Sale ends todayGet 30% off any course (excluding packages)

Ends in --- --- ---

Harness design and power supply questions

Practical Motorsport Wiring - Club Level

Forum Posts



Tech Articles

Discussion and questions related to the course Practical Motorsport Wiring - Club Level

= Resolved threads


Hi all, got some questions below for my engine and body wiring harnesses in a 1977 Celica with 1UZ with VAG coils and a Link G4+ Xtreme ECU.

I'm looking to run a 47 pin Deutsch HD30 series bulkhead connector at the firewall, this has 5 size 16 pins that I'll use for coil and injector power supply (4) and ECU Ground to engine block (1).

1. To supply power to the coils and injectors, is it OK to use 4 of these size 16 pins to run 18AWG TXL wires (from a relay) and splice these after the bulkhead connector?

2. Do I run the Alternator connector wiring (not the main post to battery wire) with the engine harness, or is this better to be run with the body harness? Is it the same for the Air Con connector?

3. Running a Vehicle Speed Sensor from the gearbox, is there any benefit in running this to an ECU input?

4. Using Speedhut Gauges for Oil Pressure and Water Temperature, can these be run off an ECU output rather than running separate sensors just for the gauges?

5. Will be running a new mains power from the battery, what size wire should this be (10AWG?), and what size Main Fuse (100A?) is appropriate for a general road car? (no big stereo install.)

6. Will be powering the ECU off a 40A relay and splicing this off to the coils/injectors relay, should the fuel pump and thermo fan relays also be powered from the main /ECU relay?

7. Do I run 1 Fuse after main relay then from there to ECU and other Relays? Or do I run a seperate Fuse between the main relay and ECU, and the main relay and coils/injectors relay? Or do I just run a Fuse between the main relay and ECU then run a Fuse for the coils/injectors after their own relay?

1, What sort of coils and injectors are you using? 18AWG will carry a fair bit of current.

2, I would run them in the body harness.

3, Yes, you will find that there are a number of uses for this such as gear determination, Launch Control and Coolant Fan operation.

4, I haven't used these gauges, so I cannot specifically comment on their operation, from my experience you can share the Voltage signal from the pressure sensor without too much offset occurring as long as the gauges are bot using the same power supply and ground sources. If the Temperature sensor is using the resistance change of the sensor, then that will be skewed by the presence of a second device reading the signal.

5, How much total load will there be on the battery (worse case, cranking a cold engine when the battery voltage is low and everything is cold) and how far away is the battery going to be mounted from the main load?

6, Fuel pumps and Thermo Fans should have a separate relay and fuse. What size fuse are you using for the ECU?

7, Fuses protect the wires.

1. VAG Coils, as used on Audi R8, and other Audi/VW's. Denso high impedence injectors

4. Rather than 2 sources reading the same signal, I was hoping that the ECU could read from sensor on input, and the gauge can read from the ECU's output

7. The attached diagram doesn't show it, but the ECU will need a Fuse, it's whether it goes before the Splice off to sub-relays (Coils, etc) or after Splice. I'm assuming after Splice otherwise it'll need to be a Fuse too big to protect the ECU and protect the Coils/Injectors etc, if it goes after the Splice then if it blows it only shuts off the ECU and the other Relays still remain powered thereby making the whole point of running these other Relays triggered by the Main Relay obsolete

Attached Files

Those coils and injectors could be powered off of a single run of 16AWG. I tested one of those VAG coils recently (I do the coil calibrations for a major ECU company) and one of the tests that I do is to see if the coil can run a V8 through a distributor at 10,000 rpm. Under this load, the current draw for the coil averaged out to just under 4A/sec. The injectors will be drawing less than 1A per firing.

If the gauges can read the CAN stream from the ECU, then you could use them, but most ECU's will not output a signal that could be used to replace an analogue sensor signal.

6 & 7 - do your relays as per the diagram in the help file - copy below. This is the best way to avoid potential for a back feed and ideally you want the ECU and injectors connected to the same source so it can apply correct deadtimes. The fuse on the main relay in this diagram as it is below would be 10A in most cases which is still ok to protect the ecu and injector wiring. If you have low imp injectors then you may want a separate lower amp fuse for the ecu after the main relay.

Note wherever possible your fuses should be as close to the battery as possible, in the diagram you attached in your last post your fuel pump fuse being after the relay means you have a whole lot of unprotected wiring. The fuse is to prevent a fire, not protect the fuel pump.

Thanks guys, think this is enough info for me to go off.

We usually reply within 12hrs (often sooner)

Need Help?

Need help choosing a course?

Experiencing website difficulties?

Or need to contact us for any other reason?