flywheel tachometer?

[Smokey Eddy]

I have a sensor that will spit out a signal when a magnet passes by it.
If I inset a strong rare earth metal magnet into the flywheel, could I read rpm through the peep hole on the transmission?

I would be feeding this output signal to a tach I bought years ago that just looks for a signal on a wire. Its a Sunpro tach.

[CrazyAndy]

I don't see why it wouldn't work, but are you sure the tach needs to see only one voltage spike for it's signal? Not saying it won't work, but most tachs take a signal off a distributor, and they're timed for a specific cylinder number.  It might work if you put 4 magnets in the flywheel to replicate a 4 cylinder engine then it might work.  The other thing is making sure the sensor is a certain distance from the flywheel, but you probably already know that.  Are you sure the sensor will have a sufficient output to trigger the tach?

[Smokey Eddy]

No, I have no idea. I also have no idea how I could test anything without a frequency generator and scope. If I recall I remembber there being a switch on the back of the tach to specify the number of cylinders. 3 more sensors would be pricey. It might be easier to make a multiplier.
I forgot the fact that the tach is looking for a signal from a distributor. Flip.

[libbydiesel]

A 4-cyl sees two pulses per crank rotation.  Two magnets would be easier than two sensors.

[Smokey Eddy]

Deeerrrrrrr of course. I'm not thinking clearly today.

[92EcoDiesel Jetta]

You're in Canada, wouldn't it be easier to to find a factory VW diesel tach to run off the alt W terminal?

If you want to run off a magnet(s) on the flywheel, here are some tips from actual experience:

In my EcoDiesel, I have been running a gasser VW CE2  tach with 2 magnets on the harmonic balancer for a few years now. It works great as long as the magnets stay put. One of them keeps flying off and the tach would read 1/2 actual rpm until I glue another magnet back on. If you put magnets on the flywheel, the centripetal force is much greater than on the harmonic balancer so you would need to secure the magnet(s) much better for it to stay put. I'd suggest tapping the flywheel and use a machine screw instead of glue.  Before you do all that work, I'd strongly suggest testing in with a disc with magnets on a drill to make sure the tach works first.

[Smokey Eddy]

Re: point #1, they go for around $100 if you can find one.

Re: point #2, very sound advice. I did try this tach set up with the pulley from the crank shaft but i also had a hard time keeping the magnets in place. What kind of glue do you use? I tried to make a tapered hole and press the magnets in which lasted for a few days of driving but the magnet dissapeared before i finished the mount for the sensor. The pulley is alluminum however which after being replaced with a neodymium magnet i would imagine the balance would be off. Not enough to really matter though i don't think... (thoughts on that?). I think i'll change my timing belt and water pump gasket soon (when it really heats up) and at the same time i'll drill 3 more holes for magnets and give it a go. since the pulley is aluminum i think i'll peen the alluminum over the magnets a bit to help keep them in place. Maybe some JB weld in there for glue would help also. Suppose I'll have to rough up the magnet too so it has something to grab onto.

[bajacalal]

I vote for using a tap and bolting them on, with a small screw.

Using a tap to make a threaded hole is easy once you get the hang of it and you don't have to buy a whole tap set if you don't want to, just the size you want and something to turn it.

[CRSMP5]

loosing them off crank.. glad they not in your timmingbelt...

on flywheel... i see hole in tranny case... not lots of room for it to fling off..

i think alt fan best.. heat up red hot.. dimple for mag spot.. let it cool.. press magnet in..

[92EcoDiesel Jetta]

Re: point #1, they go for around $100 if you can find one.

Re: point #2, very sound advice. I did try this tach set up with the pulley from the crank shaft but i also had a hard time keeping the magnets in place. What kind of glue do you use? I tried to make a tapered hole and press the magnets in which lasted for a few days of driving but the magnet dissapeared before i finished the mount for the sensor. The pulley is alluminum however which after being replaced with a neodymium magnet i would imagine the balance would be off. Not enough to really matter though i don't think... (thoughts on that?). I think i'll change my timing belt and water pump gasket soon (when it really heats up) and at the same time i'll drill 3 more holes for magnets and give it a go. since the pulley is aluminum i think i'll peen the alluminum over the magnets a bit to help keep them in place. Maybe some JB weld in there for glue would help also. Suppose I'll have to rough up the magnet too so it has something to grab onto.

If I were you, I'd spend the $100 and buy a W terminal tach.  

I used crazy glue and epoxy. Strangely, only one of them keeps flying off (the same one) so it must be bad surface prep. My harmonic balancer pulley is steel (magnet sticks to it). You sure yours is aluminum? I don't think there is enough meat to tap the pulley for a screw, it would go through and hit the belt. Not sure if you can tap the flywheel either. Wouldn't it hit the starter motor gear?

[bajacalal]

I'm sure there is plenty of meat on it. The belt only goes around the edge. Do the magnets really have to be on the edge? As long as you get 2 passes/revolution and the sensor is mounted above the surface, wouldn't it work? With long enough magnets or a bracket it would still reach the edge. I would worry more about it not being balanced.

[92EcoDiesel Jetta]

They don't have to be on the edge. My magnets are on the edge of the pulley (actually sticks out beyond the edge about 1/8"), which makes making and mounting the bracket for the pickup and changing the pwr steering belt much less complicated .

[92EcoDiesel Jetta]

loosing them off crank.. glad they not in your timmingbelt...

on flywheel... i see hole in tranny case... not lots of room for it to fling off..

i think alt fan best.. heat up red hot.. dimple for mag spot.. let it cool.. press magnet in..

My magnets are on the harmonic balancer, not crank, so no fear of magnet jamming the timing belt.

[R.O.R-2.0]

http://www.vwdiesel.net/forum/index.php?topic=30442.0

hope some of that helps.. because it worked for me.

[fatmobile]

 Kind of off topic but I've been wanting to use a reflective surface,.. like the laser tachs use.
 Completely different sensor,.. but no magnets to fly off.