Using a voltmeter, check battery voltage. If less than 5.5VDC, recharge the battery with an AC adapter. If the battery doesn't recharge, replace it. If no AC adapter is available, replace the battery.
Inspect the speed sensor and its adjustment. The gap between the speed sensor and the teeth of the speed disc should be .025 (+/- .005), about the thickness of a credit card. Adjust if necessary. Inspect the speed sensor and wiring for damage. Replace the sensor if necessary.
Using a voltmeter, place your negative lead to the cathode side of the diode labeled D1 and the positive lead to TP3. (The cathode side of a diode is opposite the anode side which is determined with a stripe) The voltmeter should read 7VDC. If improper voltage, replace the lower board.