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Author Topic:   Tachometer Diagnosis and Repair
kenyon posted 05-20-2002 03:43 PM ET (US)   Profile for kenyon   Send Email to kenyon  
I have a 1984 Outrage with what is probably the original tachometer. I get no reading. What is the procedure to troubleshoot to repair?


Bigshot posted 05-20-2002 03:57 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Type of engine would help but usually it is related to the rectifier. Didi the tach start freaking out before it died? Is your engine charging the battery?
jimh posted 05-20-2002 09:49 PM ET (US)     Profile for jimh  Send Email to jimh     
The TACHOMETER gauge needs three things to work:

1. Source of pulses from engine sender;
2. Wiring connecting sender to gauge;
3. Gauge.

It is unlikely, given your apparent unfamiliarity with the circuit, that you will be able to make repairs to the gauge portion, nor would most.

These leaves two areas to inspect: the sender unit and the wiring.

If you are familiar with simple electronics and possess some simple test instruments, you can assess the sender unit. Measure the voltage and frequency of the pulses it produces. A train of pulses which are clamped between ground and battery postive should be seen. The frequency of the pulses should be an integer multiple of the crankcase speed, typical 4 to 8 times, depending on the arrangement of magnets and pickups.

If you have even a slight notion of electrical circuits you can check the wiring between sender and gauge for continuity or shunts.

If you require greater elaboration of any of these procedures please advise and I will attempt to provide it.

Whalerdan posted 05-21-2002 07:55 AM ET (US)     Profile for Whalerdan  Send Email to Whalerdan     

Without an O-scope, how do you read the pulses? Do you use a meter and watch the needle bounce? Are the pulses slow enough and the meter fast enough to see them or will you get some averaged voltage?

jimh posted 05-21-2002 09:23 AM ET (US)     Profile for jimh  Send Email to jimh     
Last year I was diagnosing some tachometer problems on my boat. I hauled a 120 VAC powered Tektronics 100MHz, dual trace, triggered sweep oscilloscope out to the boat. Using it to check the signals coming from the engines to the tachometers, I could quickly see the problem.

Where I work, we have oscilloscopes like that lying around unused (because in these days of high speed digital video we need much facier 'scopes to see what the heck is really happening). As I was making these observations on the tach pulses, the thought came to mind that this might have been the first time in the history of outboard engine diagosis and repair that anyone used a Tektronics oscilloscope to fix a tach!

Typically, on traditional 2-stroke engines, the tach pulses are derived from the battery charging circuit and are just the raw, pulsating DC coming off the coil winding, ahead of the rectifier.

The rectifier does not make the pulses. In the typical configuration the rectifier is wired so that it provides the ground reference for the coil and also clamps the pulse amplitude at battery positive voltage.

When the rectifier fails, there are many possible failure modes. If it opens to ground, it may disable the tach. If it shorts the coil, it may diable the tach (and burn up the coil).

I had a bad rectifier on my old 1976 Merc 50-HP, but it had failed in such a way that the tachometer kept working just fine. The circuit that resulted was not doing any battery charging, but it made nice tach pulses.

The other component of the tach sender circuit is a coil, usually located under the flywheel. This is often called a stator, because it is stationary. Permanent magnets glued to the flywheel rotate above the coil and induce current flow in the coil when they pass over it.

The stator is subject to damage in several ways. Usually too much heat causes them harm. Since they are at the top of the engine and under the flywheel, they are in one of the hottest areas of the engine and not well ventilated. Heat from current flowing in them also builds up. If you are chronically in need of maximum charging current because of low battery voltage and high electrical load, the effect over time can be to overheat the stator coil.

Connecting the battery to the engine with the polarity reversed is another way to practically instantaneously damage the engine charging circuit. You will likely destroy the rectifier and possibly the stator coil if you have done this. It seems quite a common mistake.

If you can disconnect the stator coil leads from the rectifier, you can measure the resistance of the stator coil and check it for shorts to ground (if it is floating electrically, as they usually are).

You can also disconnect the rectifier and measure it, although on occassion a rectifier will measure as "good" but will fail in the real circuit. The failure may only occur with elevated temperature, for example.

The good news is that an OEM rectifier costs $35 (Mercury) or ($65 Yamaha---but of course those never fail, eh?). You can replace it in 5-minutes with simple tools.

The bad new is that a bad stator coil is more expensive, and much harder to replace. On some engines, the "stator" is part of a larger assembly that contains other coils. Sometimes these can be expensive. A friend with a MARINER engine (a Merc-Yamah hybrid) had to pay over $300 for a stator coil assembly. I have new stators on both my Yamaha engines and the coils only cost $90, in part because there are three different assemblies for coils under the stator and I only needed one of them. (And the one I needed was the cheapest, too!)

Even getting the part, there is still much work ahead to replace the stator. You have to remove the flywheel. From my experience with a little 70-HP Yamaha, it is impossible to remove the flywheel without the proper tools and an air-driven impact driver.

Once the flywheel is off, replacing the coil should be straightforward. When you put the engine back together you may need to retime the ignition pulses. That is probably more true if your stator coil assembly was an all-in-one unit which also had the ignition timing pulse coils in it.

