Digital speedometer troubles

GoldenMotor.com

azbill

Active Member
May 18, 2008
3,358
5
38
64
Fountain Hills, Arizona
when buying one...keep in mind the wheel size
ie...a 20" speedo won't be accurate with a 26" wheel
I figure you already thought of that, but had to add it just in case :)
 

Clotho

Member
May 25, 2008
304
2
18
I had my share of problems with speedo's too. Then one day I noticed that the ones that always worked used a shielded wire from the sensor to the speedo. The ones that didn't all used just 2 wires (like speaker wire, it looks like 2 wires stuck together). I now only purchase speedo's with the shielded type of wire and I don't have problems anymore.
 

whodathunkit

New Member
Nov 1, 2008
11
0
0
Lansdale, PA
OK guys, here'e the root cause of the problem and how to fix it:

The little black CDI unit creates a very short duration, very high voltage, and relatively high current pulse that feeds the spark plug. This short, high-intensity current pulse travels along the spark plug wire from the CDI to the spark plug. Whenever you have short duration pulses of high current in a wire, the wire radiates an electromagnetic field that can interfere with nearby sensitive electronic equipment. You might be familiar with this when a hot-rodded car pulls up next ro your car and you hear his "ignition noise" in your AM radio. This is produced by the same mechanism, but in our case, the noise interferes with the computer's ability to receive the small signal that the wheel sensor sends to the computer over the interconnecting wiring.

The solution known for years to silence ignition noise is to put some resistance in the high-voltage wiring, or inside the spark plug. This limits the peak current in the spark wiring to a level which attenuates the amount of noise radiated, but it also limits how 'hot' the spark is. It's a tradeoff.

My Target speedo computer would reset every time I started the engine, and often when I was riding too. To prove my theory, I went out and purchased an Autolite 425 plug, which has a built in 4000 ohm resistor. Voila! The interference problem disappeared! Unfortunately, due to the weaker spark, the engine didn't run very well until it got good and hot, and even then its top end under load wasn't as good. I figured that 4000 ohms was probably a little much for the amount of energy our little CDI's put out (as I said, it's a tradeoff), but at least I proved what the problem was.

My next approach was to go back to the original plug and replace the stock spark plug wire with some automobile spark plug wire (this stuff usually has a resistance core). I measured the resistance of a 2 foot ignition wire I had laying around and found it to be about 2000 ohms. I figured that if I cut it down to about 8", the resistance would drop to about 600-700 ohms or so, which was right about where my gut said I'd like it to be. This is a huge increase over the original wire which was only about 50 ohms, yet much less than the troublesome 4000 ohms built into the resistor plug. As expected, the bike now runs as good as it ever has, and my computer is 100% reliable.

If you do this mod, here are a few things to keep in mind:
1) You want a 7mm diameter automobile plug wire that has a right-angle cap on the plug end.
2) You'll have to install one of those little screw-on metal connection tips that come with new spark plugs, since your stock plug only has the threads and no tip for the new plug wire to mate properly with.
3) Remove the original wire by unscrewing it from the CDI body. It comes out pretty easily.
4) Cut the new wire about 6-8" from the plug cap end (or whatever length you need to reach from your plug to your CDI).
5) Push the cut end of the wire into the CDI body while turning in a clockwise direction. The CDI has a pointy screw sticking up inside that will thread into the core of the wire. Once the wire has gone in about 1/4", you're done - Don't over-tighten!

This should fix your problem. Let us know how you make out

Joe
 
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Ghost0

New Member
Mar 7, 2008
763
1
0
Bellingham, WA
That's funny, someone said in the second post in this thread that the best solution was to replace the plug wire, wait that was me, but what do I know I'm just an engineer.:)
 

whodathunkit

New Member
Nov 1, 2008
11
0
0
Lansdale, PA
Ghost,

Sorry if I stole some of your thunder. My objective was to provide an understanding of the underlying causes of the problem as well as a step-by-step solution. I also wanted to dimension the parameters involved so others could understand and improve the solution. I looked for a place to post this tutorial, and not wanting to start yet another thread, I found this one and went right to the end.

BTW, like you, I'm also an engineer, and a physicist by education - I guess similar minds work alike!

Joe
 

breno

New Member
Aug 19, 2010
411
0
0
Syd. OZ
Are you referring to one of these Norman?

Snap Choke Core - RadioShack.com

You can also replace the wire for the sensor with a twisted pair (like in ethernet cable - + and - twisted around each other) and that should shield out interference too.

