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Hall Effect - Rising or Falling edge?

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I am mixing brands and having some issues with the correct way to set things up. The build is a Big Block Ford FE V8. The distributor is a Holley 565-205 Dual Sync distributor which uses Hall Effect sensors for signal and sync. Their documents address using the Holley EFI system but no input on using a different brand ECU. The literature does mention both sensors are 'digital falling'

I am using a Performance Electronics PE-8400 ECU which defaults to 'digital falling' when using Hall sensors. It does allow changing this to 'rising' by checking a box. Information I've picked up seems to indicate the unit should be set to 'rising'

I've started the engine and matched timing, using the 'rising' edge. Seems to run OK but haven't done any dyno work yet. I am getting mixed input as to whether this should be set to 'falling' or 'rising'. Some say "Doesn't matter", others warn about incorrect dwell, others say the rpm stability might not be good.

I admit this is way past my pay grade and I only know enough to be dangerous.

Any comments or suggestions to help me understand this would be appreciated


In theory it should not matter, but there are exceptions where it does. If you've got it working with a stable ignition timing, that is all that matters. Issues like coil dwell can be handled in the PE ECU. I would trust info from PE with respect to their ECU above other sources.

Depending on the trigger pattern used with the engine, the edge used can be very relevant with a Hall effect sensor. Some patterns will have one edge equally spaced around the disk, but the other edge is not equally spaced (there is a number of Cam patterns like this, not so much Crank triggers). If the wrong edge is chosen for these patterns then the timing will be moving constantly with the different edge placements. If the ECU manufacturer specifies the edge that the trigger needs to be set to in the help for the mode being used, then this should be used.

The more generic patterns with equally spaced teeth of the same width usually are not specific, other than ensuring that the sync edge is not in a position that a relative movement of the edge to the Crank trigger teeth cannot cause the Sync tooth edge to swap sides of the nearest Crank tooth.

My ECU manufacturer (Performance Electronics) defaults to 'falling' however they have a check box to change it to rising. Talking to Holley, I get little to no help, their documentation says the sensor is "Digital Falling"

Do Hall Sensors come in differing styles? Or is that part of the circuit design? I am about to wear out my Google button trying to understand this.

The difference in the rising / falling edge is which edge of the trigger wheel you are detecting. Yes, it can be inverted with some sensors / wiring. So the key is to understand which edge you want to trigger on and set the ECU for that.


Your comment "which edge you want to trigger on" -

Is there a mechanical preference? Is the preference dictated by either the sensor or the circuit design?

If the teeth are evenly spaced, then no. With a Hall effect, there typically isn't a preference for the edge that you use, you pick the one that will give you the cleanest trace and lowest chance of an edge swap happening. Where you need to get the edges correct are when there are variances used in the tooth widths that allow for the ECU to use a specific edge to calculate it's location. This information should be provided by the ECU manufacturer for the trigger pattern that you are using, as it will be based on how the trigger pattern mode has been programmed into the ECU.

The Trigger is located inside a Holley Dual Sync distributor. It is a rotating shutter with eight equal spaces/shutters. The ECU manufacturer will allow me to select all kinds of trigger patterns, to match a particular style wheel, this one included.

I've been looking for a technical bulletin on hall sensors to learn if the edges are steep or if there is any slope to them, making that particular edge hard to read. No luck finding this.

Hall Effect sensors by their nature are a filtered output from a Magnetic sensor, so they should have a square edge signal. Sometimes you will get roll over at the top of the square, but this is why you have a threshold for triggering the edge to be read that is around the mid point of the rise or fall, there is also hysteresis values so that the ECU can recognise that the signal is true and to detect the direction.

Honeywell Hall Effect Sensor datasheet

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