ECU Upgrade : Chassis Harness, the Rabbit Hole

TL;DR nerd gotta nerd

As a follow-up to the post about the ECU upgrade, I will detail some discoveries about the Caterham 170 and its electronics. This blog is above all my way of taking notes, in a more polished version than the scraps I store as Post-it® notes in Google Keep.

While the engine harness and its pin-out was compatible with the Caterham 160 (or easily adapted from it), the documentation provided by SCS Delta was not so effective with the chassis harness.

I won't go into the details on who got which information in which order (or how), but here are my observations - and some just look bonkers.

• Except for the speedometer that gets its signal straight from the left rear wheel sensor, all the dashboard is managed by the tachometer. The tacho is the brain of the operation, receiving sensor data from the ECU via Canbus, it pilots all the other gauges by providing what I assume is an analog voltage : oil pressure, water temperature, fuel gauge.
• Connection of the shift light was a known quantity with its green+purple wire coming from the ECU.
  
• The "tacho" output pin used on the 160 is not used for the 170 tacho at all. That line is to provide the "FPC" signal to control the fuel pump module. That Ford (Focus) unit expects a 100Hz signal with a duty cycle between 5 and 51%, with the fuel pump speed being twice that percentage, it would go from 10 to 100%.  Look for F1FZ9D370C, it's the same or has the same pin-out.
• There is a "FPM" return signal from that fuel pump module that would provide 3 kinds of duty cycles for 3 different states : one nominal "OK", and two options for "Not OK".
• The fog lights are piloted by the ECU. I kid you not. The momentary switch is connected to an ECU input, and the relay is driven by an output. Allowing for a proper and regulation-compliant logic, 2 other inputs are feeding the dipped/full beam headlights status.
• The seat belt warning light and buzzer are also managed by the ECU : the belt buckle sensor, the handbrake / brake fluid warning light are connected to 2 inputs, as well a the right rear wheel speed sensor. All that is also managed in software to define whether or not the warning light / buzzer should be triggered.

PURE. SIMPLE. FUN. they say

The fuel pump debugging required us to remove the aluminium part of the bottom of the boot to access the fuel pump control module and put the oscilloscope on it. We actually never found the other side of the FPM (feedback) signal, and temporarly fixed the FPC by feeding "10 times the tacho" to the module, as "1 times the tacho" let the engine die at idle.

Now that we have a better understanding, we will adjust it at the same time as fixing the fog lights. 

Calibration of the gauges was also a pain point, especially the fuel level, I will end up mapping the sensor reading vs actual level by filling the tank 2 liters at a time.

For reference, in my notes I noted that (with the original EFI ECU) : 

• Water temperature was most of the time 85°C
• Cooling fan kicked in at 98°C and stopping around 91-92°C in 30 to 60 seconds - temperatures read with the bluetooth OBD2 dongle and the "Torque" Android app.
  
• Oil pressure read 4-5 bars when revving, and 1-2 bars at idle, with the engine warm
• Fuel gauge was not perfect but predictable enough, getting down to zero meant two liters left in the tank