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Rev counter wobble with added pinging


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I've developed a wobbly tacho needle on my Isuzu.. It's happening at lower engine speeds ( just off or at idle).

 

When the needle drops I get a flash of the warning light, and a ping from the buzzer.

 

I've checked the obvious stuff - the belts not slipping, can't see a broken wire, and the battery is still charging.

The engine's done about 600 hours, but the alternator has probably done about 200 after first one died.

 

One question which might ( or might not make me look really dumb) - should the engine be earthed onto the hull / bed etc as I've never found it.  The alternator earths onto the mounting bracket, but I can't see an earth to bypass the rubber engine mounts.

 

I've taken a video of it with my phone, but I don't think I can upload it here anymore

 

Anyone have any idea's as it's driving me potty...

 

Thanks in advance...

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I suspect a poor connection to a diode inside the alternator. It will still be charging but at a reduced voltage and current.

 

As nothing apart from just one 12V negative connection should be made to the hull the lack of an engine to hull 12V bond should make no difference to the revcounter. In any case it is probably earthling through the exhaust, control cables, and  to a degree the prop shaft (depending upon type of coupling. If the hull really has no 12V neg bond the if something shorts to the hull the hull will go live to 12V because no fuses would blow. This might cause hull corrosion.

 

 

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Hi Tony. Thanks for quick reply. 

Is there a way of testing to confirm? Possibly running wire from engine alternator to tacho feed to see if it stabilises needle etc

Its an ACR so not end of the world financially if  I have to replace it, but it’s only about 3 years old. Is its short life just a consequence of living in a damp engine hole under semi trad, or is there steps I can take to make next one last longer. 

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If there was not a cable from the W (taco) terminal on the alternator or the fly lead if the alternator has been modified to accept a tacho to the taco itself then you would have no reading so that cable is there. I have known them get erratic when there was a loose connection, often at the alternator end so engine vibration causes it to make and break but something assimilate might happen with loose/dirty connections at the tacho end on any of the three wires. Best check the cables and terminals first.

 

The only ways I know of to see if a diode is failing is to either measure amps and volts into a load on a test bench and from that infer there might be a phase failing or with a oscilloscope so you can look at the ripple on the alternator output. It might be worth inspecting the soldered diode and W terminal connection on the diode pack to see if one has cracked. this could be on a positive or negative diode. If you have a suitably large iron and something to clamp around the diode lead as you work you could take the alternator off and try re-soldering the diode connections but don't and about with a cool or small iron on the connection.

 

ACRs are well known for drilling a hole through the idiot centre slip ring, why did you not swap it for a higher output A127, direct fit but I would change the main blade connection(s) to  the stud one.

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Have you tried wiggling the large connector on the engine harness it is I think 13 pin and a major cause of wired faults. The plating on the contacts oxidises and can lead to fax no problems starting problems and a multitude of others,

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13 hours ago, Silebyman said:

One question which might ( or might not make me look really dumb) - should the engine be earthed onto the hull / bed etc as I've never found it.  The alternator earths onto the mounting bracket, but I can't see an earth to bypass the rubber engine mounts.

Mine was like this - there was continuity (probably there's carbon in the engine mount rubber), but the was a resistance. I added a flexible braided earth lead from the stud where the battery negative bonds to the engine block to the hull using a bolt through the engine bearer.  Use a locknut or nyloc to ensure it stays tight.  My boat didn't have a dedicated AC earth bond connection to the hull either, so that is now connected in a similar way next to it.  Note that the AC and DC bonds should not share the same stud.

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40 minutes ago, Brian422 said:

Beta 43 Marine manual suggest checking alternator output from 'W' connection; should be approximately 9 Volts A.C. 

 

Or the frequency. The problem is that multi-meters sample the voltage at discreet time intervals and this seems to be an intermittent fault so if only occurs between samples the fault would not register.

 

Just had another thought - would the OP have solar charging by any chance?  If so it might be fully charged batteries allowing a high solar charge voltage causing the alternator to shut down for brief period. I suspect that even MPPT controllers do not supply a rock steady voltage as theya lso seem to sample so if I am right a brief peak in solar voltage might well shut the alternator down. If this is what is going on then there will be no fault as such. When it does t again try putting on a high electrical load and see  if it stops until the load is removed. Try covering or isolating the panes, expect similar results if I am correct.

 

Again, if I am right it will need an analogue oscilloscope to measure the suspect pulses or a very expensive high speed digital one.

 

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To answer  2 replies at once,

 

The Beta comment is interesting - I've just been reading the manual.  It's far more comprehensive than the token gesture manual supplied Izusu.

 

No I don't have solar.  The problem first appeared when I disconnected from landline last Saturday morning, so first assumption was batteries were fully charged and the alternator was shutting down.

 

The problem was still there Sunday morning after an overnights worth of discharge.. So don't now think that was the cause.

 

I'm building a list of things to check when I get back down to the boat in a couple of weeks time, such as corroded connectors, voltage at W terminal, alternator charging output etc before I condemn the alternator and cough up for another.

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1 hour ago, Silebyman said:

I'm building a list of things to check when I get back down to the boat in a couple of weeks time, such as corroded connectors, voltage at W terminal, alternator charging output etc before I condemn the alternator and cough up for another.

Don't condom until you have had it tested at your local starter and alternator specialists. If its is faulty they will probably swap it fr one straight off the shelf. Other with local knowledge will tell you where to go.

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19 hours ago, Tony Brooks said:

Or the frequency. The problem is that multi-meters sample the voltage at discreet time intervals and this seems to be an intermittent fault so if only occurs between samples the fault would not register.

 

Understand limitations of digital multi-meters but if you know someone with an analogue multi-meter and can leave it connected as close as possible to alternator 'W' output ...

Edited by Brian422
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On 03/06/2019 at 17:28, Silebyman said:

I've developed a wobbly tacho needle on my Isuzu.. It's happening at lower engine speeds ( just off or at idle).

 

When the needle drops I get a flash of the warning light, and a ping from the buzzer.

 

I've checked the obvious stuff - the belts not slipping, can't see a broken wire, and the battery is still charging.

The engine's done about 600 hours, but the alternator has probably done about 200 after first one died.

 

One question which might ( or might not make me look really dumb) - should the engine be earthed onto the hull / bed etc as I've never found it.  The alternator earths onto the mounting bracket, but I can't see an earth to bypass the rubber engine mounts.

 

I've taken a video of it with my phone, but I don't think I can upload it here anymore

 

Anyone have any idea's as it's driving me potty...

 

Thanks in advance...

Before you condemn the alternator check the the multipoint connector between the engine and instrument panel.  Pull apart and spray with switch cleaner. These connectors can be a source of all sorts of problems.

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