
KASEKENNY
  • AUTOMOTIVE REPAIR CONTRIBUTOR
  • 18,907 POSTS
So if the engine only starts with this out then I suspect that wire has excessive resistance in it. I don't see how that is preventing the starter from engaging so you are correct that this is an odd one, but the PCM must be failing to ground the starter relay when the shunt is pulling the meter down. These systems are tied pretty closely together but I have never seen this so it will be a new one on me.

Have you tested the resistance of the wire?
Jun 14, 2021 at 8:30 PM