So, any EE's out there that will explain how a tach works?

[DonW]

I sort of get it. Some hocus-pocus wizardry circuit count the pulses of "on/off" from the coil, and translate that to a voltage that drives the meter needle to the correct position.

I've tested 3-4 tachs on my bike this week. One works fine and steady (it's from a '03 R1150RT). Several of the others work OK up to about 3.5K RPM, then start to dance- UP, down... steady... UP, NEEDLE BANGING UP, down... back to zero...

Similar symptoms on 2 of the tachs, and one does nothing.

Anyone got an explanation. Really just for intellectual (???) curiosity.

Don

[Major Softie]

Since two "bad" tachs exhibit the same behavior, I'd try them on other bikes before even suspecting those tachs themselves. It could be that your bike's ignition is creating EMI that is interfering with the tachs performing correctly, and the Oilhead tach has better suppression circuitry built into it, so it's able to ignore it.

[robert]

Since two "bad" tachs exhibit the same behavior, I'd try them on other bikes before even suspecting those tachs themselves. It could be that your bike's ignition is creating EMI that is interfering with the tachs performing correctly, and the Oilhead tach has better suppression circuitry built into it, so it's able to ignore it.

To add to that, a bad connection between the signal source and the tach could give more or less pulses for the tach to see.

[DonW]

OK, great ideas. Last week, I ran a separate lead from the coil connection to the tach to see if that changed anything, and it didn't seem to help.

Now, about EMI- anything I can do to filter or suppress it? What might cause it?

Thanks,

Don

[Major Softie]

OK, great ideas. Last week, I ran a separate lead from the coil connection to the tach to see if that changed anything, and it didn't seem to help.

Now, about EMI- anything I can do to filter or suppress it? What might cause it?

Thanks,

Don

I understand a tinfoil hat is de rigueur.


Running without resistance caps or plugs can do it. A screwed up coil can do it. A bad condenser can do it, but that would be pretty severe symptoms for that. A screwed up electronic ignition can do it too.

[blitz]

This can also be the manifestation of a bad ground. I had that problem a few years ago, and traced it to a bad ground. (Sorry, i don't remember where it was bad.) When the speedy failed for the 3'rd time, i installed a Trail Tech Vector Speedy/Tach combo.

[Major Softie]

I just don't know anything that can't be attributed to a bad ground . . . low compression, maybe.

It's amazing how many ways a bad ground can F you up.

[Ken in Oklahoma]

OK, great ideas. Last week, I ran a separate lead from the coil connection to the tach to see if that changed anything, and it didn't seem to help.

Now, about EMI- anything I can do to filter or suppress it? What might cause it?

EMI susceptibility/interference is a big deal in military electronics, especially aircraft related. If the tach has a separate ground wire in addition to the signal wire you might use the simple expedient of creating a twisted pair feeding the tach. The idea is that any interference will affect both the positive and negative leads, and since they're twisted together, the net effect cancels out.

Another approach would be to use a shielded wire, which would be a center wire surrounded by a finely braided metal jacket (and then covered by a plastic or rubber sleeve. The metal jacket would then be terminated to ground at both ends. The idea is that any radiated interference is shunted to ground, thus shielding the center conductor.

Yet another approach might be to connect a capacitor to the tach signal lead as close to the tach as practical. The other end of the capacitor would be terminated to ground. The idea is that high frequencies can pass through a capacitor to ground whereas the low frequencies associated with the tach signal would remain unaffected.

As to where to get this stuff, you can twist wire pairs using a homemade hook in your electrical drill motor. Shielded wire might be harder to come by, but just maybe Radio Shack would have it. Failing that, if you have access to a surplus electronics store you should be able to find what you need there.

The possible flaw in my reasoning is that we're talking about a motorcycle tachometer intended to be used on a motorcycle. I would expect such a tach to already have internal electrical filters to do deal with any likely spurious interference. But hey, you're desperate!

Continuing my scattershooting, do you have resistor wire or resistor caps on your plugs?

I'm going to be out of touch for a few days so I won't be available for any follow up.

Good luck.

Bonus comment: In the above I am disclaiming the likely value of my input. My old boss had a saying that I like. "In the land of the blind the one-eyed man is King!"



Ken

[barryh]

I've little hands on experience with electronic tachometers but I did train in electronics and the primary components likely to fail have to be capacitors, so if I ever had a problem with my tach that's what I would replace first.

Most capacitors are inherently subject to aging and can't even be easily manufactured to close tolerances except perhaps tantulum capacitors and they are prone to failure too.

Funny that we are only too ready to regularly replace the condenser on points ignition systems even though that happens to be a fairly reliable type of capacitor. The input capacitor in an electronic tach may be seeing the same voltage as the points condenser.

[DonW]

OK, here's more info. I test ran 2 of the tachs again yesterday. For the first 15 minutes, they worked fine, though one seemed to read pretty high. Then, they started getting erratic again. This would lead me to believe this is HEAT related, not EMI or other electronic interference- that should ALWAYS be there, if at all, not change with heat.

Another possibility is the ignition control module, which are prone to overheating and failure. BUT- the bike runs great, it's just the tach.

OK guys, now what do you think?

D