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S550 Engine Cut Off

I finally decided on an approach for Master Power cut-off for the S550. To save this post from being over-long, I’ll post in this thread on how I selected the approach and second on the install.

I wanted a solution that:
1) Uses low current switching to avoid the need to run thick (1 Gauge) wire
2) Enables both in cockpit and exterior cowl switch locations
3) Meets NASA Tech testing
4) Eliminates the risks of Alternator current spikes causing damage on Engine Kill
5) Is Fail Safe

I looked at 3 approaches:
a) A Solid State Device like this Cartek device: Isolator GT EN.pdf

b) An “all in one” 6-pole like this:

c) A solenoid based solution, like @captdistraction describes here:

Solid State
Pros: Lightweight and Small. Provides Alternator protection
Cons: Uses Chassis ground for isolation—would require rework to support S550 smart charge current sensor. Fail Safe?

Pros: FIA approved, provides Alternator protection, Fail Safe
Cons: Requires 1 gauge cable runs, 2 switch install complexity


Pros: Simple multi switch install. Avoids 1 gauge wire runs. Relatively good re Fail Safe
Cons: Need to deal with Alternator spikes and Starter current draw

Based on this I decided to pursue the Solenoid option, as there was limited durability data on the Cartek solution and I wanted to avoid 1 Gauge cable runs associated with the 6-pole switch.

The solution @captdistraction describes works well for the S197 and deals with the cons above as both the Starter and Alternator cables remain hot as the Battery Junction Box (BJB) cable is separate to both the Starter and Alternator.

However the S550 the starter cable is crimped to the BJB cable at the Battery cable. Separating these cables would address the risk of Solenoid durability issues due to the repeated starter high current draw, but still leaves the issue of Alternator spikes to deal with as the Alternator terminates at the BJB.

The solution is to repurpose the stock Battery cable to feed the Starter and take the current draw from the Alternator.
Disclaimer: Not yet tested. Use at your risk!

The first post describes why I decided on the Solenoid approach. This post is the "How To"

First, disconnect the Battery feed and Alternator Feed to the Battery Junction Box:

I then applied heat shrink insulation to the crimp and most of the exposed part of the 90* lug terminals:

Next insert a 300A fuse between the Battery and Alternator leads (OE is 275A, but couldn't find that anywhere):

Apply more shrink wrap and secure using zip ties:

Make up a new 4 Gauge cable to run from the Solenoid to the BJB:


Note you need a 90* lug connector for the BJB:


Install Solenoid (I used a 500A) and make connections as shown below:


I then ran the Blue wire via 2 switches in series (one in the cockpit, one on the Cowl to ground. Opening either will de-power the solenoid, killing power to everything except the starter and the alternator.
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There is one addition I made to the install above to allow engine power to be killed, while retaining car control (EPAS) and lights etc. at speed.

This addition also improves Fail Safe (in the unlikely event of a solenoid failure that left the circuit closed).

I tapped into the PCM relay circuit and added a second cockpit switch that just cuts PCM power (Ignition and Fuel).

This circuit also runs to the Cowl, where I will install a double-pole switch. Blue wire is Solenoid to ground, Black/Red is PCM:


The single cowl switch is in series with the 2 cockpit switches and will kill both PCM and solenoid power simultaneously, whereas the cockpit provides independent control of each.

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