my new hall effect trigger setup.

[Xander]

A while ago I was checking the timing on my 305 SBC. A was using the stock (7pin) ignition that came with the engine. It was a TBI engine so the ignition was already computer controlled. There was no advance mechanism in the distributor. When I Checked the timing at idle I fixed the ignition advance to 12 degrees with my megasquirt. The timing was jumping somewhere between 5 and 20 degrees!. When I gave it some gas it became a bit more steady but still no where near what I would consider rock solid. Also when I accelerated the engine to about 3000 rpm the timing would shift a couple of degrees. This combined with a strange trigger mis at 2200 rpm made me deside that the stock trigger is junk.

 

My old setup on my l28et consisted of 3 trigger magnet in the flywheel with a homemade trigger using a small hall effect chip. This setup was very cheap and very accurate. I build the same setup for my SBC.

 

I did not feel inclined to remove the engine to get to the flywheel so the trigger magnets had to go in the balancer. When I took the balancer of I was pleasantly suprised to see that GM already drilled 12 evenly spaced holes in it.

 

 

4 allen head bolts where bolted to the balancer and the heads where drilled to a slightly larger diameter than the magnets used. The magnets where bonded in place with epoxy resin. BUILD THE TRIGGER BEFORE YOU GLUE THE MAGNETS. The magnets only work one way. If they are installed the wrong way around they will not trigger the switch. if you build the trigger first you can check the magnet orientation by setting the output of the middle led on the MS unit to trigger IRQ. This will make the led light up when a trigger (magnet) is present. I used a blue marker to mark the trigger side of the magnet before glueing the in.

 

 

The trigger was a small hall switch which cost about 2 dollars. It is tiny but works very well.

 

 

I built an extra connector into the megasquirt unit so that I could use a shielded wire. The connector is a mini XLR connector used on guitars and such so it is built to withstand a bit of fibration and it will not easilly come apart.

 

 

The trigger itself is bonded to a piece of aluminium and bolted to the front of the engine.

 

 

Conclusion The stock ignition pickup on a tbi SBC v8 is junk. An easy way to install a new trigger is to do the above. It works like a charm. The ignition is very accurate and it does not shift at higher rpm. At idle (500 rpm wit this engine) there is still a little bounce which is probably due to the low resolution of using only 4 magnets. This is where a 36-1 trigger wheel would be better. But at anything over 1000 rpm this small bounce disapears. Did I mention that this is a very cheap setup. Anyone with a soldering iron and an angle grinder can build it.

 

this would be an appropriate switch http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=620-1047-ND

it will work within a voltage range of 4.2 to 24V. temp range of -40 to +150 deg celcius.

When viewed from the printed side pin 1 (left) is connected to a decent power suply. This can be pretty much anything from 5 volt from the megasquirt mainboard or straight battery voltage. I used a sanctioned 12 volt supply from the megasquirt board. The middle pin is the ground connection. There are plenty places on the mainboard to connect this pin. I used the ground pin just above the proto area on the v3 MS board. Pin number 3 (on the right) wil be directly connected to the output side of the opto isolator. The input side of the opto isolator is connected to a 5 volt suply. With this setup you are not triggering the positive side of the opto isolator but instead the hall trigger grounds the negative side when a magnet passes by. Don't forget to connect OPTO out to TSEL. This connects the opto output to the megasqiurt ECU.

 

This blue lead connects the opto in to a 5 volt source.

 

these bleu leads connect to the hall effect switch. XG1 is the negative side of the opto isolator. S12 is a sanctioned 12 volt supply and GND is the ground.

 

 

This is basically the same setup used by SDS. Please check there website for more information regarding sensor placement http://www.SDSEFI.com

[mikeatrpi]

Thank you for the very detailed write-up.

 

Do you have to do any kind of alignment with the magnets with respect to the ignition, or can you take care of it all in software?

 

Edit - I'm looking to do either a VR distributor trigger or a Hall-effect sensor as you describe. I'm assuming if I did this on an L series engine I can drill holes in the harmonic balancer if they weren't already present.

[Xander]

The magnets have to be evenly spaced. 120degrees for a 6 cylinder and 90 degrees on a v8. Have you read the magnet guide at SDS? it goes into detail about how to place the magnets.

 

http://www.sdsefi.com/em2def.htm#EM-2E

 

here is the link. there used to be a better page with pictures, but I can't seem to find it right now.

 

Basically what you do is set the engine to TDC and then mark the place on the pulley where the trigger sits. The first magnet is located 60 degrees from there. A great advantage with the 6 cylinder engines is that it doesn't mater in which direction you go. if you accidentally place the trigger 60 degrees in the wrong direction then the next magnet you place will be right where it should be. because they should be placed 120 degrees apart. The exact placement of the magnets in relation to the trigger is Less important that having them exactly 120 degrees apart. The trigger angle can be trimmed in the megatune software.

 

I have used a stock VR distributor too. It worked well, but the hall setup was more accurate.

 

edit: this was the link I was looking for

 

http://www.sdsefi.com/techhall3.htm

[Six_Shooter]

The bounce at low RPM is probably due to the outer ring of the harminic balancer, where you attached your magnets, actually moving, or twisting more specifically around the axis of the crank, as per the function of a harmonic balancer. The movement or twisting is probably either less, or not noticed as much due to lack of time to actually be able to see any error.

 

I'm surprised there isn't more wandering of trigger signal. I had thought about doing something similar with an engine I built, but would be notches in the outer ring, not magnets, due to the ignition system I was running and the sensor I was using with it. I opted for new disc to take any possibility of a wandering signal out of the equation.

 

Looks pretty cool though, I like it.