While waiting on the last fittings to arrive, this past week I’ve been working on wiring plans in the evenings.
Because race car I’m pulling the entire Miata harness on the chassis side as well and will make a new harness for just the essentials. For the record, if this was a street car with many more factory systems I would keep the factory harness and just add things as needed. But since that’s not the case, there’s some planning to be done here if I want to wire everything up in one nice clean harness.
The engine harness itself has already been set up nicely for my needs by V8R. They trimmed out anything unnecessary, converted it from auto to manual, added an OBDII port for me and extended a few wires at my request. They also fused anything that needed it and added a main relay. All that’s left for me to do on that side is carry a few voltage wires from the cabin/battery over to the engine side.
But, that doesn’t get me out of the woods. I still need to sort out a chassis harness which includes several systems like fuel, ignition, etc. as well as the Racepak dash plus the sensor module for the dash. I also need a cooling fan circuit, and I need to get the OBDII port and the drive-by-wire connector into the cabin. To make things easy to service/remove, almost everything will pass through a single bulkhead connector in the firewall.
One unknown I ran into during planning was the question of how much info the digital dash can pull directly from the ECU via the OBDII port. Anything the dash can get straight from the ECU saves me time and wires. The guys at Racepak couldn’t make any guarantees, they said you really just have to plug the dash into the car, start it up and see what data it can pull. Well that’s great except I can’t start my car yet – I need the wiring done before I can start it… but if I want to do the wiring all at once and not have to hack it up and add stuff later I need to know this answer before I do the wiring. Gotta love catch-22’s.
Sean had a great idea for a solution – rent a V6 Camaro and wire the dash up to it.
Turns out it can pull quite a bit of info. I don’t need most of it, but here’s the full list of what the dash is getting from the OBDII port – noteworthy items that I’ll probably be displaying on the dash in bold:
Fuel system status
Engine RPM
Vehicle speed
Calculated engine load value
Short term fuel % trim – Bank 1
Short term fuel % trim – Bank 2
Long term fuel % trim – Bank 1
Long term fuel % trim – Bank 2
Intake air temperature
Intake manifold absolute pressure
Timing advance
Engine coolant temperature
MAF air flow rate
Oxygen sensor Bank 1, Sensor 1
Oxygen sensor Bank 1, Sensor 2
Oxygen sensor Bank 2, Sensor 1
Oxygen sensor Bank 2, Sensor 2
Run time since engine start
Distance traveled with malfunction indicator lamp (MIL) on
Fuel pressure
Fuel rail pressure (diesel)
Fuel level input
Fuel type
Commanded evaporative purge
# of warm-ups since codes cleared
Distance traveled since codes cleared
Evap. system vapor pressure
Barometric pressure
Ambient air temperature
Control module voltage
Absolute load value
Command equivalence ratio
Throttle position
Relative throttle position
Absolute throttle position B
Absolute throttle position D
Absolute throttle position E
Command throttle actuator
In addition to the above bolded items, I’m adding temperature sensors for engine oil, transmission, differential and power steering fluid. These four will tie into a Racepak universal sensor module which transfers the data from those sensors to the dash via one cable. Between the stuff above and these four added sensors the dash should be able to keep an eye on everything important. I can set up warnings on the dash for any of these inputs, so I can focus on driving rather than worrying about checking gauges.
I’m disappointed that oil pressure isn’t on that list. The ECU monitors this, but it appears it doesn’t send that info to the OBDII port. I’ll need to wire up a second OP sensor or figure out if I can piggy-back on the factory sensor’s wires. Not sure yet on that front, but overall some good info gained and I can finish up my wiring plans now.
Next morning update:
I spoke with Racepak this morning. Explanation for oil pressure not coming through the OBDII plug is that the Racepak dash can only read the “standard” OBDII PIDs. There are additional “enhanced” PIDs added by GM which are not required part of the standard/required OBDII data. HPTuners and similar can read those enhanced PIDs but the dash cannot.
Worst-case scenario is having to run a second OP sensor for the dash. However, I asked about piggy-backing off the factory sensor and they said because the factory sensor is a 3-wire unit, one of those wires should be a 0-5V signal wire and there shouldn’t be any issues with splicing into that wire to connect to the dash’s OP input wire on its harness. Then, it’s a matter of configuring the dash for that sensor which requires knowing the scaling of the factory sensor – info with should be in the factory service manual. So, sounds hopeful-ish…