Tach question

[jhodge]

After replacing the recitifer the tach started working. BUT when the power is turned on to the tach it jumps around 200 rpm when the engine is not running. Is this an indication that the wrong setting is being used? I set it on the 6 position. When going wot the reading seems to max out at 6000 rpm, and the motor is rated at 5300 to 5800 full throttle rpm, so I'll throttle back to keep it in range. There is one higher setting on the back of the tach, I think it was the H setting. I guess there is no harm in changing the setting, but was wondering how I can verify the rpm that the tach is showing. How would you use a regular automotive meter to check this, since there are three coils and no common pole to read the three primary charges going to the coils? Just measure with one and multiply by three? The tach is an OMC model and the engine is an 86 60 hp 3 cyl Merc. Any help would be appreciated. Thanks JH

 

[Chris1956]

Re: Tach question

Tach signal comes from the stator, not from the coils. Although you can probably read some AC voltage that cooresponds to a particular RPM, I do not know what that is.

OMC and Merc had quite different Tach sender signals for a while, some years ago. I think they are more standard today.

I would disconnect the signal wire and see if the tach jumps around. If so, it is likely defective.

 

[j_martin]

Re: Tach question

The tach uses the distorted ac off the rectifier, shapes it into pulses, and counts the pulses. On a 6 pole system that amounts to 1 pulse per second per 10 rpm. (100 hz at 1000 rpm)

If there's some noise injected, it will count that also. If there's a fault in the tach it would add to the count.

If it reads upscale with the engine stopped, I would not rely on it for accuracy.

A common source of noise is the selector switch itself. Move it back and forth several times to clean it.

hope it helps
John

 

[sschefer]

Re: Tach question

123

 

[j_martin]

Re: Tach question

Good explanation John, just to add if you don't mind, the way to correctly determine the correct setting for your tach is to either know, gain knowledge of or count the poles (coils) on your stator and divide that by the number of cylinders.

Ex.. 12 pole stator with a 3 cyl engine = 4 pulses. Set the tach to position 4.

Ah but you ask what if the stator only has 8 poles and you have a 3 cyl engine. THAT's 2.6666666666... This is where the H setting comes into play.

If the tach seems to stick on something other than zero when you turn the key off it is just residual voltage that is hanging around. In cars we have balast resistors that eliminate this.

To check your tach you need to get a Dwell/Tach, hook it up to #1 coil and multiply by the number of cylinders just like you mentioned. Better Dwell/Tachs have a multiplier switch.

HUH?
What does the number of cylinders have to do with the price of beans?

The magnets go by x number of poles regardless of how many cylinders are pushing it around.

Anyway, you count the number of coils on the stator, and divide it by 2, and that's the number of cycles or pulses per revolution. Some calibrate switches are marked in poles, and some are in pulses.

If you have a distributor ignition, where one coil is supplying all the fire for the engine, and you use the coil signal off'n it for a tach signal, then cylinder count comes into the calculation.

The ballast resistor in a contact point ignition system has absolutely nothing to do with any of this.

hope it helps
John

 

[sschefer]

Re: Tach question

456

 

[sschefer]

Re: Tach question

789

 

[sschefer]

Re: Tach question

10.. Whatever ....

 

[jhodge]

Re: Tach question

Thanks everyone. A bit of clarification. When I was writting about coils and their primary circuit I was asking about using an automotive dwell / tach unit to check the rpm against the OMC tach in the dash. Unlike older automotive, with a single wire going to the single coil for firing all the cylinders this motor has the switch box with an individual primary wire going to a dedicated coil for each cylinder. I know the signal pulses come off the rectifier and I have by passed the grey wire for now and have a separate wire running from the rectifier to the tach. I will move the switch around on the back of the tach to see if I can get a good contact.
Also, I will remove the signal wire and see if the power on still causes a bump when the power is turned on.
I recently replaced the stator with a Sierra unit and unlike the old stator the coils are encased with what ever they use. The two coils that are dedicated for power to the switch box are also encased. It seems like a couple of coils for the rectifier are encased with the two for the switch box. this makes it difficult to get an accurate count of coils. My thought is use an automotive tach meter and adjust the OMC to be as close as possible. Is this crazy? The tach may be a bit of an antique but if I can get it to respond some where close I'd be satisfied, should be better than no tach at all. Thanks for any help on using the automotive tach meter to check the rpm on this Merc engine. JH

 

[j_martin]

Re: Tach question

Actually, even in a 12 pole system, there might be only 6, or 10 coils. It has to do more with the number of magnets than with the number of coils. It's just normally easier to count coils.

The coils are usually wired in series, and each coil has a magnet going by it at the same time as all the other coils. If some coils are missing, it doesn't have any effect on the output frequency, only the voltage.

Looking at the picture at mercruiserparts.com, it looks like yours is a 12 pole, 16 amp stator. Setup would be 12 poles, or 6 cycles or pulses.

hope it helps
John

 

[jhodge]

Re: Tach question

I remember there were a lot more coils visable on the old defuct open coil stator than on the Sierra model that's in there now. I salvaged the copper out of the old stator so I'm familiar with the windings, if I remember correctly they were wound opposite of the adjacent coil and were one continous length of copper. Thanks again JH

 

[jhodge]

Re: Tach question

OK> I checked the tach without the pulse wire connected. It made no difference and showed a 200rpm jump in all positions 2-6 and H on the back. Something about the 'power on' causing a small displacement of the tach needle. Gonna live with it, but will try to use an automotive tach / dwell meter to find the closest correct setting for my stator. Thanks for the help. When my ship comes in I'll get a tach that works with this engine. JH