Based on my experience, I would let a shop handle the flywheel removal, unless you happen to have the right tools lying around.

To sum up: TACH doesn't work will probably be:

--Bad wiring; a $5 fix.
--Bad gauge; a $75 fix.
--Bad rectifier; a $35 - $65 fix.
--Bad stator; a $90 - $300 parts plus $75 - $150 labor fix.


Whalerdan posted 05-21-2002 10:19 AM ET (US)     Profile for Whalerdan  Send Email to Whalerdan     
I guess when you looked at the pulses with the scope you were just checking to see if they were there, and not that they were the right period. Would make sense that any pulses at all would at least give some kind of tach reading.

I was going to use a Techtronic scope on mine last year when I had a problem (we have them at work too). I was too caught up not really knowing what the waveform "should" look like instead of just seeing what was there. Wished I'd have tried it as it would have been interesting.

Good job Jim!

Basscat 1 posted 08-12-2005 10:27 PM ET (US)     Profile for Basscat 1  Send Email to Basscat 1     
Do all OMC engines emit 6 pulses per revolution? Or is it one pulse per every two stator coils?
jimh posted 08-13-2005 08:24 AM ET (US)     Profile for jimh  Send Email to jimh     
[Changed TOPIC; was "tach doest (sic) work."]
jimh posted 08-13-2005 08:47 AM ET (US)     Profile for jimh  Send Email to jimh     
The pulse output per revolution depends on the number of magnets in the flywheel and the number of poles in the coil winding over which the flywheel passes. Historically it was typical that there be a single winding and a single phase rectifier, but newer engines (such as the Evinrude E-TEC) may have multiple windings and use a polyphase rectifier.

Aftermarket tachometer gauges usually have adjustments for calibrating them to work with different engines what may develop tachometer pulses at different integer multiples of the crankshaft speed. An OEM tachometer may not have such an adjustment.

I have frequently read that OMC engines work best with OMC tachometers, but the precise reason for this is usually not mentioned, nor do I know why this preference is often repeated. It may be there is something about the tachometer signal from an OMC engine which is slightly different. For example, they could clamp the voltage to a lower level which might affect the reading on some gauges not expecting that.

Basscat 1 posted 08-18-2005 05:24 PM ET (US)     Profile for Basscat 1  Send Email to Basscat 1     
I have been researching this and the verdict is still out. I called a couple of local boat shops and I got various answers. The first shop didnt have a clue. I know this old man that I was talking to on the phone. He is an excellent mechanic, but he dont know anything about tach settings. The second shop said that most were set on 6 but he seemed to remember that some were set on other settings, but he didnt remember which ones. Then I called the OMC hotline. The first technician I talked to on the phone said to set them all on 6, but he seemed a little unsure. I waited a couple of days and called back. The second technician said most were set on 6P, however prior to 1978 there we some models that were set on 4P and 5P, but he didnt know which models. Then today, I talked to a buddy of mine who has been working on outboard engines for over 20 years. He said one of his books says for every 2 poles = 1 pulse, which would mean we are correct. He suggest to leave it on 4P. He has a shop tach, and said he would hook up to my engine and check it whenever I can bring it by. The problem is that he lives about 60 miles away, so I dont when I will get to do that.

Thanks Jimh for your reply. You have explained it in more detail than most.

Sal DiMercurio posted 08-18-2005 09:13 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
Start the engine, put a voltmeter on the gray wire that connects "TO" the tach, if it reads 12 volts the tach is bad, if it reads less or nothing, your rectifier is gone with the wind.
The correct number to set on the back of the tach for that engine is,...#6.
Basscat 1 posted 08-24-2005 12:33 PM ET (US)     Profile for Basscat 1  Send Email to Basscat 1     
I understand how to troubleshoot the charging and its relation to the tach. Its the tach settings on the back that there seems to be some confusion.
You say to set them all on 6, how do you know that?
Basscat 1 posted 08-24-2005 12:51 PM ET (US)     Profile for Basscat 1  Send Email to Basscat 1     
I didnt mean for my question to sound so blunt. I am just gathering specific information on how the tach recieves its pulse signal in relation to the flywheel, stator-to the tach and its various settings. OMC tachs have various settings on the backs of their tachs for a reason. It seems strange to me that all of them would be set on 6.
Nobody seems to have an answer as to why set it on 6, they just say set it on 6.
Basscat 1 posted 08-26-2005 11:52 AM ET (US)     Profile for Basscat 1  Send Email to Basscat 1     
jimh posted 08-26-2005 01:51 PM ET (US)     Profile for jimh  Send Email to jimh     
The number of pulses on the tachometer signal lead produced per revolution of the engine crankshaft is proportional to the number of magnets in the flywheel and number of poles in the coil overwhich the magnets pass. Manufacturers of tachometers make their instruments universal by allowing the calibration to be changed to match the number of pulses produced per revolution by various engines.
where2 posted 08-27-2005 11:07 PM ET (US)     Profile for where2  Send Email to where2     
Since one of the main manufacturers of marine instrumentation puts plenty of their knowledge on the web, I consulted them.

Tach settings for the shade tree mechanic:

The real fun is seeing the O-scope sweep in the bright sunlight... (why do you guys borrow these from work? my father keeps one in a closet at the house).

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