Coyote EFI, Starter, and FPPDB Wiring Question

[32J]

Okay, I'm going a little batty going between the Roadster manual, the Ron Francis Chassis Harness manual, the Coyote Fitment manual, and the Ford Coyote Controls Pack manual. And the phrasing in some of the instructions has me second-guessing.

My battery is mounted in the trunk, and between the seats I have the battery cut-off switch. Do I run the battery positive cable 'just' to the 250 amp fuse that comes with the Coyote EFI harness? Or, do I also run (branch) off this fuse terminal to the starter? Or, I suppose I could run the battery positive cable to the starter first, and then from the starter run another 4 AWG cable to the 250 amp terminal. See attached sketch.

20220622_202539.jpg

Like I said, all the manuals have me turned around a bit. On p. 234 of the roadster manual, it says, "Run the battery cable forward on the right side of the transmission tunnel to the starter." Then, in the Coyote fitment manual, p. 98 says "From the other inline fuse stud run a battery cable to the starter battery cable or to a battery cut-off switch."

I'm hoping someone can clear this up for me. Thanks in advance.

32J

[edwardb]

The Ford instructions also say to run the power to the Coyote PDB from an uninterrupted power source. So the Coyote PCM is always powered on. That saves fuel trims, error codes, etc. The current drain is very low as it's just powering memory chips. Your diagram would shut everything down when the disconnect is off. This is how I've wired mine (Coyote and LS builds, which have similar requirements): 4 gauge from the battery to the disconnect. Cable to the Coyote 250 amp fuse off the unswitched side. Cable to the starter off the switched side. So by definition they're separate wires, which is part of your question. I'm assuming that switched battery power to the starter would also pick up the power leads to the RF harness at some point. I attach those to the switched side of disconnect or in one case to a feedthrough on the firewall.

Hope that helps.

[Nigel Allen]

I suppose you could put a 5amp fuse across the battery isolator to maintain a low current supply for the PCM when the isolator is in the off position.

With the isolator in the off position there is power to maintain memory. However if you short something out, or someone tries to start the engine, the fuse will blow.

[edwardb]

I suppose you could put a 5amp fuse across the battery isolator to maintain a low current supply for the PCM when the isolator is in the off position.

With the isolator in the off position there is power to maintain memory. However if you short something out, or someone tries to start the engine, the fuse will blow.

That certainly works. I personally just never understood the concept though. As long as the Coyote PDB is the only thing wired to the unswitched side (and the starter and RF harness on the switched side) with the switch off trying to start the car will do nothing. Why have a maintenance item (e.g. the 5 amp fuse) that potentially needs to be replaced? Maybe I'm missing something.

[32J]

I did see pictures of your set-up (such a clean build!). I might be able to replicate your wiring if I had my switch on the firewall, but my disconnect is between the seats on the back wall. For HAAT to the PDB, I suppose I could run a small 12 GA wire from the un-switched side of the cut-off along the transmission tunnel, to the start of the fuse block. I'm guessing that the ignition wire that connects to the starter won't allow the solenoid to close, if there is no power coming to the starter (i.e. disconnect is open). So, I wouldn't need a small fuse on the 12 GA HAAT line to the PDB.

20220623_183120.jpg

I would then run the 4 GA battery positive cable from the switched side of the disconnect along the tunnel, and right to the starter. And then, off the same post on the starter that I have the battery cable connected, I would run a second #4 GA cable up to the start of the fuse block. Is this correct? I have to run battery power simultaneously to the start of the fuse block AND to the starter?

And the power leads to the RF harness... are you referring to the power leads that they indicate being connected at the starter post, where the battery positive cable connects? Where my pen is pointed in this photo:

20220623_184608.jpg

Thanks!

32J

[edwardb]

I did see pictures of your set-up (such a clean build!). I might be able to replicate your wiring if I had my switch on the firewall, but my disconnect is between the seats on the back wall. For HAAT to the PDB, I suppose I could run a small 12 GA wire from the un-switched side of the cut-off along the transmission tunnel, to the start of the fuse block. I'm guessing that the ignition wire that connects to the starter won't allow the solenoid to close, if there is no power coming to the starter (i.e. disconnect is open). So, I wouldn't need a small fuse on the 12 GA HAAT line to the PDB.

20220623_183120.jpg

I would then run the 4 GA battery positive cable from the switched side of the disconnect along the tunnel, and right to the starter. And then, off the same post on the starter that I have the battery cable connected, I would run a second #4 GA cable up to the start of the fuse block. Is this correct? I have to run battery power simultaneously to the start of the fuse block AND to the starter?

