tach not working on 1991 135hp Merc outboard

My 1991 V6 135 HP Merc outboard starts quickly and runs great. I recently put in two batteries, and selector A/B switch, and an isolator so the batteries would recharge properly. All my accessories run off the batteries. Now I am experiencing 2 conditions: 1, my tach is not working. It senses electricity when the power is on, moving from a dead position to zero, but does not work when the engine is running. Second problem is that the batteries seem to be under-charged. I have a merc owners manual and I followed the steps to test the voltage regulator. The voltage to the common terminal on the switch with the motor running was approx. 5 volts. When I disconnected the Yellow wire from the voltage regulator, the voltage spiked to 19-20 volts, which according to the manual means the voltage regulator is bad. It is an expensive part and don't want to make an un-necessary purchase. Also, I have been told that the only component that supplies information to the tach is the stator. That testing seems very complicated, plus I would need someone to convince me that getting to the stator is not as hard as it looks. I am not a wimp, but sometimes you need to pull out the checkbook.

Thoughts?

Thanks to all who will respond.

Your tach gets its signal from one of the yellow leads coming from the stator. These 2 yellow leads also supply AC to the rectifier/regulator. When a diode in the rectifier shorts it kills the signal going to the rectifier. If you have ever hooked the battery up backwards or created a spark anywhere (even for a second) you most likely fried a diode or 2 in the rectifier. Your manual should also tell you how to check those diodes. You do not have to access the stator to do a static test. Just the leads going to it. You have all the symptoms of a bad rectifier which is part of the regulator.