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Cyclecomputers can be serviced when they malfunction, and most problems are not that difficult to fix. You will need a multimeter (volt-ohm-milliammeter) which can be purchased at your local Radio Shack for under $15.
Some repairs may also require a soldering iron and knowledge of how to use it.
Repairs to the innards of cyclecomputers are generally beyond the capability of bike shops, but the printed circuits and chips used in modern units are extremely reliable, so they rarely give trouble. The most common problems are fairly easy to fix. There are three basic causes of the vast majority of cyclecomputer malfunctions: Battery problems, wiring problems, and misalignment between the magnet and the sensor.
Battery failure is easy to diagnose. If you see clear, black numbers on the screen, the battery is OK. As the battery weakens, the liquid-crystal display may go dim. When the battery fails, the display will go completely blank. Open up the unit and remove the battery. Examine the terminals to see whether there is any corrosion. If the terminals look fuzzy, clean them with a cotton swab dipped in ammonia. Put in a new battery. If the display is still blank, it is time to give up unless you are handy with a soldering iron.
When the battery is replaced, power is restored to the computer. Some units are a bit fussy about how cleanly power is restored. Ideally, the battery should make instantaneous, solid contact with its terminals, but in some cases, this is not so easy to do. If the battery makes contact on a "bouncy" way, the irregularities of contact as the battery is pushed in can send confusing messages to the circuit. This can cause "garbage" readings to appear. The unit may show all 8's, or random fragments of characters, or may come up blank. This is a particularly common problem with the Cateye Solar, which has very tight spring contacts and uses two small batteries that are a bit hard to install.
Many Cateye Solars have been discarded as defective, when they only need to have the batteries re-installed correctly. Genuine Cateye batteries, which are no longer available, came with a special little piece of thin plastic that can be inserted between the two batteries and then yanked out to establish contact cleanly.
Some other units have a special procedure to "re-boot" the processor, usually by pushing all of the buttons at once.
Electrical wiring on bicycles is often subjected to serious abuse, and it is very common to have breaks in the wiring, particularly if the wire is not properly routed and secured.
The usual problems occur around the headset, where the wire can be pulled too hard if the handlebar is turned farther than usual. Front-wheel reading computers should be more reliable, because when they are properly installed there are no loose loops of wire. The wire should go up the fork blade to the front brake, and follow the brake cable to the handlebars, without being attached to the frame at all.
Rear-mount units require a bit more care, because you have to allow a loop of wire sufficient for the handlebar to be turned fully in both directions without overstressing the wire. The wire loop must also be kept safe from contact with the tire, which will wear it through in just a few miles. Another area of potential difficulty with-rear mount units is the wiring under the chainstay, which can be damaged by mounting the bike in some trainers. On the other hand, a front-mounted sensor doesn't do much for you when your bicycle is mounted on a trainer!
There is a detailed article on this site about proper Cyclecomputer Installation.
A wireless computer has a second battery in the sensing unit, which transmits a radio signal. As the battery weakens, speed readings become flaky, and finally quit entirely. To get any reading at all, the sensor must be installed on the front wheel. On a small-wheel bicycle, you might violate the rule about installing the magnet close to the hub, to get the sensor closer to the computer.
It may also help to install the computer below the handlebar. There are special brackets available to install computers (and lights) on the handlebar stem. Tilting the computer forward or backward on the handlebar also can prevent it from responding to the sensor, by misaligning its internal antenna. This can be a special problem on a recumbent.
The wiring is easy to check if you have an ohmmeter or a continuity checker (you should!). First you need to understand how these units work. What the manufacturers grandly call a "sensor" is actually nothing but a magnetically- operated switch. When the magnet gets close enough, the switch closes, making contact between the two wires in the cable. When the magnet is not close to the "sensor", the switch is open, so there is no connection between the two wires.
If you remove the computer from its handlebar mount, you will find two metal contacts (three for units with cadence function). These contacts and the mating contacts in the computer unit must be clean and make a firm, spring-loaded connection for reliable operation. If cleaning the contacts doesn't restore normal operation. you can check the wiring and magnet alignment with your ohmmeter or continuity tester.
Hold the probes of the tester against the two contacts in the handlebar mount. (if there are three contacts, one of them is just for the wheel sensor, another is just for the cadence sensor, and the third is common to both. you will need to try different combinations to discover which is which by trial and error.)
Turn the wheel so that the magnet is away from the sensor. There should be no continuity between the two contacts. If you get contact, the wires are short-circuited, and you need a new pickup/wire/sensor unit. (This procedure does not apply to Avocet computers. See below for Avocet procedure.)
The circuit should be open with the magnet away from the sensor. If it is, rotate the wheel so that the magnet is next to the sensor. Now you should have continuity. If you do, wiggle the wire a bit here and there and make sure that the continuity is maintained as the wire moves. If not, the wire is damaged, and the unit will only work intermittently at best. If it passes this test, the wiring and magnet placement are OK.
If you don't get continuity with the wheel magnet next to the sensor, try another magnet, hand-held. Hold the magnet right up against the sensor on the same side of the sensor that the wheel magnet would normally be on. If the circuit is still open, the wire is bad. If you get continuity with a hand-held magnet but not with the wheel magnet, the wheel magnet or sensor needs to be moved closer together.
The better models of Avocet computers work on a different principle from all others. The Avocet sensor is not a magnetic switch, but a coil of wire. As the 20-pole magnet ring rotates past the coil in the sensor, a small electric current is generated. You can test this with an AC voltmeter. Spinning the wheel by hand, you should be able to measure an AC voltage of around 50 millivolts at the handlebar terminals.
If you test with a continuity tester or ohmmeter, you should get continuity regardless of magnet position. Unlike most cyclecomputers, the Avocet system allows you to replace just the wire, without replacing the sensor or the handlebar mount.
(The Avocet models 15 and 25 use conventional reed switches with single spoke magnets.)
By and large, magnets used in cyclecomputers are very ordinary, and may be interchanged from one brand to another. Some are more powerful than others. The sensor will not respond if too far from the magnet. The more powerful the magnet, the less fussy its alignment with the sensor. Sometimes, substitution of a more powerful magnet can make the difference in getting a computer to work.
A misaligned magnet can also result in odd or doubled readings -- see the article on that topic. I have sometimes used small button magnets from Radio Shack secured to the spokes with transparent tape. This trick is particularly useful when you have more than one set of wheels, with different spoke patterns so you can't duplicate the magnet placement.
The Cateye solar magnet has two lines on it. One of these lines is supposed to line up with a matching line on the sensor. People who don't read the directions carefully sometimes assume that the sensor line should line up somewhere between the lines on the magnet.This mistake causes very erratic readings. Each of the lines on the magnet is located at a pole of the magnet. The midpoint between these lines is not magnetized.
The Cateye Mate uses four magnets mounted on a ring. If the ring is slightly bent, one of the magnets will be farther from the sensor than the others. This will result in under-reading-it will say you are going 15 mph when you are really going 20, etc. This can be tested for by turning the wheel so that one of the magnets on the ring is next to the sensor.
Turn the wheel forward and backward just a couple of inches as rapidly as you can conveniently, so that the one magnet keeps going back and forth near the sensor. You should get a speed reading, even if it is only 1 or 2 mph. Repeat this test for each of the four magnets. If you don't get a speed reading from all four, you need to re-position the sensor.
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