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27-06-2023, 21:02
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#1
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Registered User
Join Date: Oct 2017
Location: Australia
Boat: 37 ft Aluminium Cutter - Roberts design
Posts: 179
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Do Tachometers Die?
Hi All. I have a 110mm VDO tachometer with integrated hour meter. The tachometer reads very high (against the measurements I took with an optical tachometer) and the hour meter hasn’t worked since I bought this yacht. The adjustment screw at the back of the tachometer has no effect on the revs shown.
The tachometer pick-up is currently from the alternator (a 160A Prestolite Leece-Neville).
I know I can buy a new VDO tachometer to fit the slot of the old but it’s pretty pricey. I will buy it, however, if I knew it would work.
So my queries are, is my existing VDO tachometer dead and is it a straightforward task of wiring up a new one?
Thanks for any expert input!
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28-06-2023, 04:58
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#2
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Senior Cruiser

Join Date: Mar 2003
Location: Thunder Bay, Ontario - 48-29N x 89-20W
Boat: (Cruiser Living On Dirt)
Posts: 51,843
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Re: Do Tachometers Die?
On diesel engines, that use the alternator output as the tach input, the number of pulses per revolution is a function of the number of alternator poles, and the ratio of the alternator pulley size to the engine pulley size.
Teleflex tachs must have 12 VDC (from the ignition switch, “ON” when
engine is running) to the “IGN” stud.
When tachometer is first turned on, pointer must go to zero ±50 RPM. If not, check connections.
1. When the ignition is switched on, is there 12vdc (or 24vdc) between the positive terminal, and negative terminals, when measured with a voltmeter? If not, use your voltmeter to determine whether the problem is in the wiring on the positive side or the ground side.
2. If the voltage is correct, the next step is to check the input signal. Remove the signal wire.
Set your voltmeter to AC volts, and connect it between the signal wire [generally labeled: ‘R’, ‘W’, or ‘AC’] and ground.
Start the engine, and set the speed to idle.
You should get a reading on the multimeter that varies with engine speed. The AC voltage should rise, when the engine RPM’s rise, and fall when the engine RPM’s fall.
If the tachometer is connected to the alternator, this reading should be a minimum of about 5VAC, otherwise it will be more like 0.3VAC.
If you get any indication of a pulse stream, reconnect the signal wire, and if the problem is still evident, the culprit is most likely the tachometer itself.
3. If you get no indication of a pulse stream, disconnect the tachometer signal wire on the engine, and repeat the test there. If you detect a pulse stream at that end, the problem must be in the wiring. Otherwise, the problem is in the signal generator.
__________________
Gord May
"If you didn't have the time or money to do it right in the first place, when will you get the time/$ to fix it?"
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28-06-2023, 05:34
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#3
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Registered User
Join Date: Aug 2012
Location: Maryland, USA
Boat: 58' Sedan Bridge
Posts: 5,628
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Re: Do Tachometers Die?
Quote:
Originally Posted by Wideocean7
Hi All. I have a 110mm VDO tachometer with integrated hour meter. The tachometer reads very high (against the measurements I took with an optical tachometer) and the hour meter hasn’t worked since I bought this yacht. The adjustment screw at the back of the tachometer has no effect on the revs shown.
The tachometer pick-up is currently from the alternator (a 160A Prestolite Leece-Neville).
I know I can buy a new VDO tachometer to fit the slot of the old but it’s pretty pricey. I will buy it, however, if I knew it would work.
So my queries are, is my existing VDO tachometer dead and is it a straightforward task of wiring up a new one?
Thanks for any expert input!
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We had Faria analog tachs that came on an earlier boat... and they died. Presumably VDO gauges could die, too...
And even when they were working, the Faria adjustment mechanism still only provided a ballpark RPM.
(They offered a rebuild service, for approx half the cost of new. That process zero'd out the engine hours. And the resulting accuracy was just as underwhelming as original. But perhaps VDO might do that too...)
You might have a look at Aetna or CruzPro digital tachs for replacement... if they have something that will sense correctly from your engine... and might fit your existing cut-out.
-Chris
__________________
Chesapeake Bay, USA.
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28-06-2023, 06:57
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#4
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Registered User
Join Date: Oct 2020
Location: SE USA
Boat: Hunter 38
Posts: 1,471
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Re: Do Tachometers Die?
Check with North Hollywood Speedometer. IDK if they do marine, but they've done a good job on some old Porsche gauges for me, cheap, fast, and good believe it or not.
Maybe they can help point you to a vendor.
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28-06-2023, 07:07
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#5
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Registered User
Join Date: Jun 2019
Location: Rochester, NY
Boat: Chris Craft 381 Catalina
Posts: 6,988
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Re: Do Tachometers Die?
I had a similar experience to ranger58sb with Faria tachs. I had an older one that worked fine and was accurate. When my other original tach started getting flaky, I replaced it with a matching Faria. The thing was never accurate. I could get it accurate at low or high RPM, but not both. Warrantied it, same story with the replacement.
So I gave up and replaced both with Autometer tachs. In my application, they were dead on accurate right out of the box, no calibration needed. For use on your engines, I think you'd need the adapter they sell to convert the alternator tach signal to work with their tachs.
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28-06-2023, 17:29
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#6
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Moderator and Certifiable Refitter
Join Date: Jan 2008
Location: South of 43 S, Australia
Boat: C.L.O.D.
Posts: 21,518
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Re: Do Tachometers Die?
WO7, I assume you are still in Australia -
Almost all car / boat instruments can be repaired but it is not always cost effective. There is blistering array of auto instrument repairers in Australia and there are many VDO suppliers.
