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Old 06-03-2014, 09:44 PM   #1
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Tach problem

For the last two trips op the ICW my port tachs have not worked. They read zero both fly bridge and helm tach. I was told they operated from the alternator. It appears that this is in fact correct. I found a broken wire that once was attached to the port tach. A similar wire is attached to the starboard tach. I reconnected the wire, started the port engine and the tach moved. At this point I figured I solved the problem. Well, not true. hen under way and running cruise RPM's the tach starts to show lower RPM readings. In simple terms the tach will go to around 1400 and then start to go back down while increasing throttle. Also noticed that after shutting down this evening all four tachs show about 1000 RPM's even when the engines are shut down. That is right all four. Can't fault a tach. How does the alternator drive the tach and what could cause a residual indication? What causes the tach to go backwards?

Ken
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Old 06-03-2014, 10:00 PM   #2
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Alternator drive tachs by stator pole pulses. What kind of alternator? What year and type of tach? Do you have an advanced regulator and does it have alt or batt temp input?
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Old 06-04-2014, 09:41 PM   #3
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Check alternator ground wire.
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Old 06-05-2014, 12:22 PM   #4
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The broken wire: Did you get it back where it came from? Sure? If so, checking your alternator grounds as mentioned above, is a good start. If you have serious output alternators, they should not just ground through the block anyway; they should have their own ground wires, same size as the + output wire directly to -bus bar or - bat term.

You could have a problem with one or both of your alternators causing them to "drop" pulses as they warm up thus give lower tach readings or some such crazy thing.
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Old 06-05-2014, 10:35 PM   #5
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The tachs use to work. Yes it was reattached o the same place. The starboard tach seems to work fine, it is only the port the has crazy readings. I will look at the ground, it seems reasonable.

Ken
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Old 06-06-2014, 09:31 AM   #6
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I've been chasing inaccurate tach problems for over a year now.

My experience (granted, only about a years' worth) is that poor connections and noise are generally the problem. Adjustment can be an art form in itself, but you can't even start that until you get a clean signal.

I've finally got ONE good source of RPM data; I bought new mechanical sensors that talk NMEA to my MFD. So my motivation to get the alternator-driven tachs working correctly has gone down. When I do get around to it, my plan is to:

1) Add a ground wire from the alternators to the tachs. Currently they're using a common ground.

2) Use shielded cable for ALL tach wiring.

I'll probably replace the brand-new wires from the new senders with shielded cable, too. Even with all-new parts and wiring, they still go whacky when I turn on the florescent ER lights. One of them does exactly what you saw; it displays lower RPM as you bring the throttle up.

It's funny; I expect fuel gauges and other analog displays to be inaccurate. You'd think a pulse-driven gauge would be right all the time.
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Old 06-06-2014, 01:36 PM   #7
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If your genset is charging your batteries at the same time as your tach alternator, the RPM readings can drop out. Turn the genset off and see if the problem goes away.

Also as discussed, two alternators on the same battery bank can scramble the tach reading. Try one engine at a time to check.
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Old 06-06-2014, 03:22 PM   #8
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I have a port side tach that only works sporadically too!
So far, I have noticed that when the port engine mounted, high output alternator is charging the house batteries with gusto, 90 amps or so, the tach will often, but not always, work. Other times, nothing. I have become quite used to listening to the harmonics between the engines and matching operating temps to keep the loads even between the engines. Who really needs more than one tach? I have two that work, both on the Stb engine.
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Old 06-09-2014, 09:38 AM   #9
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Ford/Sabre uses oil pump drive for tach, others may as well.

