Only runs one way

Tiny
ERIC ROPER
  • MEMBER
  • 1993 FORD F-150
  • 5.8L
  • V8
  • 2WD
  • AUTOMATIC
  • 156,000 MILES
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.
Saturday, June 12th, 2021 AT 7:58 AM

5 Replies

Tiny
ERIC ROPER
  • MEMBER
  • 4 POSTS
So I'm not the only o e puzzled I see.
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Sunday, June 13th, 2021 AT 10:54 AM
Tiny
ERIC ROPER
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  • 4 POSTS
Only one puzzled.
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Sunday, June 13th, 2021 AT 10:55 AM
Tiny
KASEKENNY
  • MECHANIC
  • 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
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Sunday, June 13th, 2021 AT 7:32 PM
Tiny
ERIC ROPER
  • MEMBER
  • 4 POSTS
Yes the ammeter shunt.
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Monday, June 14th, 2021 AT 9:51 AM
Tiny
KASEKENNY
  • MECHANIC
  • 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?
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Monday, June 14th, 2021 AT 8:30 PM

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