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2 5/8", adapter, converter, rubbermount, tach

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  #1  
Old 3rd October 2011
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Default How to wire a tachometer on a carbed rubbermount

Some of this may be review... skim if need be!

If any of you with a carbed rubbermount (Roadsters excluded) have ever priced a tach from Harley - you might have jumped a little at the price. While the MicroTach offered by one of our supporting vendors is definitely a great option, some of us still want to stick with an actual gauge. My choice was pretty much made for me as I happened across a free 2 5/8" Harley tach.

Without getting into too many details, Harley tried a little "experiment" with the 04-06 models involving a serial data bus. I won't pretend to know a ton about this, but from what I've gathered it seems that for these models, vehicle speed, engine speed, check engine light, and oil pressure light data was all sent digitally on ONE wire from the brain box. The gauges "listen" to this wire and display the appropriate information. The trouble is, every Harley tach I've ever ran across is an analogue tach and has no idea what to do with this one magic wire. This is the reason you see the same tach kits fitting up to 03 models and 07+, but not 04-06. Our beloved carbed rubbermounts require an additional, expensive, ugly, magic black box to decode the digital signal and convert it to analogue so the tach displays correctly. I don't recall the exact part number for the magic black box, but it comes in the 04-06 kits along with the same old Harley tach. It also bumps the price to the tune of around $180 over the regular kit. Not cool Harley.. not cool. Thankfully they included a new pink wire off the ECU in 07+ bikes that serves as an already decoded, analogue signal for tachometers. Hence why the tach install on my SB was ridiculously easy by comparison.

So why don't we just run the analogue signal wire from the tach directly to a coil like everybody used to? Well because Harley switched away from the "wasted spark" model and you'll need to splice into both coil triggers for an accurate signal. This requires a single fire tach adapter to provide an accurate signal without letting the coil trigger signals "bleed" over into each other (as I understand it). These adapters are available from all over - google is your friend. But in reality, they're usually nothing more than a couple resistors and diodes wrapped in heat shrink. So we're going to make one after taking a trip to Radio Shack. You'll need a couple 1/2 watt 100k ohm resistors and two 1N4007 diodes. You'll need to buy packages of each, but they're cheap. Go for the multipacks if they don't have exactly what you're looking for - particularly with the diodes.





This link has been thrown around a lot when it comes to making your own tach adapter. I know it's for metrics, but the guy has good pictures and diagrams. Unfortunately, they're backwards. Most websites I've ran across that talk about this have diagrams with diodes and resistors pointing the opposite way from his. Maybe there's something different with the bikes he's talking about, I have no idea. We're both trying to make tach adapters for negative coil trigger systems, so I really don't understand what else could be different. Don't follow his diagrams - the tach won't work. Instead, follow this diagram.

So to start, twist up a pair of these with one diode and one resistor each. Note the orientation of the color bands on the resistor as well as the silver band on the diode. Don't mix this up or it won't work correctly. Next, twist the ends of each pair together (the ends in the LEFT of this picture, AWAY from the silver band) this is where you'll affix a single wire to run to your tach. The two remaining ends on the other side will be run to each of the negative coil trigger wires.



Heatshrink the whole affair up after SOLDERING the connections. While twisted might work in a pinch, it will eventually vibrate loose and you will not be happy.



Regarding actually attaching the wires to your coil triggers, you've got a lot of options. The connector to your coil pack will have three wires - Yellow/Green, Orange/Blue, and White/Back. The W/B wire is the common positive connection - leave it alone. The other two are the negative triggers we're after.



You can tap them back at the ICM under the seat, or really anywhere up to the coil pack. Some might choose to use the "t-in" connectors commonly found at RS or Wally World or wherever, but I don't trust these on a bike. I've seen them cut right through a wire before, and that would not be much fun on the side of the road in this particular situation. In order to keep things simple, I chose to splice into the wires from the harness under the tank which I had previously hidden on top of the back bone (tank lift). This seemed to be the cleanest place to me, and can't be seen unless you lift the tank. There's all sorts of wires there anyway! I used a direct soldering/heat shrink method so I don't have to worry about it again. Tons of tutorials on how to splice into things properly online, so I won't bother with that here.