Yes thats the thing thanks.
Norman
I know this is an old post but can anyone confirm if the ethernet trick works. I replaced my spark plug lead while ago with an 8mm silicone based somethingarather for something to do so I can rule that one out coz it aint workin for me (cant b bothered replacing it again as I epoxied it back in again). If I were to use the ethernet cable or Cat 6 cable I'm assuming I would just cut the wires as close as possible to either end, solder, heat shrink yadda yadda. I would try the ferrite thing but I'm trying to do this with things lying around at work. Thanks guys.
 

2door

Moderator
Staff member
Sep 15, 2008
16,302
175
63
Littleton, Colorado
Breno,
Yes its an old thread but the problem still pops up every now and then, so no worries.
After some time living with digital speedos and their problems, then finding a solution that worked for me, I should have posted here much sooner.
I've had wired and wireless speedometers that would reset themselves and/or just stop working. I used one expensive wireless, cost $50.00, and a couple of cheapo types. I'm currently using the Schwinn 12 function wired speedos from Wally. About $10.00
I mount the CDI to the seat tube, as far from the front as possible and keep the speedometer wires away from the clutch and throttle cables. One speedo has accrued nearly 1000 miles without a reset and another one about 800 miles. I still use non resistor plugs and ignition wire. I've never tried the ferite core. I think the seperation between the ignition and the speedo wiring is the reason for my success. Just my experience.
Tom
 

whodathunkit

New Member
Nov 1, 2008
11
0
0
Lansdale, PA
I know this is an old post but can anyone confirm if the ethernet trick works. I replaced my spark plug lead while ago with an 8mm silicone based somethingarather for something to do so I can rule that one out coz it aint workin for me (cant b bothered replacing it again as I epoxied it back in again).

You can try putting multiple EMI absorption devices or using twisted/shielded wire to every affected electrical device, or you can cure it once at the source. The "source" is insufficient resistance in the stock spark plug current loop (see my previous). Your "8mm silicone something" substitution is not necessarily a controlled resistance wire. You need to measure the resistance with an ohmeter - it should be between 500 and 1000 ohms for optimum performance and least EMI radiation. Almost any el-cheapo meter sold at Radio Shack or your local hardware store will probably do the job. I have one that I bought for $3 from Harbor Freight. If you don't have one, get one - you'll use it for other useful things.

Joe
 

breno

New Member
Aug 19, 2010
411
0
0
Syd. OZ
Thanx for the replys I'll give the shielded twisted a try 1st failing that I'll move the CDI back to the seat pole just means I'd have ta redo the lead for the plug coz she's pretty short from a mishap of ripping the plug cap off the plug and breaking it at the joint to the CDI. Thanks again I'll make sure I post up how they individually work out for me.
Cheers Brendan
 

breno

New Member
Aug 19, 2010
411
0
0
Syd. OZ
Well I ended up having enough plug cable to move the coil to the seat pole. Also tried using a ferrite choke right near the speedo device. Still no love from the little bugger.

I might try replacing the cable tonight with a 1 pair twisted shielded tonight and hopefully thatwill do the trick.

Is there any other suggestions or tricks with this??

I have the kill switch for the motor going to the frame.. I was thinking this could be contributing to the probleem is it worth changing this around as in putting the green wire to the black instead or however it goes?
 

breno

New Member
Aug 19, 2010
411
0
0
Syd. OZ
Just a thought on the twisted shielded cable if i were to do this I cant really think of a way to earth the shield. So i dont know if it would work. Maybe being twisted that could help but i dunno.
 

breno

New Member
Aug 19, 2010
411
0
0
Syd. OZ
The fix after trying just about everything inc.3 different speedos, CAT 6 cable, ferrite chokes, holding it away from the hangers with the wire not touching anything, moving everything around i.e coil to the seat post etc. etc.....

Got a SIGMA 5 function speedo from blackmans bicycles for $39 no mods just chucked it in on and its never skipped a beat..
 

Toadmund

New Member
Jan 19, 2012
792
6
0
Saint John, New Brunswick, Canada
...and keep the speedometer wires away from the clutch and throttle cables.
Well, that worked.
I got one of those ebay wired 'bicycle computers' for about 8 bux. It was malfunctioning with the motor on.
I figured I'd need a ferrite core choke.
I hopped on my bike, then headed to 'The Source', formerly 'Radio Shack' and to the liquor store to get some beer. I just went to the beer store because I realized (thinking of this post I read previously) that if I just try moving the speedo wire to the left, then the kill wire to the right, what will that do?
It did nothing...to the speedo, it worked great!
Pulled out two tie wraps from my back pack, fastened them down on my handle bars, and just got beer.

I'll drink to that, as a matter of fact, like a lawyer, I've got another 'case' to work on.


PS, 56 kmh top speed so far! (34.8 mph)