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Old 06-21-2010, 02:29 AM   #1
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Tacho Generator

I have 2 tachos that don't work although they try to

I have the 120hp Lehman however I cannot see where the tachos would be driven from

I can see the driver on the side of the motor near the fuel filters but there is nothing on that.

Can somebody throw some light on this for me please

Allan
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Old 06-21-2010, 04:18 AM   #2
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RE: Tacho Generator

Tachs will either be driven by a pulse generator , mounted on the engine , or by the output of the alternator.

It counts AC pulses before their rectified into DC.

Although there are mechanical, but its hard to miss the drive cable.

IF the tach was built to feed a tach there will be a seperate output post .

Lots of times a boat tack craps out and a car cheapo replacement is used.

If that's your case , get a new pulse generator ($50-$75 or so) and keep using the car alts .
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Old 06-21-2010, 07:16 AM   #3
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RE: Tacho Generator

Quote:
AllanY wrote:

I have 2 tachos that don't work although they try to


What exactly are the symptoms? Is this a new problem? have you changed anything or had any work done on the electrical or charging system recently?
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Old 06-21-2010, 03:33 PM   #4
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RE: Tacho Generator

Hi Rick

I just bought the boat a week ago and knew the tachos didn't work as he surveyor noted it so I don't know how long the problem has existed.

The tachos just flicker when the engine runs.

I thought it might be a common problem to put both tachos out of service (fingers crossed)

I was trying to find the generator on the motor.

I am not familiar with the altenator method but I will look at that next time I am down at the boat (2 hours drive away)

If anyone has a photo of that set up I would appreciate it

Allan
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Old 06-24-2010, 07:21 AM   #5
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RE: Tacho Generator

Alan, the tachos on that boat, like mine, will be run from the alternator. It means they may have a loose connection - or they might be shot - as one of mine was. I had the upper helm tacho replaced by a marine electrician, as I was not confident to do it myself, and the VDO they used works fine running off the same output as the original Motorola.
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Old 12-02-2010, 03:03 PM   #6
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RE: Tacho Generator

If I could jump in here and steal this thread....

I have a bad tach, or more like flaky.* At times it reads way high - just kind of floats up to about 3000 RPM when the real RPM is probably closer to 2000.* I've had the mate check the lower helm and it reads correctly.* My logic tells me this is more likely a bad guage than bad sender since one reads correctly at the same time as the other one is way high.*

Reasonable?
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Old 12-03-2010, 04:45 AM   #7
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RE: Tacho Generator

"I have a bad tach, or more like flaky."

All the alt driven tachs are only rough approximations of the RPM.

None are accurate as the units that have a pulse generator and tach head that can be calibrated.
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Old 12-03-2010, 07:22 AM   #8
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RE: Tacho Generator

Problems could be,
Loose or corroded wires at the gage of alternator.
Bad tach gage.
Bad alternator.
bad tach sensor on the alternator.
Problem with the oil pressure sensor switch.

Our tach does not get going until the oil pressure starts to increase and the sensor activates.

Chuck
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Old 12-03-2010, 09:26 AM   #9
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RE: Tacho Generator

I would check the V belt tension first.
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Old 12-03-2010, 10:39 AM   #10
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RE: Tacho Generator

"My logic tells me this is more likely a bad guage than bad sender since one reads correctly at the same time as the other one is way high.*

Reasonable?"

Logical but to my experience it can easily be a loose connection, bad ground connection, etc. in the circuit going to the flaky reading gage.
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Old 12-03-2010, 11:36 AM   #11
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RE: Tacho Generator

If the tach reading is from the alternator, in all probability the problem is loose V belts. If tightening the V belts doesn't correct the problem, then start looking at the expensive solutions. On the Perkiins we had on our previous boat and on the Lehmans in the current boat the symptoms you describe have been corrected by tightening the V Belts.
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Old 12-04-2010, 06:14 AM   #12
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RE: Tacho Generator

One way to check for the belt slipping would be to watch the volt or ammeter when the tach reading drops right down. If it's the belt, then the charge will drop as well.
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Old 12-04-2010, 10:06 AM   #13
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RE: Tacho Generator

I had an issue with my tach working intermittently and checked the electrical connection several times, sometimes it would correct it, but other times it would not. Finally decided to replace the old connector, and that was it, now the tachs work full time.

BTW...my tach connection is mounted on the starboard side of the engine near the flywheel or rear of the engine (Yanmar)
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Old 12-05-2010, 06:02 AM   #14
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RE: Tacho Generator

A V belt loose enough to matter to the Tach will probably need the alt replaced.

The heat of the slipping V belt usually overheats the front bearing , the lube runs out and its over .
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Old 12-05-2010, 06:43 AM   #15
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RE: Tacho Generator

FF - very interesting analysis. I've never experience bearing failure caused by a slipping V belt. I have had belts slip and squeel, slip and make no noise - leave just powdered rubber on the front of the engine and I've had 60 amp and 160 amp alternators too hot to touch. I have never had an alternator fail due to lube running out of the front bearing. I have had tachs run to 600 or 700 rpm and stop or show no reading at all. I have corrected this by tightening the V belt. Just my experience.
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Old 12-05-2010, 07:43 AM   #16
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RE: Tacho Generator

If the belt is old and glazed, tightening it may not help, it can still slip and put excessive loading on the front bearings.

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Old 12-23-2010, 07:28 AM   #17
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RE: Tacho Generator

Quote:
Larry M wrote:If the belt is old and glazed, tightening it may not help, it can still slip and put excessive loading on the front bearings.
Which is almost certainly the cause of the failed front bearings, not heat from the pulley.

*
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Old 12-23-2010, 12:17 PM   #18
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RE: Tacho Generator

"Which is almost certainly the cause of the failed front bearings, not heat from the pulley."

Gee if a simple load will kill a ball bearing , I guess its scrap yard time for cars , trucks m'cycles table saws and about everything that goes round!
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Old 12-23-2010, 12:33 PM   #19
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RE: Tacho Generator

Quote:
FF wrote:Gee if a simple load will kill a ball bearing , I guess its scrap yard time for cars , trucks m'cycles table saws and about everything that goes round!
FF, are you really that* stupid or do you just play a moron on the internet?

Repeated tightening of a belt to eliminate squeal will overload the front bearing and destroy it in short order. Heat from a belt slipping on the pulley will not destroy the bearing.

*
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Old 12-23-2010, 02:23 PM   #20
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RE: Tacho Generator

Given the heat being applied to a water pump bearing by the coolant and the heat of the engine block, I cannot see any heat from a slipping belt even being a factor in the life of the bearing(s).

However, I have heard warnings from many sources--- including the Smiths at American Diesel with regard to the FL120--- our diesel shop and our marine electric shop (for the generator) not to over-tighten a drive belt for a water pump because this can shorten the life of the pump bearing. In fact on our Onan MDJE the "normal" setting for the belt driving the engine's coolant pump is extremely loose. So loose it worried me the first time I saw it until the shop explained that the design of the flywheel prevented any belt slip and putting a lot of tension on the pump would wear out the bearing pretty fast and with a generator this old replacement pumps are hard to find and expensive when you do. So keep the belt slack and the water pump will be much happier.

In all cases we were told that if a pump drive belt begins to slip--- evidenced on the FL120 by soot-like rubber deposits on the front of the engine case behind the belt---- the belt should be replaced, not tightened more unless it has become very loose.
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