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Tachometer powers on, but does not show RPM

Apr 25, 2020
13
Catalina 30 Long Beach
I'm the third owner of a 1982 Catalina 30 with a Universal diesel. The tach that came with the boat wasn't working when I bought it. It's not a big deal, just kind of annoying and I'd like to get it working. I've replaced the tach with a newer one that has a digital display for engine hours, and it powers on correctly including when it firsts gets power and the needle moves through the full range of readings.

Diagnosis I've done so far:
- Normal alternator output to electrical system (I think 14.2V DC) and charges batteries normally
- Testing the AC output on sending line to the tachometer, the multimeter on AC reads 4.0V and on DC reads 2.3V. It doesn't change when I increase the throttle (I had heard that it was supposed to).
- The voltage readings are the same at the alternator and where the sending line enters the tach
- Mentioned above, I replaced the tachometer and still not getting a reading, even after programming appropriately according to the new tach manual
- Wiring to the tachometer is correct

Since I've tried to isolate every source of the problem, I'm a bit lost. Could it be that there is something wrong with the signal that alternator is putting out? Any advice would be helpful!
 
Jul 23, 2009
497
Beneteau 31 Oceanis Grand Lake, Oklahoma
Do you have a picture of the back of the alternator? What is the "sending line" labeled on the back of the alternator?

4 volts seems low. I have not experimented with a stator tap but I would think the AC voltage should be very close to the DC output of the alternator.
 

Tom J

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Sep 30, 2008
2,004
Catalina 310 Quincy, MA
I believe the tach is looking for frequency change at the stator of the alternator. Maybe check for hertz vs. rpm.
 
Feb 6, 1998
11,436
Canadian Sailcraft 36T Casco Bay, ME
The stator signal from your alt AC TAP should be half of the regulated voltage or 7.1V if you’re getting 14.2V.. What model is the alt? Photo?
 
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Likes: LloydB
Feb 26, 2004
21,959
Catalina 34 224 Maple Bay, BC, Canada
Have you checked the continuity of the wire between the alternator and the gauge?
 

19thol

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May 2, 2014
111
Hunter 30 St.Petersburg, Fl
Don't know if it's related to your motor, but mine has a sensor on the bell housing with two wires on it, my tach wouldn't work, so I cleaned the connection on the sensor, all good. Good luck with yours.
 
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y knot

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Jun 5, 2004
15
Oday 272 Sarasota Bay
I had the same problem. Either it is a bad alternator or the wire came off the back of the alternator.
 

higgs

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Aug 24, 2005
3,485
Nassau 34 Olcott, NY
The sensor on the bell housing can go bad, but as others have pointed out, it might just be a bad connection there. Google how to test the sensor with a meter.
 
Nov 18, 2009
2
Santana 2023C Matquette Mi
The tachometer operation is off of the poles of the alternator and how fast they pulse. Your meter should be able to show the frequency change as rpms do. It's not related to regulated voltage output. BUT your alternator needs to be charging for the tach to work. That means unplug your shore power and make sure that you rev your engine above 2,000 rpms (estimated) to excite your alternator.
 
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Tom J

.
Sep 30, 2008
2,004
Catalina 310 Quincy, MA
BUT your alternator needs to be charging for the tach to work.
I learned this the hard way. We were motor sailing up the New Jersey coast one day, when my tach suddenly went to zero, but the engine was still running. The voltage stayed at 14.2 volts, and we were only running the VHF and refrigerator, so we continued on to Atlantic City, with me thinking I will need to troubleshoot the alternator. As we were tying up, a fellow boater asked how we were doing, and I mentioned the tach issue, and possibly a bad alternator. He looked up over my head and pointed out that I have solar panels and the sun is out, so the batteries would be fully charged, and the alternator would have shut down. Duh.
 
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Likes: ggrizzard
Mar 6, 2008
692
Catalina 1999 C36 MKII #1787 Coyote Point Marina, CA.
I am curious if anyone has measured the alternator current output at 2000 RPM. This is also displayed on the battery monitor. I am seeing only 3 AMPS. Is that normal?
 

Tom J

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Sep 30, 2008
2,004
Catalina 310 Quincy, MA
I am curious if anyone has measured the alternator current output at 2000 RPM. This is also displayed on the battery monitor. I am seeing only 3 AMPS. Is that normal?
Probably. Depends on what state your batteries are in, and what load there is on the DC system. In my previous post, I mentioned that the alternator output was zero, because the batts were fully charged. That was at about 2,500 rpm.
The previous posts were referencing the output of the stator tap to the tach at 2,000 rpm, and not the DC output of the alternator.
 
Apr 25, 2020
13
Catalina 30 Long Beach
Thank you all for the advice! Replying one by one:

The stator signal from your alt AC TAP should be half of the regulated voltage or 7.1V if you’re getting 14.2V.. What model is the alt? Photo?
A picture of the alternator:
IMG_1359.jpg

Sorry, hard to get a good angle. But the sending line is the one below and a bit to the left of the + output. It says "AC Tap" next to it. The diagram in the engine manual details this:
IMG_1362.jpg

So it appears to be wired correctly.

Have you checked the continuity of the wire between the alternator and the gauge?
Yes, I get the same voltage reading at the alternator and as it goes into the tach. I also double checked this by running an alternate line to jump it, and it didn't make a difference.

The tachometer operation is off of the poles of the alternator and how fast they pulse. Your meter should be able to show the frequency change as rpms do. It's not related to regulated voltage output. BUT your alternator needs to be charging for the tach to work. That means unplug your shore power and make sure that you rev your engine above 2,000 rpms (estimated) to excite your alternator.
Regardless of whether I'm connected to shore power or not, it doesn't seem to make a difference. Even when I'm out for more than a day or so, it won't read. The batteries definitely charge though. Also doesn't change when I increase the RPM.

Also, when I was referring to the voltage output, I meant what was coming out of the AC tap.

Thanks again for the feedback.
 
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Apr 25, 2020
13
Catalina 30 Long Beach
What model tach are you installing?
I'm not sure exactly, they don't have the precise model name in the manual. The engine is a Universal Diesel M2-12, and this is a link to what is supposed to be a replacement alternator:

 
Oct 22, 2014
16,135
CAL 35 Cruiser moored EVERETT WA
Have you checked the wiring of the Tach?
Power and AC Tap cable to the correct pin on the tack?
Proper Ground Pin connected|?
1618360474839.png