Spastic Tach Breaks Its Own Needle

[David Artman]

Hi, folks--boy,am I glad this forum exists! 

I have a Frankenbike CB-1 which has brand new AutoMeter ProCycle tach.

I installed it on Sunday. I immediately noticed during hard revving that the needle has a somewhat spastic "bounce" or "oscillation" as it sweeps up through 9000 - 11000 RPM. I probably should have removed the fuse right then and began investigating, but I am an impatient moron. 

On Monday, I looked down and saw that the needle had snapped off at the hub. It still turns, but half the needle is gone (fallen into the guts of the tach). And here's the kicker: that was exactly what was wrong with the ProCycle gauge that came with the bike when I bought it; something in the bike itself is managing to cause the spastic motion which, in turn, causes the needle to snap off.

My questions:
1) What part of a CB-1 sends the signal to the tach?
2) How could that signal source fail in a way to cause this spastic oscillation?
3) What's the fix?

Thank you for your help;
David

[mechdziner714]

Follow the cable, the problem is prolly on the other end. I know the speedo cable goes on the sprocket cover. The end of the cable has a plastic socket that slips over the nut holding the sproket on. If there is a misalignment that mat caure erratic needle action.

[Rusty]

THERE IS NO CABLE TO THE TACH!!! The tach is electrical!!!
Do you have a manual. If not, get one. You are going to have to trace wires on the wiring diagram.
The 3 wire on the tach. Y/G wire goes to the spark unit, the G wire is the ground, and the BL/BR wire goes everywhere on the bike! With a aftermarket tach, who knows?

[David Artman]

Yes, the tach is all electrical. It has four wires; and I don't know exactly what's connected to what because it is a replacement of the same basic tach, installed by the past (racer) owner, so I reused his wiring rather than tearing the whole thing down. And got the same apparent failure, yes (see above).

NEW INFO:
Funnily enough, my new speedo (AutoMeter ProCycle also) has this odd issue where it will "pulse" a bit, at a stop, if I rev fast (i.e. more than just bliping the throttle). Also, it will cut out at times, at higher speeds (or RPMs?) and then randomly come back on and sweep just fine through the 30s to 50s... and cut out eventually, at highway speeds.

I somewhat suspect a bad ground from the signal sender I've used on the mechanical drive cable; I am going to re-ground to the main battery strap and eliminate that as a possible cause today. OR (*shudder*) it might be a sign of a dieing stator or some other ignition/charging/spark component (pulse generator, maybe??).

I do not suspect the cable-to-sensor connection itself, mechdziner, because I have used a new stock inner cable with a shortened Harley sheath (to fit the sensor's 5/8" fitting) and some hose clamps on the "plastic socket" to avoid unsheathing issues. Thus, it's solid as a rock and doesn't even have stretching stress during turning, because I no longer route it through the steering to the front. Instead, it's routed in a somewhat looping (i.e. loose curves) route that ends behind the left front engine cover plate. The sender's wires are all that routes past the steering to the front, to go into the speedo.

SO, really, I have two, possibly related issues:
A) The tach that goes spastic at high revs, which breaks its own fragile metal needles.
B) A speedo which occasionally registers "speed" at a stop or which occasionally cuts out for a moment or two, both situations during hard revving.

THUS I am pretty sure something electrical is amiss, and I hoped someone knew which part of the spaghetti pile of that wiring diagram in the manual is a possible culprit (and how to test it, perhaps).

Oh, and yes, I have the factory service manual. All it covers is checking for power (got it) and checking continuity on the spark unit connector (could try--but it DOES sweep smoothly through lower RPMs).

I HATE electrical issues...
David

[David Artman]

FOLLOW-UP:
I re-ran the speed sensor ground wire straight to the battery terminal: no change.

I disconnected the main ground wire from the starter (?) and sanded it and reconnected with dielectric grease: no change.

Now I am thinking bad coil(s)? Intermittent ground faults? Charging or ignition component?

At this point, I am completely stumped--I can handle just about anything other than electrical issues. Is there anything I can do short of completely tracing out the wiring system (!) or sending it to the local Honda Certified mechanic to begin diagnosis (!!!)?

Please, help me save myself $500+!
David

[Rusty]

With after market gauges, you are pretty much on your own. I don't think a Honda mechanic will be of much help, unless he has played around with something like this before. You might be better off trying to find the stock gauges to put on it. Hope you get it worked out.

[David Artman]

With after market gauges, you are pretty much on your own.

So... how do all those custom bike builders stay in business? Sorry, but I am fairly sure I could find someone who would be able to trace out a standard Honda wiring harness, up to the gauges--which are using the factory wires. And the speedo is only, really, using a hot and ground from a dash lamp--nothing very "outre" about that.

Thanks for trying, but I am only "on my own" if I refuse to pay big bucks or if no one here or elsewhere online has seen similar symptoms and traced out a fault.

I don't think a Honda mechanic will be of much help, unless he has played around with something like this before.

So... Honda motorcycles never have ground/component faults which might lead to intermittent or inconsistent power to the stock gauges? Again, sorry, but I doubt it.

You might be better off trying to find the stock gauges to put on it.

Not going to happen.

AutoMeter ProCycle gauges are pretty much the Gold Standard for racing. Hell, they're the Gold Standard for just about all racing: watch some in-car cameras on NASCAR: you'll see ProComp gauges in most of the cars.

So... the only thing I'd do to "replace" these gauges is to get them repaired, sell them on eBay (prolly $500+ for both), and put on a battery-powered mountain bike speedo (say, a Catseye Eight or something). THAT would obfuscate the problem, and leave me with a working speedo (except when its batteries die). But it's basically "giving up" and leaving a problem unresolved by using a workaround.

Also, going back to the first point above, I don't see how putting stock gauges back on would change anything about my symptoms. It'd be the same three wires on the tach, and the same two wires on the speedo (and the same white wires for dash lights). OK, maybe there's something weird and magical about a stock tach--some odd resistance, some kind of signal normalizing electronics, whatever--but what cold possibly be "special" about a power and ground wire?

Again, thanks for replying--you're the only one, seemingly, even reading--but I am not trying to move backwards, after all I've dealt with to get this 90% of the way toward "done." 
Now... anyone else got an idea about where a ground or component failure might have occurred? A buddy of mine told an anecdotal story about his older Yamaha that killed headlights... until he redid the clutch and, somehow, "fixed" a poor ground in his case. THAT's the sort of trace out help I really need: someone for whom he wiring diagram isn't a total mind bomb, basically (like, say, a Certified Honda mechanic?)....

Thanks... and help!
David