After that it's just running the ground and power wires for your tach. Make sure you pick a wire that turns OFF when ignition is off and you'll be made in the shade. Total cost for me was less than $10, but the tach was free. I'd imagine this would work just fine with any other aftermarket analogue tach designed for 2-cylinder engines.
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Old 3rd October 2011
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what is the point of the diode if the resistor is connected in parallel with it?
the diode only allows power flow in one direction, the resistor allows flow in either direction. the resistor would negate anything the diode is trying to do. so unless there is something i am missing here the diode is not needed.
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Old 3rd October 2011
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Quote:
Originally Posted by caain View Post
I don't recall the exact part number for the magic black box
The D/A converter box is # 72358-04. It used to cost around $167 (eg: at Chicago H-D), but it's now discontinued.
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Old 3rd October 2011
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Originally Posted by Folkie View Post
The D/A converter box is # 72358-04. It used to cost around $167 (eg: at Chicago H-D), but it's now discontinued.
Thanks Folkie. I seem to remember you talking about the box on a thread a while back.

Quote:
Originally Posted by asasial View Post
what is the point of the diode if the resistor is connected in parallel with it?
the diode only allows power flow in one direction, the resistor allows flow in either direction. the resistor would negate anything the diode is trying to do. so unless there is something i am missing here the diode is not needed.
Your guess is as good as mine. I've heard a lot of explanations for both of them, but using both is in every tach adapter diagram I've ever ran across.

Somebody said the diode keeps the signal from bleeding from one trigger into another (grounding both at the same time rather than just when each one needs to fire), while also claiming the resistor "smooths things out". I suppose it makes sense, but then again he could have been talking about of his ass. I do it frequently
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Great write up, BTW.
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Fair enough.
It would allow full signal from the coil to the tach through the diode, and add resistance to any signal from the tach to the coil, but i'm not sure why you would need any signal/voltage going from the tach to the coil.
but hey if it works it works.
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Quote:
Originally Posted by asasial View Post
what is the point of the diode if the resistor is connected in parallel with it?
the diode only allows power flow in one direction, the resistor allows flow in either direction. the resistor would negate anything the diode is trying to do. so unless there is something i am missing here the diode is not needed.
100K Ohms is quite high impedance. The diodes allows the negative(-) tach signal to reach the tach without the coil signals getting crossed. (Remember: diodes only let electricity flow in one direction.) The high impedance resistor smooths the signal by allowing some of it to return to the coil, thus smoothing out the tach needle.

PS: LOL. I just read your second post there, caain, and I decided to leave mine up here to bolster the "smoothing" argument. And the diode part is correct too.....
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Quote:
Originally Posted by asasial View Post
Fair enough.
It would allow full signal from the coil to the tach through the diode, and add resistance to any signal from the tach to the coil, but i'm not sure why you would need any signal/voltage going from the tach to the coil.
but hey if it works it works.
The wire from the tach is in a "high" state, which means it has voltage on it. I think this voltage varies by tach manufacturer, but my guess is most of them have about 5 volts present on this wire (that would be the nominal voltage of the 555 or whatever TTL chip they use.) When the coil fires, it pulls this wire "low" (0 volts or ground). The tach counts these and displays it.
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Quote:
Originally Posted by caain View Post
Somebody said the diode keeps the signal from bleeding from one trigger into another (grounding both at the same time rather than just when each one needs to fire), while also claiming the resistor "smooths things out". I suppose it makes sense, but then again he could have been talking about of his ass. I do it frequently
Did you run test it yet? I wonder whether the needle fluctuate a lot? How does it look on all rpm's and gears?
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Clearly many of you fine gentlemen are much more learned in the resistor/diode arts than I will ever be.. Thanks for the explanation!

And yeah I've had it on the bike for a couple weeks now. I keep playing with where the tach is mounted (down low on the side under my speedometer now). It does in fact fluctuate quite a bit at idle, but it's pretty smooth once you're north of 1300-1400 regardless of gear. I'd try for a moving video but I value life too much. I'll post a video revving it in neutral once the bike's back to running. Currently torn apart extending the wiring/cables for taller apes on order.
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