Bicycle Mechanics - Astrale 8 Rear Sensor

Last week I purchased a Cateye Astrale 8 cyclocomputer based on recommendations I've seen from this board and elsewhere. Installation was easy enough, I had to short circuit the computer to get it to recognize the MPH/Distance portion for some reason, but after that it worked fine.

Yesterday it was acting up, occasionally reading 5, 6 mph while I was definitly going faster then that. It'd pop back up to real speeds (14, 15, etc) after a bit, but it'd still occasionally show that. This morning the cadence section worked, but the MPH/Distance would not work no matter what I did. When I got to work I short circuited the computer again, but it made no difference (the screen did flash briefly, the max mph now is listed at 5.2 even though it never showed that any other time).

I have two questions:
1) Is this most likely a faulty rear sensor? I can not locate any broken wires or the like.

2) The instruction state that the rear sensor and the spoke magnet should be no more then 5 mm apart. Before, when it was working, the sensors were a tad bit more then that, maybe 7mm apart. However, things worked. Could this be a potential issue? The only way I can get the two closer is to move the rear sensor stuff closer to the center of the wheel, but I thought that was a 'no no' as it could lead to inaccurate readings.

http://img141.imageshack.us/img141/3923/wheelof3.jpg
My the magnet/sensor is currently lined up on the red line, which is about the center of the spokes. This is where I was told it would receive the 'best readings' since it was in the center.
To get it closer to 5mm I'd have to move it to the green line. This is where I thought that I'd get poor readings.

Regarding the second question: Was I misinformed?

Thanks for any help:)

You might also want to check the contacts on the computer mount. Mine were getting oxidized at one point causing my cadence reading to drop out occasionally, but this was after several months of use. It sounds like your Astrale 8 is nearly new.

As for spoke position, I've had my magnet mounted in both of the locations you illustrate, and they both seemed to work fine. I originally had it closer in to the hub because this is supposed to make the readings more reliable but I had to move it because it was interfering with my bike rack. It made no difference.

One thing I'm not sure of is if the angle between the sensor and the magnet matters. My understanding is that cyclocomputer sensors usually use a mechanical reed switch to detect the magnet. Each time the magnet passes by, it pulls on the switch and bends it until it touches a contact, causing a current pulse which is detected by the head unit. I could imagine that, if the reed switch is designed such that it can only moves in one plane, and you have the sensor wand positioned so that it's angle with the magnet is too far out of this plane of motion, it could cause the reed switch to stick occasionally and miss some wheel revolutions. Just speculating here. I know very little about reed switches or cyclocomputer sensors. I know that on my bike I had to rotate the sensor a bit on the chainstay to fine tune the magnet clearance, but it never caused a problem.

Also just because your cables look good on the outside it doesn't necessarily mean there isn't an internal problem. Check for kinks or lumps in the insulation that might indicate mishandling or other defects.

I'd suggest moving the magnet down near the hub. The spokes are closer to the chain stay there and you should be able to get the sensor closer to the magnet. I'm running 3 Astrale 7s on my fleet. All the magnets are 1-2 mm from the sensors and work fine.

There are several ways to mount the sensor on the left chainstay. I've never had a problem getting it close enough to the magnet. I suspect that your 7mm is not close enough. I run mine about 2 to 3 mm from the magnet. The slot window in the sensor should face the magnet.
Keeping the contacts on the back of the CPU and the contacts on the mounting bracket clean and dry is important.

Al

Thanks folks, I'll give it a shot when I get home from work (Don't have any ties here :) ).

I went through the same experience with mine...one day I removed the computer head from the cradle and reinstalled it. Perfect operation resumed. This is after all sorts of finagleing with the magnet/sensor mounting. In my case anyway, Metaluna is right on the money; check the contacts with the cradle.