Recreating a Crank Position Signal for a Mazda RX8 - thoughts?

[radiogareth]

This is the situation: I'm putting a different (non-original) engine into a Mazda RX8 which has (had) a rotary engine. Understandably the rotary engine does not have a conventional ignition trigger wheel (such as the 36-1 Ford device), but it does have the commonly used VR sensor. Its not a problem other than the original Mazda ECU/car management ecu expects an engine signal before it will turn on the electric power-assisted steering rack. It also drives the rev counter. One fix is to mount a replica Maxda trigger wheel onto my new engine, but since my new engine outputs a suitable RPM signal I thought 'why not get a Picaxe to replicate the Mazda pattern'. The new engine runs on its own ECU and outputs a '2 pulses per rev.' tachometer signal.



I envisage a Picaxe counting these two pulses and outputting the specific stream at an engine speed related rate. The power steering is not speed sensitive, but clearly the tacho is!
The attached picture shows what I'd need to replicate, every two times an imput pulse is received.

My initial thoughts are that a simple squarewave of appropriate M:S ratio may well work. To shape the pulse a bit I wondered about a simple RC filter (or CR) or perhaps use a picaxe output to drive a spare sensor and physically place the driver next to the Mazda original.

Maybe someone has done this already. Maximum RP of the donor will be 6,500 rpm, so the styytem will need to scale accordingly as the engine speed rises.

Thoughts and comments??

Thanks

Gareth

 

[srnet]

Thoughts and comments??

So you are planning to re-engineer the steering, and possibly engine management, on a road going vehicle ?

If you mess it up, or the advice on here is poor, are there any safety implications ?

 

[radiogareth]

No re-engineering of the steering nor its management. I'm just wondering if an engine speed in one format could be re-created in another. The steering still works without assistance, its just a lot harder work as many a Vauxhall owners can testify when the factory electric-assist system goes awol. The RX-8 is a high spec sports with a CAN-bus that links all sorts and the build quality of all the components is excellent. The engine management is untouched other than a parts specific tune and the removal of its anti-theft option (software delete). Re-engine-ing cars is allowed under various dos and don't as specified by the DVLA. I've even built one form a pile of 'sundry parts' and got it registered, plus several rebuilds and a kit car that was my daily for 10 years. If it proves in any way to be unreliable, I'll revert to a mechanical solution which would equal what Mazda built. At this point its sitting in my garage pending other work such as its LPG conversion, just trying to get ahead of things.

 

[hippy]

I recall some previous discussion on using engine shaft rotation to give an appropriate signal for a tacho which may be of use.

 

[radiogareth]

As far as I have got with research, the VR sensor feeds the required ignition timing info the the Mazda Engine ECU which fires the injectors and coils and also communicates (via CANbus I believe) with the steering ECU which controls a huge heatsunk 'power controller' on a 60 amp fuse! Somewhere in there a signal gets sent to the dashboard (itself sporting several sm micros and entirely double sided sm on epoxy/fibreglass!) which them moves a good old needle.
Lots of the old threads relate to the area of rpm, but not found anything that processes it much...yet....