F-150 starter circuit

billr

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Make
Ford
Model
F-150 2WD
Year
1999
Miles
unk
Engine
4.2L V-6 auto trans (4R70W)
The typical sometimes "no-crank" problem. It is probably in the range switch (auto trans) because the shifting lever is rather finicky. However, in preparing to be able to jumper power to the starter solenoid, to force cranking, I notice that this beast has two high current contactors. One down on the starter itself, a solenoid that both engages the drive gear with the flex-plate and connects main power to the motor. But, there is also a high current contactor (heavy cables in/out) up on the firewall. I suspect that contactor is in series with the starter solenoid. Anybody have schematics that show both of those power relays?
 

Mobile Dan

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1611852975845.pngLooks like the "high current contactor" on the firewall connects B+ to the trigger wire for starter.
 

billr

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OK, I see that. It is a big relay, with battery-size cables to power that solenoid on the starter. Seems kind of odd, since the solenoid only needs about a #16 AWG wire, but it is a Ford design... I should not complain, it is far easier to get to that relay on the firewall than the starter if I need to force cranking; especially out on the road and in the rain! Won't even need a wire, just a key or coin to bridge the two studs on the relay.
 

Mobile Dan

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That solenoid may also function to "cut power to some stuff" while cranking and reconnect power to that stuff when solenoid internal contact returns to Off position.
 

billr

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The schematic you posted shows it as a simple SPST relay, and that is what it looks like. It looks the same as Ford was using from about 1930 and on, when they hadn't adopted the gear-engagement solenoid design yet. Maybe they had a vast supply to use up?

 

nickb2

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It looks the same as Ford was using from about 1930 and on, when they hadn't adopted the gear-engagement solenoid design yet. Maybe they had a vast supply to use up?

That sounds about right bill.

Things have evolved since then, but going back to this design, much have cost ford a lot of money or all that copper for all those yrs. Wonder who was the bean counter in that one. ;)
 

billr

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OK, back to the root problem. The shifter cable had broken strands that were bunched-up and making the cable hard to move as well as sometimes restricting its travel. Fixed.
 

nickb2

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I like root problems, it is safer for me in my brain space.

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