And the power leads to the RF harness... are you referring to the power leads that they indicate being connected at the starter post, where the battery positive cable connects? Where my pen is pointed in this photo:

20220623_184608.jpg

Thanks!

32J

I suspect that smaller gauge HAAT wire would be OK. But can't cite personal experience or any possible concerns. I understand you have the disconnect in a different location than I've done on my builds. But if it were me, I'd still run the 4 gauge cables off the switch. One to the starter from the switched side. The other to the PDB from the unswitched side. Simple and proven. I'd bet the length of the two combined cables wouldn't be that much more than going from the starter to the PDB. But can't say for sure since I don't know the location of your components.

The solenoid on the starter will only be energized by the Coyote start wire (which I assume you're using) when commanded by the Coyote PCM. And that's not going to happen as long as the Coyote +12v pigtail ignition wire doesn't have voltage. Which it won't if the RF wires power wires are also switched off with the disconnect.

As far as the RF power wires, yes, those are the right ones. The schematic shows them going to a solenoid, which I've always interpreted as a firewall mounted solenoid. Which isn't required as long as the starter has a solenoid. Which they've had for many years now. I've not installed a firewall mounted solenoid on any of my builds and from what I've observed, most others do the same. Yes, you can pull those wires down to the +12V battery connection on the starter. I don't because usually the wires aren't long enough and frankly it's a little messy. I hook them either to a feedthrough on the firewall or to a bus bar which has +12V battery power.

Lots of options for wiring these things. No right or wrong as long as it works properly. Good luck,

[facultyofmusic]

I did see pictures of your set-up (such a clean build!). I might be able to replicate your wiring if I had my switch on the firewall, but my disconnect is between the seats on the back wall. For HAAT to the PDB, I suppose I could run a small 12 GA wire from the un-switched side of the cut-off along the transmission tunnel, to the start of the fuse block. I'm guessing that the ignition wire that connects to the starter won't allow the solenoid to close, if there is no power coming to the starter (i.e. disconnect is open). So, I wouldn't need a small fuse on the 12 GA HAAT line to the PDB.

20220623_183120.jpg

I would then run the 4 GA battery positive cable from the switched side of the disconnect along the tunnel, and right to the starter. And then, off the same post on the starter that I have the battery cable connected, I would run a second #4 GA cable up to the start of the fuse block. Is this correct? I have to run battery power simultaneously to the start of the fuse block AND to the starter?

And the power leads to the RF harness... are you referring to the power leads that they indicate being connected at the starter post, where the battery positive cable connects? Where my pen is pointed in this photo:

20220623_184608.jpg

Thanks!

32J

Hey 32J, I don't think the wiring diagram shown in your post above is what you need. Let me explain:

Here's what Paul's and mine (and many others) look like, note that the 250A fuse is on HAAT and ONLY goes to the coyote PDB. Like Paul mentioned above, the coyote PCM will only energize the starter motor when given the signal from RF harness. Therefore if the RF harness is not receiving power then the starter will never start. The coyote PCM does NOT power the start motor, it merely gives it the signal to turn on (signals are indicated by arrows).
correct.jpg
Diagram 1.

Here's an equivalent diagram to what you have above. Note how the 12GA wire is powering the FFPDB, but it's also at the same time powering everything else. This effectively makes the 12GA wire a weak-point in the system because the RF harness still gets power and can therefore tell the PCM to energize the starter motor. A.k.a. The diagram you've shown above is NOT the equivalent to Diagram 1.
wrong.jpg
Diagram 2.

In order to make the circuits equivalents in its current state you can two options. There are most definitely more but here are the ones that I can see:

Plan A: Use a large diode to ensure the 12GA wire doesn't back-power anything but FFPDB. If you try to trace from battery to RF harness through the 12GA wire in Diagram 3 you'll see it's impossible due to the diode.
planA.jpg
Diagram 3.

Plan B: Use a DPST (dual post single throw) switch to separate the circuits so they are independent when the switch is open. It's effectively an on-off switch that switch 2 circuits at the same time, which makes the FFPDB and the rest separate circuits so 12GA wire can't power anything but FFPDB.
planB.jpg
Diagram 4.

[32J]

To Sir EdwardB and FacultyofMusic... thank you both! I'm getting a clearer understanding of what I should be doing and some pitfalls to avoid. Now I just have to figure out why my disconnect switch won't actually disconnect (separate issue... I'm getting continuity when it's both on and off).

Thanks again,

32J