My tachometer Seems to hunt around even though the engine RPM's are not changing. What is the cause of this and what is the best way to trouble shoot it?
tachometer not accurate
- Thread starter Hayden Watson
- Start date
When that happened with mine, it turned out to be a dirty electrical contact. Checking the wiring is where I'd start.
Tachs are typically driven by a sensor that is pulsed by a magnet on the flywheel or are driven by an output on the alternator. You need to know which type you have. First I would check the engine speed directly with an optical tachometer to be sure it is running at a constant speed. Compare that reading with the tach reading. They should agree closely. If your tach uses a sensor counting rpm from the flywheel the fault could be the sensor or the instrument head. If it is driven from the alternator it is unlikely that the alternator is bad. If you have a digital multimeter tha TV will measure frequency, check the output of the sensor or the alternator to see if it is changing l I'll keep the tach is reading. Of course I assume that you have checked all wire connections and they are tight. This should lead you to the fault.
Some old engines have a spinning cable driving the tach. If the cable needs lube it can hang up and read a slower speed, then spring back and read faster than the actual speed and then continuously repeat. Exactly how the OP describes his. Lubing the cable fixes it and you can get the special lube and tool it inject it at a motorcycle shop. Probably like $10.
My tach is alternator sensing. The engine is running at a constant pitch and very steady. The tach fluctuates by 600+ rpms. Alternator is putting out the expected current and voltage. I have checked the connections in the engine but have not yet torn apart the engine panel in the cockpit.
I guess that is the next step.
I guess that is the next step.
@Hayden Watson , while you're in there: you might consider "resetting" your tach if it has the older dip switches. Over the years some have reported that merely moving the dip switches and back to their original positions "clears" the tach. Our C34 tech wiki has the Teleflex manuals, IIRC.
Also, is your hour meter seeming to run properly? Of course you'll not only visually check but actually physically remove and replace them at the back of the tach. Consider using dialectric grease or at least T9 on them.
Good luck.
My understanding is that dielectric grease is a non-conductor so it inhibits the flow of current but it helps to prevent corrosion.
Yeah, I used to think that until I did "my research & homework." Like I've been suggesting for decades.
I use it on my model railroad, too. Choo Choo....
I often also suggest RTFM. So I read the label!
Case closed --- that's why they make it. Lousy name, great performance!
What alternator?
How many poles?
What is driving it belt wise?
How many amps is the alt?
Internal or external regulation?
What batteries?
What tachometer?
Any other charge sources on the bus?
Occurring in bulk only?
Occurring in absorption only?
Have you compared it to a digital photo tach?
We Can't help if you don't provide enough information....
That was what I did. I have used dielectric grease on my boat for years but then I decided to look it up and found this.Yeah, I used to think that until I did "my research & homework." Like I've been suggesting for decades.
I use it on my model railroad, too. Choo Choo....
I often also suggest RTFM. So I read the label!
"...What is Dielectric Grease?
Dielectric grease is a silicone-based grease that is used in an electric circuit to protect components from dirt, moisture, and corrosion. Dielectric grease is also known as silicone grease.
It is a non-conductive material used in the electrical circuit to transfer heat from the device. It is a waterproof grease and made by adding a silicone oil with a thickener.
What is Dielectric Grease Used For?
Dielectric grease is used to disturb the flow of electric current with the property of good lubricants. It is used in many applications including home electrical work, vehicle wiring, and automotive tune-up.
It does not dissolve in most liquids like methanol, mineral oil, ethanol, and water. Therefore, it is used in the marine application and outdoor application to make electrical components waterproof. But the dielectric grease can be dissolved in xylene, mineral spirits, and Methyl Ethyl Ketone (MEK). ..."
I will answer below in red as best as I can. This is a new condition on a boat that has not had any changes made to the charging system in the 23 years I have owned her. I have not been out yet this year and have seen the behavior when running the motor at the dock with the battery fully charged.
Charge bus simply means; anything else charging the batteries when you ran this test?
A 3/8" belt is way undersized for a 105A alt charging AGM batteries. However slippage would not rear its ugly head when charging full batteries as current would be extremely low.
That old Xantrex regulator is slow to react and offers no way to limit alternator output to match the belt. If something else is charging the bank when you ran this test the Xantrex may be cutting the field temporarily to try and keep the voltage where it needs to be. If the batteries were truly 100% SoC this can also happen as the Xantrex often can't react quickly enough to prevent voltage over-shoot. When alternator field stops, so does the tach.
Try applying a decent load to the DC bus and see if the tach stabilizes.
Unless the isolator is a FET based low voltage drop-unit, diode isolators are a poor choice for alternator performance but it has likely saved your belts and alternator from heat damage.......
A 3/8" belt is way undersized for a 105A alt charging AGM batteries. However slippage would not rear its ugly head when charging full batteries as current would be extremely low.
That old Xantrex regulator is slow to react and offers no way to limit alternator output to match the belt. If something else is charging the bank when you ran this test the Xantrex may be cutting the field temporarily to try and keep the voltage where it needs to be. If the batteries were truly 100% SoC this can also happen as the Xantrex often can't react quickly enough to prevent voltage over-shoot. When alternator field stops, so does the tach.
Try applying a decent load to the DC bus and see if the tach stabilizes.
Unless the isolator is a FET based low voltage drop-unit, diode isolators are a poor choice for alternator performance but it has likely saved your belts and alternator from heat damage.......
Thanks Maine Sail. I did not have any other charging when I see tach fluctuations but suspect that voltage overshoot you mention is the most likely candidate. The battery is toast and will be replaced shortly and the internal resistance is really high. It hits set voltage as soon as I start the engine and the capacity is nil.
Along with a new house bank, I plan to install a serpentine belt kit. The battery sense voltage is taken from the house battery and my understanding was that would overcome the normal 0.7v drop across the diodes. Is that not correct?
After I have the serpentine belt and a new battery that can actually except the current, there will be times that I will probably want to reduce the current being generated. Can I just add a rheostat to the field wire to cut the current manually when wanted?
Hayden - if you want rock solid rpm - epoxy 4-8 rare earth magnets evenly around the back of the flywheel. Then use a hall sensor to sense the magnets as the engine turns.
you’ll need to adjust the tach to read correct rpm but it won’t vary with slipping belts, or go to zero when batteries are topped off.
you’ll need to adjust the tach to read correct rpm but it won’t vary with slipping belts, or go to zero when batteries are topped off.
early M25’s won’t accept any serpentine kit. Your crank pulley needs to have the 3 bolt holes for a pto. I don’t know when it was introduced as standard, but not on my 83 and not on @Stu Jackson 34 (87??). Good news is if you can source an XPB crank pulley it should fit.
you can’t turn dow charging voltage with a rheostat. If you tote the regulator will just boost the field to compensate for lower output voltage. It’s called a voltage regulator not a current regulator for a reason
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