Tachometer Question

I am working on my electric conversion for my XJ (still on paper in design phase), but I am trying to incorporate a flashing brake light. By itself, not a difficult task, however, I am interested in a different approach. I want the brake light to be triggered obviously by the brake switch, BUT also, timed with the tachometer. This way, as you are slowing down, the flashing gets slower and slower until it just stays on. If you are traveling above 3600 rpm, the light will appear to be solid, (good for highway braking) and as you hit 3600 rpm and go down the light flashes slower and slower until it stays on.

ANYWAY, the question is, is there an electrical tap for rpm goin to the tap. I didn't see it on my wiring diagram, and as it is about 6 degrees here, I am not enthused about taking my tach apart. Almost as much as my girl was when she came home to find the bike in the kitchen. Needless to say, I need someone who knows to help me out.

 

The electric tach on the XJ gets it's signal from the primary side of one of the coils so you could tap in there. However, the XS1100 uses one of the three phase outputs from the alternator for the signal and that might be easier to deal with since it would require less filtering and so on.

 

most tachs are fed from the neg side of the coil primary, but there may be large induced spikes there
the pick up coil to the igniter maybe but that may load it down

i like a wire on the spokes

 

Didn't the original signal canceller use a reed switch in the speedometer? Count that.

 

It would be infinitely easier to adapt the output signal from a "Wireless" (magnet system) bicycle speedometer triggered by wheel rotation sensor.


The triggering strips along the wheel rim could be cut from "Refrigerator magnets" and "patterned" if needed for both Pulse Generation Duration and frequency.

Bingo!

 

I could be wrong, but I don't think my bike has an auto canceling unit. However, you are right that the speedo would be a better thing to count, as it is based off of wheel rotation and not engine rev's. I gave it some thought last night at work and realized that the tach would fall off dramatically once you put the clutch in and the flashing wouldn't be going on during the full process.

As for the magnets, I really didn't want to go that route, Hall sensor and so forth. I kinda wanted to utilize the existing wiring system, for cleanliness. Guess I'll just have to break a speedo open and see what there is to work with there.

 

There's really nothing much to work with inside a speedo since it only has one moving part. If you take one apart you will be greatly disappointed but at least you'll know what's inside there.

 

Just took mine apart to fix something else. There is a little bell looking thing with a round magnet that goes up inside it. The magnet is what is spinned by the cable. The bell is then moved by the magnet and countered by a spring. The faster the magnet spins, the more force it puts on the bell moving it around until it is balanced by the spring. Any thing you did to this system would effect (probably severely) the reading of the speedo.

 

I happened to have an old broken XJ speedo around so I did a quick check: the reed switch mounted on the back of the speedo housing is normally closed. It appears to open every 90 degrees or four times per cable revolution. That could definitely be used as a vehicle speed sensor (which is what is does for the self cancelling unit).