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Hi all,
Wanted to make this post as I wasn’t able to find much in the way of information when I encountered my issue. The issue I had was my speedometer, odometer, and Trip meter had stopped working, but the hour meter was fully functional. For reference, this is on a 1999 Foreman 400 S foot shift, with the analog display. This may also apply to other Foreman/Honda atv models.

This post is how I fixed my specific issue, not from a repair book, and cannot guarantee that this will fix your specific issue. I am not responsible if your bike is damaged. Now onto the repair.

Tools used:
10mm wrench
flat head screw driver

Speed sensor part number:
37700-HM7-751
My replacement sensor was purchased in used condition from ebay. However, it should be available to purchase elsewhere online.

The first test I did was to check if the sensor was getting power. I did this by disconnecting the 3 prong plug located on the frame behind the right rear tire (pic one), then turning the key to the on position. I took my multimeter and inserted the prongs into the holes of the connector coming off of the bike (pic 2). (the connecter attached to the sensor is female and the bikes' is the male). I don’t recall which one was the power wire and which was the ground, (I want to say the green and red wires?) but there’s only 3 of them so two of them should eventually produce about 12 volts, the third being what I am assuming is the output wire.
1. 796554EB-4C89-41F5-A85E-99A66C3E84AC.jpeg 2. 40A765EA-2AB1-442A-BC17-8E01F9892F25.jpeg

After ensuring that the sensor was getting 12 volts, I turned the key off, and unclipped the connector from the frame. From there I began to remove the sensor, which is located to the right of the oil dipstick on the left side of the bike. The rest of the sensor wiring can be accessed from the back of the bike, near where the pull starter is.
3. 1614028228871.jpeg
Before removing the sensor, I first removed the wiring running to it. The plug on the frame (pic 2) should be disconnected and pushed down in order to detach it from the frame. The wiring itself is held in by two clips, which I pried open with a flat head screw driver.

Before moving on to the removal of the sensor, I drained the engine oil from the bike, as the sensor seems to sit in it. I could be wrong, but it was time for an oil change to begin with and didn't want to risk spilling all of the oil. It is also worth mentioning that even with the oil drained, some still will come out with the sensor. Keep some paper towels near by.

At this point I removed the actual sensor. There are only two 10mm bolts holding it in, the bottom of which will require an open wrench. The top can be undone with a ratchet. Upon removing both bolts, I gave the body of the sensor a tug, and it came loose, letting some oil out. I then pulled the sensor free of the bike along with it's wiring (pic 4).
4. 1614029877415.jpeg
After removing the sensor, the black spacer(? not sure what its called) stayed in place. This does have an O-ring on the other side that faces into the gearbox/motor, however I did not attempt to remove the spacer to change it as it was not leaking (pic 6)
5. 1614028579294.jpeg 6. 1614031189438.jpeg
Pictured above (pic 5) is the sensor that I put in. To test the new sensor, I plugged it in to the bike, turned the key to the on position, and then carefully spun the shaft with a drill. Also be sure to fully plug the sensor in. I didn't the first time and it didn't work. Upon fully plugging the connector in and spinning with the drill, the speedometer needle indicated 5 mph, whereas with the old sensor it would not move the needle at all.

After verifying that the replacement sensor was good, I turned the key to the off position and disconnected the plug. I slid the new sensor into its place. Ensure that your replacement sensor has the depicted O-ring (pic 7).
7. 1614031114823.jpeg

It may take a few small rotations of the sensor shaft in order to get the flat head of the sensor into its hole (flat head visible in pic 7 at the end of the shaft). Upon proper installation the body of the sensor should sit flush against the black spacer, with the plug facing upwards. Do not try to force the sensor to go into its hole, as if properly aligned it will slide in smoothly. I then threaded both bolts back into their holes, and tightened them to a reasonably snug level, where I was confident that the sensor would not leak (about as tight as when they were removed). I then routed the wire in the same way as when I had removed it, closed the clips around it, then clipped the head into its place on the frame, finishing by plugging it into the bike.

After all of this was done, I filled the bike back up with oil (2.2 quarts of Honda 10w-30 as the local dealer does not carry Honda 10w-40) and started the bike. Went for a test ride to confirm that the odometer, speedometer and trip meter were functioning as they should, which they were.

Hopefully this is somewhat helpful for someone. This is my first post here so apologies if I missed something. Thanks.
 
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