My tachometer recently started acting erratically with large swings of the RPM’s shown when increasing the throttle to near zero when idling.
I connected a voltmeter on the Tach and Ground connections on the Tach and at idle, the DC voltage was 1.5, and at higher RPMs (about 2,500), the voltage was 10.5. So, it looks like the alternator Tach output signal is working OK.
Is there anything else I should check before replacing the Tach? My tach looks like the original equipment for a 1986 Nonsuch 30 Ultra. All the alternator’s wire connections and the instrument panel’s tach are clean and tight.
There is a separate wire for the tachometer connection on the alternator. My understanding is it sends an AC current to the tachometer meter. There should not be a DC voltage reading on the tach connection. Check that the wire is good and connected to both the tach and the alternator.
I recently had a similar issue on my new-to-me NS36. I have a Balmar SG200 to see the current in/out of the batteries, so I could see that the alternator was not charging - it seemed to coincide when the tachometer stopped indicating rpms. I took the alternator to a shop to get it tested and found out the diodes were fried. For $125 I had it rebuilt. I’ll find out this weekend if everything’s working correctly again. I did buy a tach on Ebay for $20 just in case to connect and test if it’s the old tach or not.
Good luck -
Bob Gehrman
NS36 #52 “Fortunate”
Greenwich Bay, Rhode Island
The W-connection on your alternator is not a variable DC output. Instead, it puts out an AC voltage. The frequency of this AC voltage is converted by your tachometer to an indication of the RPM of the engine. To be short, a frequency to voltage conversion takes place in the tachometer (which is intended to be connected to a W-connection on the alternator). The higher the frequency coming from the alternator, the more the needle comes up.
If you are absolutely sure your alternator works like it should, It’s probably time to replace or repair your tachometer.
Also, my tach was acting wonky this past summer too. It turned out the exciter wires were loose on blade connecter, then it fell off the back of the alternator.
“There should not be a DC voltage reading on the tach connection”. This is true, but most (cheap) multimeters will tell you there is a DC on the W connection. This is, because the AC signal has a different frequency than 50/60hz. A multimeter is build to measure AC in the frequency range of 50/60hz. A 30hz or 500hz signal will give “false” DC readings in AC mode of most multimeters.
The W connection of an alternator is connected directly to the coils. It is not rectified, hence it being AC.
The hour counter on my tachometer failed. I used to use this device to schedule maintenance, so I’m in the process of fixing it right now. I could buy another tachometer but the Westerbeke control panel wouldn’t look as nice by with a modern part, so I decided to fix the old Motorola tach.
The Motorola tach can’t be opened without cutting it, so I did. I 3D printed a holder for a new mechanic hour counter to fit exactly to the front display plate. Nobody will see the difference.
The original electronics, however, did not fit anymore. So I designed new electronics (f to DC - LM2917 integrated circuit IC) and I’m printing a bracket for it to go in the old tach housing as we speak.
Nice work, Sprio. It is certainly beyond my abilities. When I first learned about electricity and electronics, transistors were just starting to appear.
Joe Valinoti
S/V iL Gatto NS30U #221
Sea Harbour YC
Oriental, NC USA
It may be as simple as checking the altenator belt tension.
Every once in a while my tach shows 0, then jumps up high and settles back down. It will occassionally fluctuate a bit after it has been running for a while…
Once I tighten the alternator belt slightly to remove excess play in the belt, the problem disappears.
Peter Grabow
S/V CAKE WALK III
1987 30U 430
Jersey City, NJ
A couple of things about “V” belt style alternators – If you start to see dust in that area around the pulleys, the belt may be coming apart or there is rust on the pulleys. Tightening the belt too much will decrease the life of the bearings in said alternator so look at a video on how to make that adjustment.
Joe Valinoti
S/V iL Gatto NS30U #221
Sea Harbour YC
Oriental, NC USA
I took my alternator into a repair shop and they said the diodes were burned out, but they could clean it up and rebuild it. This fall I noticed that the alternator was putting out anywhere from 10 to 12 volts. The alternator is a PowerLine brand with a PowerLine external regulator. The regulator indicator lights were the first indication of trouble. I’ll need to wait until spring to try out the rebuilt alternator. Hopefully, this also solves the erratic tachometer.
