CPS Tone ring question

Does a crank position sensor sense the little gaps, or just the break in the ?tone??

I have an older single mass flywheel replacing a newer flex plate, and the solids vs gaps are opposite. With this affect anything? I.e, does anything electronically need to be changed?

I think it senses the transitions (first derivatives?), or as you said the breaks.

The gap vs. solid should be preserved. Otherwise, the timing reference position may be off, and there may be electrical noise problems detecting the missing tooth section reliably. Below is what the LH2.4 flexplate should look like:



I think your flywheel is OK (STS would know), but the flexplate is bad - has it run with this flexplate? If so, was the timing at idle correct?

The flex plate is from an ‘01 V70, the flywheel is from a ‘93 850.

When I looked up a flex plate for the 93, this is what comes up. It has a gap for a break.

What engine is this going onto? Auto/Manual? And what ECU - LH2.4, or something newer?

My plan is to run the ‘93 flywheel on the ‘01 engine, adapt a truck transmission and transfer case to it and run it in a Jeep.

I really have no idea what ECU is in it, or how to even check? It was a running (auto) V70 that we had as a parts car. I pulled everything out of it as one unit.

Oh, I thought you were talking about a redblock. I guess I should have counted the crank bolt holes in your picture. Is this going to be a stable mate for your blue Willys?

I know very little about the later white block engines. I'm sure the ECUs became more complex, and are probably different between manual/auto trans, plus OBD-II requirements came into play in '96. In your picture, are the crank holes exactly aligned, meaning that the flex vs. fly holes are exactly lined up? If so, then simply swapping the 2 VR sensor wires, to flip the signal polarity, might work.

This thread may be helpful (I haven't read it)
http://forums.turbobricks.com/showthread.php?t=163457

bobxyz said:

Is this going to be a stable mate for your blue Willys?

Click to expand...

Well... the blue one will get the stable, and this one will stay out in the pasture.
It?s for my plow Jeep. I?m tired of dealing with a choke and 80 year old technology when it?s 20? outside. I have the parts to make it work, why not....

Thanks for the info and the link. I?ll check it out later.

In your picture, are the crank holes exactly aligned, meaning that the flex vs. fly holes are exactly lined up?

Click to expand...

Yes, the flywheel has solids where the flex plate has gaps. They line up perfectly.

As metal passes the magnet, it induces current, causing a spike. A hole causes an inverse (negative) current/voltage.
Just like an ignition coil will fire when wired backwards, it will fire the computer input. Do the math on width of a hole vs circumference/360, that should tell you how far the timing MIGHT be off.
This is my guess.

I honestly don’t recall what my 850 had on its flex plate but countless manual swaps (including mine) have done the reverse swap with out issues. As long as they are both 5 cylinder flex plate / flywheels I doubt it would be an issue.

Hank Scorpio said:

I honestly don?t recall what my 850 had on its flex plate but countless manual swaps (including mine) have done the reverse swap with out issues. As long as they are both 5 cylinder flex plate / flywheels I doubt it would be an issue.

Click to expand...

Right?!! I?ve searched every possible thread on this subject, and I haven?t seen one reference to this being an issue. My only real pause is from seeing every clutch conversion kit that is on the market has a flywheel with 58 teeth and a gap, rather than 58 holes and a solid.

I guess the only true way to tell is to drag the fuel tank inside, dig out all the wiring, hook everything up to a battery, get it running with the flex plate, then try it with the flywheel. Better to do it now than deal with it after it?s all in the rig.

I got enough of the harness hooked up to a battery today for this to run. First I tried it with the flex plate to establish a baseline. I didn't have all the sensors and vacuum switches hooked up, so it wanted to run lean with a high idle, but it would run smooth.

After that, I swapped the flywheel on. At first I was happy. It started right up. But then, the idle jumped up to about 4k and I shut it down. I tried it again, and it wouldn't start; it felt like the timing was all over the place. It would fire once, then it would crank a revolution hard like the timing was way advanced, and so on. Tried again, and it eventually fired, but this time the RPMs shot up fast to about 6+k before I could kill it. I swapped the flex plate back on again, and it again ran fine.

I'm thinking this ECU is not figuring it out.

This spring, I'm planning on retiring my daily driver so I can use the license plates and insurance off of it for another vehicle. It's a '95 Volvo 850, and this flywheel should work just fine with it. It was a fun experiment, and hopefully this might save others from some headaches.


TL;DR, a '93 850 flywheel does not seem to play nice with an '01 V70 engine.

Try swapping the 2 wires from the CPS.

A 2-wire CPS is a VR Variable Reluctance magnetic sensor. It's actually a really simple sensor.

Do you remember back in science class when they demonstrated generating electricity from a magnetic field? Wave a magnet near a coil of wire, or spin a coil of wire inside a magnetic field, and it generates electricity.

A VR sensor consists of a magnet and an iron rod wrapped in a bunch of really thin wire. When a tooth gets near the end of the sensor, the magnetic field sweeps out towards the iron tooth. The outward sweeping magnetic field generates a positive voltage pulse. As the tooth passes and the empty hole goes by, the magnetic field collapses back into the sensor. This generates a negative voltage pulse.

When you change from a flexplate with an empty region for the missing tooth alignment feature to a flywheel with solid steel for the alignment feature, it flips the signal polarity. Swapping the wires should get it back to the correct polarity. There may still be ECU differences between manual and automatic.

Here's an oscilloscope picture of an 18-1 tooth wheel with a VR sensor (VR is blue trace, yellow is ignition coil):

Thanks for the detailed reply, and it makes sense, but I’ve already disconnected everything, wheeled it out of the shop and put the battery back in the dump truck. My motivation for this just tanked today. I’ll have to leave it up to the next guy to try it.

Part of my frustration is just the ungodly amount of electronics attached to this compared to a model just a few years earlier. I just don’t have the patience or ambition to proceed. The wind has left my sails.

Too bad. I would have suggested to go to pirate4x4 for jeep help. And a megasquirt/speeduino/etc board

I’m going to revisit this. I’ve had this whole mess on marketplace for a few weeks and had zero inquiries. It’s worth more to me as parts than what I have it listed for. I’m gonna drag everything back into the shop today and try the wire swapping trick. I’ll post back with results.

If it still misbehaves, see if you can get any diag codes out of it before hauling it to the back pasture and putting it out of your misery. Good luck and best wishes.

I’m regretting this.

Everything is hooked up the way it was, and I can’t get it to fire.
I’ll try again later. Hopefully the battery was just too low.

Ok. I got it running with the flex plate and the CPS wires in standard orientation. Runs well.

With the flex plate still in place, I swapped the wires on the CPS... and it runs... the same?!

I swapped the flywheel in place of the flex plate, with the wires still reversed, and it runs the same! Sounds great!

I swapped the wires back to standard orientation just for science, and it runs, but it doesn?t like to be revved. It stumbles, then the idle hangs high.

I swapped the wires back to reverse orientation, and this time it ran with high idle, I bumped the pedal to see if it would adjust itself down, and it did somewhat, then it died. It did that no matter what I tried with the flywheel on.

At this point, I grabbed my cheap scanner. At first it was just a trans position code and a (810) clutch position control malfunction. (??) Odd, since it?s automatic. I cleared the codes, ran it again until it died (8 seconds?), and this time I got 4 codes. 446 (EVAP), 1500 (my code book doesn?t even go that high), 453 (EVAP), and 73 (ambient air temperature sensor circuit high).

I swapped the flex plate back on, and it runs fine again. I?m now convinced that this system needs a flywheel with the gaps, rather than an earlier flywheel with the holes.