"half-rpm" indication from Lightspeed
 

Hi everyone,
Any of you lightspeed drivers ever seen your *indicated* RPM drop by half? Apparently my lightspeed loses track of one of the two (flywheel) magnet sensor signals, stops firing two of the cylinders, and reports half RPM. When I turn off my mag, sure enough, it's only running on two cylinders.

Oddly enough, it usually happens at startup for the first minute or so then goes away. But it's now happened briefly in flight too, so I really want to get this figured out. Heat expansion is an obvious suspect, but it can happen when the engine is hot too.

I've been working with Klaus from Lightspeed on this, and have changed my flywheel-sensor gap many times with no change. He's running out of ideas and so am I.

I thought I'd check here to see if anyone has seen anything similar. Thanks for any leads!

Polarity
 

Paul,

I'm not a Lightspeed user, but I understand hall effect sensors, and I'm guessing that's what is used here.

Check to make sure that both magnets are installed with the same polarity. Unidirectional hall sensors really care about this. You could use another small magnet for this test.

Check to make sure that the gap for both magnets is in spec. You may have already done this.

Check that both magnets are roughly equal strength. A small screwdriver and calibrated fingers could perform this test.

Hope this helps.

Ted Johns
RV7 plans preview

Good ol' intermittent problems... How did you measure minimum (crankshaft aft) gap? Did you use clay or something like that? I was initially struggling to get my gap correct, and when I did, it would not operate when the crank was forward all the way. I finally mentioned to Klaus that the clear sealant on the hall effect chip seemed rather thick, and he said "oh, you should take a razor blade and shave it down to the chip" as if it was in the manual or something. In any case, the sealant was probably about .020 to .030" thick, and when removed, I found that I had margin.

I recommend finding the setting for the gap per the book (except do shave off the sealant). BUT, then I believe it is very important to establish that there remains some margin by shimming the sensor board aft by one thin washer from this nominal location. If it works, remove these washers and put it back to nominal. If it doesn't work, one needs to look for something else out of whack.

Thanks everyone for the ideas.

I have played with the magnet gap quite a bit, including taking it all the way down to touching the ring gear (I didn't run it like that), then going up from there. I've put clay in there as well to make sure I'm understanding the size of the gap and that the tracking is correct.

The weird, predictable yet intermittent nature of the problem has got me baffled. Klaus keeps trying to blame it on the fact that I had to cut his sensor connector off to run it through the firewall. Well, I didn't really want to drill a 1.5" hole in the firewall, sorry. I re-wired the connector very carefully using crimped pins. (open barrel pins, unfortunately -- hard to find a DB-15 that takes the good ones)

At the risk of stating the obvious, check the coil wiring on the coil that is not firing. Both connectors at the coil, and the BNC connector at the brain box. The only problem I ever had with my Lightspeed was my own doing...the coil connector wiring, at the coils.

Have you swapped the coils to rule out a coil itself?

Well, I've looked at the coil wiring and it is in good shape, but have not swapped it. I don't think I would be getting the weird tachometer readings if there was a wiring problem there, but I guess anything is possible. Klaus hasn't gone down this road either, maybe we're both wrong. I'll give it a try...

Radial alignment of the magnets.
 

Paul, I agree with your assessment that the half rpm indication puts blame up stream of the coils. It sure appears that the magnet is marginally triggering one of the sensors, and with warm-up or whatever, it goes out of range.

This is a pain, but I'd recommend removing the prop, and using clay to determine not just the min/max gap, but also the radial alignment of the magnets. When the magnets are not flying directly over the chip, the margins are rapidly reduced. By rotating the crank until a magnet is over the chip (requires trial and error with clay), you should be able to measure both gap and alignment, as the magnet should leave a small ring impression in the clay. It may be that your pickup board is slightly high or low. Be sure to check all combinations - both magnets on both sensors.

Good luck!

Agreed, but just to make sure I created another BNC-to-coil wire and swapped it in -- but of course the problem wasn't happening this morning. Then I accidentally ran the engine with both coils *disconnected* and still got a normal RPM reading. So it seems like it's a problem at the sensors, in the sensor wiring, or in the brain box.

I also ran a string from the sensor connector at the brain box to see if there was some intermittent connection in there. Tried tugging on it with engine running -> no problem. I'll keep it there and will try again next time it acts up.

I have put clay in between the flywheel and sensors to see that the magnets are passing over the sensors. They look close enough for me, though one is about 1/32" off. I have asked Klaus if that's a problem, haven't heard back as I believe he's at OSH.

I just remembered another theory from a while ago...at least once, the problem occurred when I turned on the alternator, then went away once I turned it off. Electrical noise or coincidence? The sensor wire runs parallel to the alternator lines.

It is possible other instances of the problem have started when I turned on the alternator, as that's one of the things I do shortly after starting the engine.
However, it usually goes away on its own without turning off the alternator.

I need to experiment with turning off the alternator, next time it happens.

Klaus hasn't heard of alternator interference causing problems with his product and was skeptical of the idea -- nonetheless I tried improving the shielding at the brain box, and moving the signal cable a little further from the alternator leads, to no effect.

Paul, good luck finding the problem
 

I haven't had any problems yet. Mine run next the alt line also.

Kent