Just so its covered, not all tachs are driven off the alt.
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Old 06-09-2014, 09:58 AM   #10
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Quote:
Originally Posted by SCOTTEDAVIS View Post
Just so its covered, not all tachs are driven off the alt.
Sorry, yes, that was my big change: alternator-driven to mechanical sensors. My Perkins engines came with a blank plate over where the mechanical sensor used to be on earlier models. I replaced them with this:


Now I've got good solid RPM data to feed into my NMEA 2000 bus. I still want to try replacing the wires from the alternators to the old tachs with shielded cable to see if that makes them a little more reliable.
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Old 06-09-2014, 10:57 AM   #11
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Quote:
Originally Posted by CaptTom View Post
Sorry, yes, that was my big change: alternator-driven to mechanical sensors. My Perkins engines came with a blank plate over where the mechanical sensor used to be on earlier models. I replaced them with this:


Now I've got good solid RPM data to feed into my NMEA 2000 bus. I still want to try replacing the wires from the alternators to the old tachs with shielded cable to see if that makes them a little more reliable.


My Ford/Sabre is electric, it just gets it's impulse off the oil pump drive.

Folks troubleshooting may not be aware of the different ways the tachs get their information, this can cause many hours of head scratching messing with the alternator.

Good service manuals are good.
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Old 06-09-2014, 11:18 AM   #12
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The alt v belt needs to be tight and not bottomed out.
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Old 06-13-2014, 04:48 AM   #13
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CaptTom, which perkins engine do you have and where is the plate located? Also, where did you get the plate and connections?
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Old 06-13-2014, 08:01 AM   #14
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Quote:
Originally Posted by CaptTom View Post
Sorry, yes, that was my big change: alternator-driven to mechanical sensors. My Perkins engines came with a blank plate over where the mechanical sensor used to be on earlier models. I replaced them with this:


Now I've got good solid RPM data to feed into my NMEA 2000 bus. I still want to try replacing the wires from the alternators to the old tachs with shielded cable to see if that makes them a little more reliable.
I'm interested in these sensors where do they bolt ion to a Perkins 4-154m motor?
Thanks,
Bill
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Old 11-26-2014, 03:45 PM   #15
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I would very much like to speak with you. I am in the market for a 66 and saw your boat in Sausalito.
Please call or email
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619-997-7722 or rbhaines@gmail.com
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Old 11-26-2014, 06:02 PM   #16
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Quote:
Originally Posted by kpinnn View Post
........... Also noticed that after shutting down this evening all four tachs show about 1000 RPM's even when the engines are shut down. That is right all four...............
You never noticed that before? Mine stay at the RPM the engine was running before the key was turned to shut it down (idle speed). They will stay at that reading until the key is turned on again and will drop to zero until the engine is restarted. My previous boat did the same.

As for your other problem, you don't need to replace the wiring with shielded cable or anything else. If it used to work, the type of wire is fine. Check the connections and check for breaks,
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Old 11-28-2014, 12:28 AM   #17
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Somebody else said it: check grounds!
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Old 11-28-2014, 06:03 PM   #18
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tach

I would run a new wire. It's probably just a bad connection.
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Old 11-28-2014, 07:23 PM   #19
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I would run a new wire. It's probably just a bad connection.
Wouldn't it be simpler to check the connections and repair any that are suspect?
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Old 11-28-2014, 08:32 PM   #20
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On some systems, it could take hours just to find the fault....if you ever find it. I chased an intermittent electrical start problem for months before I finally gave up and started replacing wires. I got it on the first try, but I had a couple of clues.
1. The red wire
and
2. There was a butt splice about 4 inches from the switch.

I had a recent intermittent power loss on my fishfinder that turned out to be an amateurish wire twist under electrical tape by the PO. I found it right away and it was right in front of the fwd ER access door so I was lucky. I swear my baby niece could wrap a tighter wire twist! I didn't want the splice as a future potential point of failure, so I ran a new, oversized and properly fused wire.

In both cases, it was less effort to run the new wire rather than break apart wire bundles to get to the bad splice and the job got done right. OK, it would have been done a bit better if I had pulled the bad wire and rebundled the entire loom with the new one, but that wasn't happenin!
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