Tandem ECU's

How would I go about running an aftermarket ECU in tandem with the factory ECU? I understand the concept but I don't understand how I would wire it so that certain inputs can be shared between the two ECU's and still have the factory ECU run the stock auxiliaries as normal.

G'day Timothy.

Can you give us some more specifics on the application? What's the car, and what's the aftermarket ECU?

Essentially though, you splice into the sensor wires you need from the factory harness, and run them to the aftermarket ECU. However, there are some things you need to take into account, like which ECU is powering the sensor, as it will also need to be providing the sensor ground. For 2 wire temperature sensors you might need to wire them to an analogue input channel on your aftermarket ecu, instead of an analogue temperature channel, as the OEM ecu will be providing a pull-up resistor on the factory signal wire.

The car is a Suzuki Swift Sport ZC32S. I'm looking at installing a Haltech Elite 1500. I want to retain as much factory function as possible while having full control over my engine. The main reason I want to run tandem ECU's is because the gauge cluster gets its inputs via CAN and from what I've dug up so far, is extremely hard to decode for aftermarket (damn Bosch). This brings the concern of the stock ECU throwing a CEL even though I know the engine is controlled by the Haltech. If you think another brand of ECU might better suit my application, please inform me. I only chose Haltech because it's my preferred ECU and I feel comfortable tuning with their software.