Changed coolant and now no RPM

[wdbriner]

I just changed the coolant - and burped it as described here - thx Stu- and now I'm not registering any RPMs.

Any ideas about what happened?

PS - the temp stabilized at about 170. Is that normal?

[David Sanner]

My guess is you may bumped the tach wire (connected to the alternator) or splashed some
water where it shouldn't have gone like the wiring harness connector.

You might try (turning off the battery first) cleaning the connectors on your
alternator and checking for corrosion in the wiring harness connector.

[Ron Hill]

Bill : There is no connection between getting the air out of the heater line and the engine tach.
 
I agree with David that you probably bumped and have a loose/poor connection of the tach wire on the alternator.  If not at the alternator look at the instrument. 

[Stu Jackson]

Was your boat plugged in at the time you read your tach?

[wdbriner]

Hi everyone,
Thanks for the responses... I knew that the two systems weren't connected which was why I was so mystified.  Today we sailed to Sausalito and the tach was working. Yes Stu I was plugged in yesterday. When I was running the engine at the dock. Would that over ride the alternator?

[Ron Hill]

Bill : I've mentioned it before - I don't think it's a good idea to be plugged in with the battery charger ON and then start up the engine. 
You'll have 2 competing systems trying to charge the same battery bank.

I always turn OFF the shore powered battery charger(when plugged in) if I decide to run the engine at the dock. 

[Stu Jackson]

Yes Stu I was plugged in yesterday. When I was running the engine at the dock. Would that over ride the alternator?

It's not so much "overriding" the alternator.  What happens is this: your battery bank is full from being plugged in, so the regulator on the alternator sees this and tells the alternator there's no reason to produce charging current.  The tach is fed off the alternator.  No alternator output (AO), no tach reading.  Add a load once you disconnect from shorepower and it'll start right up, just like your cruise to Sausalito.

[wdbriner]

Thx Stu - mystery (to me) solved.