tachometer stopped working

captran · August 07, 2013, 03:24:53 PM

Many years ago the hour meter stopped, so hooked up a separate hour meter that I mounted by the refrigeration unit. Today I replaced my oberdorfer water pump (on the M35, 1997- N202M-15) without a hitch. but when I went to test it out, the tach isn't working. there's no wiring on the starboard side of the boat where the pump is. when I start the motor, the tach pops to 5 to 7k and instantly goes to zero. It was working fine on the way in this morning. I think we can finish out the summers cruise without it working, but am wondering if there's an easy way to check it/problem solve it. have an ohm meter but I need a course in ohm meter for dummies. I looked on the back of the alternator. no loose wires . coincidence? I have noticed my alt belt is a little loose, but that hasn't changed in the time we motored in and got the pump changed out.

Stu Jackson · August 07, 2013, 03:32:38 PM

Randy, was your house bank completely charged up? If so, the regulator wasn't being instructed to make juice. You already mentioned that the wiring at the back of the alternator "looks OK" but I suggest checking carefully. Do you still have the OEM alternator or have you upgraded and/or installed an external regulator? (We may have discussed this in the past, but I plumb forgot!

) I've found many of the factory crimps for the ring terminals on my Balmar MC-612 failed, repeatedly, until I ended up redoing each one myself. If you still have the OEM alternator, I posted sketches of the wiring at the back when I did my external regulator. If you want the link, let me know.

Stu Jackson · August 07, 2013, 04:13:47 PM

Randy, here are those wiring diagrams:

http://c34.org/bbs/index.php/topic,4548.0.html

Ron Hill · August 07, 2013, 05:07:05 PM

Randy : The tach wire to the alternator is the white one. I'll guess that you might have a bad connection there or if you have an external voltage regulator you might have a disconnect in that wiring.

Worth a check, a thought

captran · August 07, 2013, 05:55:12 PM

thanks Ron, for the wiring diagrams. I'll take a look. Yes, the house batteries are fully charged, but when we ran the motor the volts show that the engine is charging. if the volts say it's charging, would the tach end of things be any different?

I was talking to someone about my newly developed tach problem and he said it sounds like a ground issue. I checked the back of the panel and everything is tight, but then had a thought. The only difference between this morning and now is that we hooked to shore power, so I turned the shore power off and restarted and guess what? the tach works! I turned on the shore power and left the motor running and the tach was fine, but when I restarted with the power on, the tach doesn't work. Guess he next time I'm bored I'll start taking apart the AC side and check for any loose grounds.

Stu Jackson · August 07, 2013, 07:00:26 PM

Randy, this comes up at least once a year. If the battery bank is full, the alternator's regulator will not demand any output. No output = no tach. The voltage is there because the battery bank has retained its voltage. You do NOT have a tach problem, nor, it seems, an A.C. ground issue at all.

Stu Jackson · August 07, 2013, 07:03:45 PM

Here's one of many posts on this same issue:

http://c34.org/bbs/index.php/topic,6573.0.html

captran · August 07, 2013, 10:49:43 PM

Stu, you seriously made my day!!!! I have so much to learn and everyones combined knowledge is an amazing thing! Thank you. I wish I had a wifi hotspot everywhere I went. I could read every day and still not touch a fraction of what's available.

Jim Hardesty · August 08, 2013, 05:55:26 AM

I've been told that's it's not good for the starter and glow plugs to start the engine with the shore power battery charger running. That's due to the higher voltage from the charger. Makes sense to me. It's just one of the things that I think of as "why do it".
Jim

Ron Hill · August 08, 2013, 01:40:55 PM

Randy : Along with what Stu said, sometime when the batteries are full and you are motoring - the tach will fluxuate.
If you turn on the blower (a load) the tach will stabilize.

lazybone · August 08, 2013, 02:07:40 PM

Quote from: Jim Hardesty on August 08, 2013, 05:55:26 AM
I've been told that's it's not good for the starter and glow plugs to start the engine with the shore power battery charger running. That's due to the higher voltage from the charger. Makes sense to me. It's just one of the things that I think of as "why do it".
Jim

Jim, Glow plugs and starters are brutish, low tech animals that could care less about your battery charger voltage.
On the other hand a remote digital alternator regulator could be damaged by starting your engine while on a shore powered battery charger.