First you need to follow Gord's advice in post #2 to determine if the alternator is outputting a AC signal on the W (or R or AC) terminal. Then confirm this is appearing at the Tach input along with the 12VDC supply to the Tach.
Assuming the AC and DC lots are OK, contact a Tach repairer and discuss. There are many to choose from but I have had good dealings with NAMISS (Gauge Shop) https://www.gauge-shop.com.au as well as Otto Instruments. https://www.ottoinstruments.com.au I believe Howards are pretty good as well. https://www.howardinstruments.com.au
There are many many VDO tachs around and they have many subtle differences. You need to know the exact part number and a service book to understand the what can be fixed and what can't. For instance, there are 2 marine VDO tachs that look the same to the user but one can be field adjusted (calibrated) and one needs to on the bench to be calibrated (with propriety VDO software etc). Again, I have found the NAMISS guys know this stuff backwards and are helpful but I sure the others are OK, it is their bread and butter after all.
If you go for the replacement route, changing out the instrument is easy but make sure you get one that can be field calibrated, suitable for the RPM range of your engine as well as accepting an AC input (W). No point in getting 6,000 rpm tach for 2,500 rpm engine.
On most engines, you can sight the crankshaft pulley and use a free smartphone tach app to determine engine speed. Different tachs have different methods of adjustment, make sure you can do it yourself before buying.
__________________
All men dream: but not equally. Those who dream by night in the dusty recesses of their minds wake in the day to find it was vanity: but the dreamers of the day are dangereous men, for they may act their dreams with open eyes, to make it possible. T.E. Lawrence
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29-06-2023, 00:38
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#7
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Registered User
Join Date: Oct 2017
Location: Australia
Boat: 37 ft Aluminium Cutter - Roberts design
Posts: 179
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Re: Do Tachometers Die?
Hi All. This is very helpful-thank you. I’ll first give a go at what Gord May suggests. That will (hopefully) tell me if the tacho is defective or not. Having said that, I know the tacho reads very high (I’ve measured the revs from the flywheel with a tach meter) and I know that the running hours gauge isn’t working. Hence that’s why I’ve been thinking that the tacho is broken. I’ll have a look at the tacho models suggested. But I need a 110mm version so that it can be accommodated in my instrument panel without adjustments.
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29-06-2023, 00:46
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#8
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Registered User
Join Date: Oct 2017
Location: Australia
Boat: 37 ft Aluminium Cutter - Roberts design
Posts: 179
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Re: Do Tachometers Die?
Quote:
Originally Posted by Wotname
WO7, I assume you are still in Australia -
Almost all car / boat instruments can be repaired but it is not always cost effective. There is blistering array of auto instrument repairers in Australia and there are many VDO suppliers.
First you need to follow Gord's advice in post #2 to determine if the alternator is outputting a AC signal on the W (or R or AC) terminal. Then confirm this is appearing at the Tach input along with the 12VDC supply to the Tach.
Assuming the AC and DC lots are OK, contact a Tach repairer and discuss. There are many to choose from but I have had good dealings with NAMISS (Gauge Shop) https://www.gauge-shop.com.au as well as Otto Instruments. https://www.ottoinstruments.com.au I believe Howards are pretty good as well. https://www.howardinstruments.com.au
There are many many VDO tachs around and they have many subtle differences. You need to know the exact part number and a service book to understand the what can be fixed and what can't. For instance, there are 2 marine VDO tachs that look the same to the user but one can be field adjusted (calibrated) and one needs to on the bench to be calibrated (with propriety VDO software etc). Again, I have found the NAMISS guys know this stuff backwards and are helpful but I sure the others are OK, it is their bread and butter after all.
If you go for the replacement route, changing out the instrument is easy but make sure you get one that can be field calibrated, suitable for the RPM range of your engine as well as accepting an AC input (W). No point in getting 6,000 rpm tach for 2,500 rpm engine.
On most engines, you can sight the crankshaft pulley and use a free smartphone tach app to determine engine speed. Different tachs have different methods of adjustment, make sure you can do it yourself before buying.
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Thanks Wornit. Good resources that I’ll follow up. Yes, still in Australia although that wasn’t quite the plan!
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29-06-2023, 00:56
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#9
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Registered User
Join Date: Oct 2017
Location: Australia
Boat: 37 ft Aluminium Cutter - Roberts design
Posts: 179
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Re: Do Tachometers Die?
Quote:
Originally Posted by Wideocean7
Thanks Wornit. Good resources that I’ll follow up. Yes, still in Australia although that wasn’t quite the plan!
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My apologies Wotname.
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29-06-2023, 01:38
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#10
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Moderator and Certifiable Refitter
Join Date: Jan 2008
Location: South of 43 S, Australia
Boat: C.L.O.D.
Posts: 21,518
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Re: Do Tachometers Die?
Quote:
Originally Posted by Wideocean7
My apologies Wotname.
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Accepted
__________________
All men dream: but not equally. Those who dream by night in the dusty recesses of their minds wake in the day to find it was vanity: but the dreamers of the day are dangereous men, for they may act their dreams with open eyes, to make it possible. T.E. Lawrence
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29-06-2023, 14:08
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#11
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Registered User
Join Date: Nov 2019
Location: Vancouver
Boat: Ericson 27
Posts: 570
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Re: Do Tachometers Die?
My tach is long dead. It worked by counting the teeth on the flywheel. This is on a Yanmar 1GM10.
These days, I use the nmea2000 capabilities of my wakespeed alternator regulator and just display the tach data on my Garmin instrument head while motoring. It works by monitoring the alternator.
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