Speedometer not working in cold
 

My speedo (aka "cyclocomputer"), a wired Cateye with cadence, has started dropping the speed reading in cool weather (<50F). Checked the magnet alignment first, and it's fine.

My first thought was the battery must be getting old, although the display isn't fading. I thought I had a spare, so I brought it inside. Didn't have a spare, so the next morning (45F), put it back on and it worked... for about two minutes. Cadence worked fine. In the afternoon all was fine.

So I replaced the battery anyhow that evening and cleaned the contacts. Same thing: worked about 3 blocks (maybe 48F?), then dropped out for another 3 miles or so, then started working.

My thoughts are: harness, contacts, magnet, and reed switches must be working, since the whole thing works when it's warm. New battery, so the temperature can't be causing the problem.

Sigh, everything's exonerated but the problem persists. I must be missing something. What could it be?

Did you clean the contacts between the computer and the mount, from both sides? DeoxIT Gold is a good product for the purpose, followed by dielectric grease for lasting effect. The temperature may be causing condensation around some dirt. Regarding battery, the screen normally fades out before the counts. If you want to make sure, you can put the computer into a freezer and see how it operates thereafter. Instead of the reed switch, you can use a paper click and close and open the computer's contacts.

^That. Sounds like condensation forming somewhere in the system. Dielectric grease on clean contacts should solve your issue. I had a similar issue several years ago, and this was the fix.

I've hit it with contact cleaner, then globbed on some dielectric grease. I'll have to wait a few days to see if that solves the problem, since it's warmer this week.

Why, though, would the contacts be temperature sensitive? If they work at first, or work at 60 degrees, why they stop working or nor work in colder temps?

It could be many things, if something like a magnetic switch can move by just the power of the magnet, it can easily get stuck or become inconsistent. Lube or springs, or pivots like a hinge can all become stuck. All of those things expand and contract with temp changes. Lube can get stiff. It's really very common. Try a new sensor.

Wired, it sounds like?

Where is your sensor placement? The closer to the hub, the better. If you're near the rim, the magnet pulse on the sensor is very very short and if it's borderline the cold might affect it more. Closer to the hub the sensor has much more time to react.

Alas! Speed/distance totally did not register again when it got cold, even after the dielectric grease treatment. I'll have to dig around and see if I can find a spare mount in the Spare Parts Archive tonight. If that doesn't work, it may be time to retire it.

To respond to a few points, the magnet is where it's been for the last 35,000 miles; I don't think that's the problem.

I'm not sure how the reed switch could be bad, when it worked for half a block the day I brought the warm head out of the house.

My best guess, if the mount doesn't fix it, is a weak spring or something jamming the wheel sensor contact in the head. It's got to be specific to that contact, since the cadence sensor is working. I think that would mean the ground and the cadence contact are OK. I just can't wrap my head around gunk in the contact being temperature sensitive.

People who travel the whole world diagnosing and repairing electrical equipment know that mechanical relays fail. It's so common I used to carry some with me. The most likely thing to go bad is the switch in the sensor, especially if it is that old. One starts by eliminating the most common problem first.

This is not complicated at all. Take the computer off the bike (not the harness) Find someone with a multi meter keep the probes on the contacts where they would normally contact the computer. Set meter to measure ohms. Test contacts by passing the magnet past the sensor and see if the meter tells you the switch is working. Then leaving the bike in the cold, or riding it in the cold, try and get the thing to not work. Test the connection again with the meter, one can then know if it is the switch in the sensor or not. I know this hard to explain on a forum. Find some one who understands this to help you. If the thing started not working in the cold, and I was there, it would take me a minute or two tops, to see if the switch in the sensor is sticking.
35,000 miles it probably is.

I know it/s a pain to try another sensor and harness. That does not mean it's not likely to be the relay. If you have someone who can solder the wires, you can cut the wire down by the sensor and, connect, a new one.

It may not be the sensor
switch but it can be eliminated in a couple of minuted by an expert in that kind of thing.