Bouncing Tachometer

[Eyeman]

Put the muffs on and fired her up. Ran perfectly, except I noticed the tach would read out as if the RPM was increasing when it wasn't. Then it would bounce around on its way back down to the correct (lower/accurate) reading and be fine. Then every so often it would repeat the same pattern. Sometimes it seemed to be corrected by a left middle finger knuckle wrap. HELP.

 

[fastimz]

Re: Bouncing Tachometer

I like the repair method. Sounds like a loose or bad ground. If you are able to get easy access to the back of the tach, run a jumper wire from teh ground on the battery to the ground on the gauge.

 

[mcastles]

Re: Bouncing Tachometer

Yep, bad ground contact. Remove the ground wire on the back of the tack, clean, add di-electric grease and reattach. Should take care of the problem.

 

[pvancorenland]

Re: Bouncing Tachometer

Second that. I had the same symptom. I opened it up to check the insides (more for fun/learning than for fixing) and after putting it back, it's nicely stable. I'm betting it was just the contacts.
start with cleaning them an dif that doesn't help, run a good ground from the battery to the unit (use car jumper cables if you wish, they're long and thick.

 

[the vision]

Re: Bouncing Tachometer

My did the same thing yesterday on the river. I checked my connections when I got home but nothing was loose. Might be a bad wire.

 

[pvancorenland]

Re: Bouncing Tachometer

Don't check for loose wires. Stuff corrodes. Corrosion = insulator. Remove connectors, clean all the connections and reattach.

 

[Eyeman]

Re: Bouncing Tachometer

Thanks folks, I'll check the connections this upcoming weekend.

 

[fossill]

Re: Bouncing Tachometer

Had that problem. Ended up replacing the tach.

 

[earlandamys180]

Re: Bouncing Tachometer

If cleaning doesn't work there is a pulse dial on the back ie 4,6,8 cyl engines give it a turn or two and put back on your setting, fixes mine when it does the same thing.

 

[Bondo]

Re: Bouncing Tachometer

Thanks folks, I'll check the connections this upcoming weekend.

Ayuh,... Don't forget the Grounds...

 

[Eyeman]

Re: Bouncing Tachometer

Clean the contact points and connectors first, then turn the pulse dial a couple time. If that doesn't do it, take a wire directly to ground. Got it, thanks.

 

[mike165]

Re: Bouncing Tachometer

Put the muffs on and fired her up. Ran perfectly, except I noticed the tach would read out as if the RPM was increasing when it wasn't. Then it would bounce around on its way back down to the correct (lower/accurate) reading and be fine. Then every so often it would repeat the same pattern. Sometimes it seemed to be corrected by a left middle finger knuckle wrap. HELP.

Had the same on a new speco tach,put a 1 amp diode in series with the wire from the coil to smooth the signal

 

[pvancorenland]

Re: Bouncing Tachometer

Had the same on a new speco tach,put a 1 amp diode in series with the wire from the coil to smooth the signal

How do you smooth the signal and why would you need 1 amp?
What you're building is an envelope detector http://en.wikipedia.org/wiki/Envelope_detector which will interfere with the tach signal if it has a lot of input capacitance. even if your tach signal is smoother now, I bet the reading is off.
These tachs work as charge pump based pulse counters. Very simply put: The more pulses they see, the more charge they dump in a capacitor, the higher the output voltage, the more the needle goes up.
Cleaning the tach wiring connections (all of them) is the real fix unless there's a problem with your tach or with the wiring running to the tach.

 

[mike165]

Re: Bouncing Tachometer

How do you smooth the signal and why would you need 1 amp?
What you're building is an envelope detector http://en.wikipedia.org/wiki/Envelope_detector which will interfere with the tach signal if it has a lot of input capacitance. even if your tach signal is smoother now, I bet the reading is off.
These tachs work as charge pump based pulse counters. Very simply put: The more pulses they see, the more charge they dump in a capacitor, the higher the output voltage, the more the needle goes up.
Cleaning the tach wiring connections (all of them) is the real fix unless there's a problem with your tach or with the wiring running to the tach.

This is a factory mod from the manufactor check their website

 

[longjohn119]

Re: Bouncing Tachometer

What they are doing here is using inherently slow switching speed of a rectifier type diode to smooth out the pulses. That's why they spec a 1 amp diode which is going to be a rectifier diode rather than using a high speed switching diode (Like used in a envelope detector circuit) like a 1N918 or 1N4148

Also most of the frequency to voltage IC's in these tachs have to have a voltage level on the input that's 1.5V or more lower than the supply voltage so the .6 - .7 volt drop across the diode can help too.

BTW I'm just poking around the net trying to find any info on digital tachs. Basically I need to convert that pulse down to 5 V. to match the input on a microprocessor IC that will count the frequency and convert it to drive four 7 segment LED display's. If someone has already done this I don't want to waste time 'reinventing the wheel'. Personally I don't like analog gauges and I can put a digital one together for less than 15 bucks in parts, actually less because I have most of it on hand already. Plus it will match the dual battery voltmeter I installed last year.

Netbook to Chartplotter Project

 

[Eyeman]

Re: Bouncing Tachometer

Finally fixed it. I had the admiral watch the tach while at idle, I climbed under and wiggled the connector, she told be the read out was bouncing. YES. Obviously a simple connection issue. Unplugged it and plugged it back in several times and all is good. The simple fixes are the best. Thanks all for the guidance.