Erratic Tachometer

[Shazaam]

Electronic tachometers give erratic readings when the battery or tach connections are loose or corroded. It can also be caused by a low system voltage from a discharged or weak battery, so It is often seen at idle when alternator output is at it’s lowest. If your running high-wattage accessories such as heated clothing or auxiliary headlights, your base voltage at idle can be reduced. Just using your brake lights at idle can drop the voltage below the threshold needed for tach operation.

If the bike starts normally, the battery and its connections are likely not the problem. Rev the engine and check the voltage across the battery terminals to make sure that your charging system is functioning properly and the voltage stays well above 12 volts. Adding an on-board voltmeter to your bike will tell you what’s going on when you’re having electrical problems.

If the problem appears at random, it’s probably a loose electrical connection. Check for a loose or corroded connections at the battery, the fuse, and any in-line connector. Check the two battery connections first, next the connector at the instrument cluster, then at the fuse, and lastly at the wiring harness connections.

Check the connectors between the computer and the tach. The tach gets its rpm signal from the computer that in turn gets engine rpm from a sensor. If the the bike is running OK, the rpm sensor gap, the sensor itself, and wiring back to the computer are not the problem.

However, if the rpm sensor gap is incorrect, or there’s an intermittent connection problem, the computer will receive (and send) erratic rpm readings and this will result in running issues, such as a 6-7K rpm brief cut-out condition. If you check the tachometer while it’s happening, if it's the sensor, the rpm reading will drop to zero.

The rpm sensor is located near the water pump where the alternator leads come out of the casing. It's shimmed out to a specified clearance from the flywheel. If the gap is incorrect or fluctuates due to a loose flywheel or cracked engine casing the rpm signal will be erratic.

Check the tach for internal damage. On earlier bikes, check the three brass screws on the back of the tach for tightness. They position and ground the circuit board and often vibrate loose causing damage to the electrical connection to the board. One test is to shake the tach and listen to determine if you have loose components inside. If so, you may have to disassemble the tach to repair the connection. However, before you attempt an internal repair or buy a replacement, try to borrow a working tach to verify that your tach is actually the problem.

[94CobraR]

Good post, Shaz. Just to add on, since I've actually taken them apart and repaired them, solder does break lose after years of vibration and a resistor or two may come loose. If you have to cut your Tach apart, you can use a dremel and cut the bezel off by cutting about an 1/8" to a 1/4" below the bezel around the Tach. Plastic welding works great to put it back together.

[Rob998]

Great post Shazaam. I had an erratic tacho needle on my 998. Turned out that the ECU wasn't seated properly.

[brod5150]

I have issues with the tach on my 2004 S4R

I have noticed my panel will ramdomly do a full sweep (like it does when turn the key to ON) when the bike is running (on idle) and sometimes the needles go wild for a fews seconds. This happens only if the bike is warm/hot (80c or more). When it gets really bad, the whole panel shuts off (even the backlight) and my turn signals stop working. It will not do it if the bike is not running and the key is ON. It only does it on idle. it will not do it i am driving at 20 mph+

I have noticed that if disconnect the front temp sensor the problem happens less often (but still happens) as if the more components attached to the Tach the more often the issue will manifest itself.

Somebody has a idea ? or at least what i should do next to troubleshoot ? Could it be the ECU who doesnt like the heat or something (overheats)?