Tracing Out Faults at Gauges

[David Artman]

Background
AutoMeter ProCycle tach and speedo.
* Speedo run by a speed sensor attached to the original mechanical speedo cable. Powered by an unused dash light's lines.
* Tach connected to normal tach power and ground lines and signal wire, plus a white wire (I suspect another unused dash light power wire, to run the tach's internal light).

Symptoms
At around 6000-7000 RPMs, the tach needle begins to vibrate--so badly it's broken itself off!--and the speedo cuts out, dropping to below zero and vibrating as well, a bit.
NEW DATA: I noticed today that, if I pull the clutch at highway speeds (high RPMs, when the speedo is spazzing or cut out) and drop RPMs, the speedo will sweep up to the right speed, just as smoothly as it should every time. Likewise, the tach--being revved down to around 2000 RPMs--also smoothly sweeps if I blip the throttle (but NOT if I hard rev back over 6000 RPM!).

Current Theory
After much discussion with various car and bike tech heads--but not one Honda motorcycle expert--the consensus is that my speedo and tach are suffering from my bike not being properly grounded. The NEW DATA above pretty much convinces me that it's all related to power changes as RPMs increases. So now, I have to find the culprit....

Somewhere in the circuit that includes the tach sensor wire, tach power wires, and dash light wires, I have a ground fault or bad component causing "dirty" power.

*flip through service manual; goggle at the wiring diagram*

So does anyone know how I would begin to go about checking for this potential ground fault or component failure?

I have an analog and a digital multimeter, but I barely have used them and I really just don't know where to begin. What do I set a multimeter to, to check for bad grounds? Where do I poke with the leads, to find the fault? Do I have to disconnect anything, to do the checks?

Thanks in advance for any help you can provide;
David

[clarkdw]

If the ground fault only appears at high rpm you may have a VERY difficult time finding it. My suggestion would be to physically go over every ground you think may be related and clean it up with an abrasive where it meets the frame. Check all the spade connectors for corrosion and clean them up. Most of the time if you have an intermittant problem like this it is just as well to cover the basics first. If after you have done all the grounds and connectors the problem still exists maybe talk to an electronics guy and see if you can fit something to clean up the power from the Reg/Rect as it may be too dirty for the tach supply. At lower speed the battery may smooth out the power spikes but as the rpm climbs the charging voltage exceeds the batterys ability to damp the spikes.

I am certainly not an expert in this but I know from my English Lucas Electrics equipped cars that sometimes instruments require cleaner power than is supplied by the charging system. They were equipped with a voltage stabilizer on the tach, gas gauge and temp gauge.

[David Artman]

So--assuming there is no ground fault at all--the coils are not likely to be culprits? It's the RegRect or nothing?

That's odd because I have had to replace the RR twice in four months. I paid a local mechanic a LOT to replace the second one and to *guarantee* me that no other electrical fault exists. He was supposed to go over tthe whole bike with a fine tooth comb, looking for reasons why the RR was blowing.

It hasn't blow in the two months since he "fixed" it, and I have good charge. I don't even have battery overheating, which is a common symptom of too-high RR output.

Could the RR be sending "dirty" power but still work OK? Of so, then I might never fix this problem... and I might go with the voltage regulator idea of yours. (Gee, wouldn't it be nice if AutoMeter made electronic gauges that were self-protecting, rather than self-destructing, when some wonky power issue crops up?)

Thanks for your advice. I'll check that out further. I have done the RR tests many, many times; and I know what it should be doing at all RPM ranges....
David