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Ht/lt Interference And Megasquirt


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Apologies for this being here but it doesn't fall into any of the tech forums we've got. Mods: feel free to move it.

I've been helping to build a megasquirt system over the last few days and the thing is pretty much working but the system requires a rpm trigger from the coil which is picking up interference (noise) from the HT system.

At the moment the megasquirt module is reading rpm as anywhere between 600 and 2000 at idle, and it starts to "fall of the end" of the fueling map at 3000 rpm.

If a 3A choke is fitted in the tacho wire the megasquirt module gets no rpm signal.

Bar screening the whole of the wiring loom has anyone any ideas for a cure??

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I imagine lots of more grown up people have better answers to this - but I've had similar problems.

Things to try:

Use screened cable (only connect the screen at one end).

Try a balanced feed - use a small 1:1 audio transformer at each end of a twisted pair cable (like cat 5 network cable) and connect the primary of the transformer between the LT lead & ground. the other transformers secondary between the input & ground.

Stick a load of ferrite cores on the cable (will damp high frequency components)

Decouple the power lines on the MS board a bit more with 10nF capacitors between +ve & -ve rails.

Move the MS board closer to the coil

Make sure that none of the LT & HT leads run in parallel. If they need to cross, cross at right angles.

or all the above!

May help?


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