[Diy_efi] Problem with cam mounted optical trigger sensor and RFI

Charles Slack slack
Sun Apr 2 16:35:16 UTC 2006

Damian, Looks like you may be having RFI running back through the DC 
supply lines.
Put a large capacitor say 500 Uf @25 VDC and a small cap ( 0.01 uf @ 25 
VDC) across
the dc power leads  to the CDI ignition module.  You can try a 0.01 uf 
@100Vdc as
a HF bypass across the pick up leads, and across the incoming air temp 
My 2 cents.
Charlie Slack
Austin, TX

Damian Long wrote:

>  I am installing a Walbro TDD ecu. It uses a motronic style 60-2 wheel. I have cam mounted this wheel. Basically what I have done is removed the standard cam mounted trigger disk (found inside the distributor) and replaced it with a custom disk. 
>  The sensor used to read the disk is a common standard Mitsubishi optical sensor that is found in the likes of Hyundai Excels, Nissans and Mazdas. I am using a sensor from an Nissan SR20 engine.
>  What is happening is that everything works fine from a cold engine. The ecu reads the signal perfectly and the engine runs great. As soon as the engine gets warm it seems like the sensor (or wires) gets some RF interferance from the high tension leads. The rpm reading goes nuts like 54000rpm and ecu obviously complains. At first I thought maybe a broken sensor as I have seen these have problems before when they get hot but the problem seems very dependant on how and where I run the wires to the sensor. Also if I connect a high power CDI ignition the problem is 10 fold. It does it from cold or hot. This leads me to believe it is not a broken sensor and the temperature has something to do with letting the RFI in.
>  Looking at the signal going into the ecu on an oscilliscope there is a nice square wave even when the strange RPM begin to happen. I don't totally understand why this is. It could be the RFI is causing the circuit inside the optical sensor to send false triggers (there is a small conditioning circuit inside the optical sensor).
>  All help appreciated.
>  Oh and Matt, if you are reading this. I live right near you. I am in Wollongong. I have seen your AFR gauges in a friends car. I like them and may buy one. 
>  cheers
>  Damian
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