tachometer needle stuck at 400 rpm

[ckiyooka]

70 hp. Suzuki, 2006 Mac. Even when I turn the ignition off and on the tach needle stubbornly stays put at 400 rpm, doesn't even quiver. I first noticed it immediately after launching the Mac last year for the summer. I assumed that a wire got loose as I was driving the rig down our somewhat rough roads on the way to the ramp. Since all of the other gauges worked incl. tach light I just boated all summer long with the stuck tach and had no other problems. However, I thought I'd see if it could be fixed before launching this year (I'm up in the B.C. Gulf Islands). I'm not particularly handy with the electrical components of the engine but wonder if there might be something I could try to diagnose the problem. So far this year I've opened the access panel on the pedestal to check if there are any obvious loose connections - couldn't see any. Also attached the engine mufflers, started up the engine and everything works just fine except for the stuck tach. Battery has lots of charge - have had it on a BatteryMinder desulfator all winter long. Any suggestions?

[March]

Try switching the key on, one click, without engaging the engine. See what happens. Then turn it off and remove the key.

[Shane]

Cliff,

Glad to here your getting her splashed soon. We'll watch for Pura Vida as we cut through Georgeson in a couple of weeks -a LPS rendezvous at Port Browning. Don't forget that Todd is just a phone call away; toll free even I've noticed the tach in ours sometimes sits up a couple ticks, but not permenantly, so I haven't looked to hard for a problem. Good luck.

Regards,
Shane

[Hardcrab]

It sounds like a mechanical return spring problem in the gage to me.
(The tach is just a voltage meter being sent variable voltages to be read out as rpm's).

To test this fully to see if any "stray" voltage is holding it at 400, disconnect any and all batteries.

That means no "stray" electricity is possible in the circuitry, therefore a 400 reading can not be possible, would be the logical conclusion.

If it drops to zero, more troubleshooting is in order.

If it's still stuck at 400 it is a mechanical problem; either replace the gage, or live with it.
(Since the engine will not idle at 400 rpm's anyhow, no real information is actually lost, IMHO).

If you are really good at such things, there might be a "zero" adjustment behind a cover on the back of the gage, don't know for sure.
( If you go this route, there are a few generic points to consider beyond the scope of this responce).

Does it seem to give normal readings above 400?
If it does, and with your experience of last year, I'd just sail on down the road and ignore the no-big-deal cosmetics.

[opie]

A tachometer is not just a voltage meter. Tachs have inside them solid-state (transitorized or IC) based circuitry that discrimates (or captures) voltage cycles and produces a signal internally that moves the needle up and down. It connects to the stator coil on most models or, on newer models, connects to the ECM or CDI unit that adds an additional layer of discrimination from the signal gained from the stator coil. It can work as well by connecting to the pulser coil or by wrapping loops of wire around the spark plug wire and sending that signal to the tach. (caution - don't try these methods this unless you are sure of what you are doing or don't care if you blow something up, ( like me.)) The fact that the voltage regulator also connects to the stator coil adds the regulator into things to check out for problems.

So, what is the problem? The above suggestions from other members are right on the money. Does the tach go to 400 with no voltage? Tach bad. Does the tach go to 400 with just the ignition on and without the engine running? Tach bad. But if the tach goes from zero to 400 and stops, but the engine sounds fine as it winds up in speed, then it could be the voltage regulator as in my case, or could be the tach itself, or it could be the CDI or ECM. Maybe try a $20 tach from Harbor Freight or a similar tach from Autozone and see what happens. Hook it up parallel to your current tach.
Electronics can be frustrating.

[Hardcrab]

Not trying to pick nits, but, as you state in your description, the results of the "stuff inside" is nothing more than a variable dc voltage; hence my statement of a RPM gage being no more than a voltmeter.

It's called a D'arsonval meter movement.
Google it to see how they work and see how the return spring works to mechanically return the needle to zero.

Almost all needle-type meters use this as the model.
Dare I say all "needle -type"meter movements?

They are dc voltage driven devices.
The more dc voltage applied equals more needle deflection.

Ammeters,voltmeters, ohmmeters,tachometers are exactly the same movements and scaled on the faces for the units measured.

Yes, the circuitry/signal conditioning from the thing being measured is case specific, but the output from that specific circuitry/signal conditioning will be a variable dc voltage applied across a D'arsonval meter movement (voltmeter).

After 30+ years in the electronics field, it tends to be not quite as frustrating.

[Don T]

Hello,
I believe the meter is an opposing magnetic field model and there is no return spring. There are two fields, one for pulling the needle to zero and one to pull it to full scale (think GM automotive analog dash). When the motor is idling and you turn it off, the meter looses both fields and does not move to zero. These type of gauges are not affected by variances in voltage and temperature because they affect both fields uniformly. As suggested, try turning on the key without cranking or starting the engine and the needle should fall to zero. If not a diagnosis is needed to resolve a mechanical problem like a rusted or corroded internal movement.

Good luck
Don

[Hardcrab]

DonT,
Good point about balanced field movements. I'd forgotten about those type.

GM fuel gage for sure, but perhaps not the tach or oil pressure?

The tach must stay indicating idle rpm as you exit the car, as you should never indicate zero rpm's when you kill electrical power while at idle rpm's.
Key off is key off.
Power is gone.

Only gets driven to zero on restart.

Is that how they do act?

I'll still vote for D'arsonval spring return to zero for a tach.

[ckiyooka]

Thank you all for your responses. I tried turning the ignition on and off a couple of times to just the first click. Buzzer sounded and the four gauge lights lit up each time. All seemed normal except that the tach needle remained stuck at 400. Never does move from this spot. Then disconnected all wires from the 2 batteries. Tach needle still at 400. Unless there are some other ideas at this point I'll try another season without the tach. I live on Mayne Island half way between Vancouver and Victoria. Enjoyed lunch dockside today watching the boat traffic in Active Pass. Time to launch Pura Vida...

[Don T]

Hello,
If the tach is always at 400, even with motor running, then it has lost electrical power. Only two types I have seen, one has 12v power and a signal from the engine control or ignition module OR a charge pump type that counts the generator pulses (AC before rectifier). If the former then it has lost 12v power or ground, if the latter the wire from the generator output or ground is open. There is a switch (usually) accessed from the rear of the tach that sets how many pulses to count per rev (2,3 or 4). Try rotating it between the settings to clean the contacts then return to proper setting and it may begin to work. You will have to ascertain which type of tach it has via inspection or circuit diagram.
Barring that the movement may just be stuck mechanically and will have to be disassembled.

more luck,
Don

PS: I'd be happy to help but it's a bit of a drive.
However I will be sailing up in the Gulf islands the last week of June and we always spend some time at Montague Harbor. That's pretty close.

[Catigale]

sounds like loose wire or ground. Check at both ends - both at tach and at engine. Chances of bad wire over bad tach are 99:1 in Vegas right now...

[opie]

Hardcrab,
A vigorous discussion is a boon for the brain. Now, nitpicking? How about taking your tach and separating the gray wire (or color of yours that is the rpm signal) and now flicking that wire across a voltage source (12v, 9v, 15 v) as fast as you can. If you can do 2 per second the tach will read 120 rpm and if you can flick it 7 times per second the tach will read 420 rpm. Now explain how your varying voltage idea works with the above fact...... thanks...

[DaveB]

Opie,
On My BF50 I just changed my Rectifyer/regulator and started engine and went to 1900 rpms and would not go lower at Idle and it jumped around other RPm's throttleing higher. I thought I had 2 gray wires crossed but switched them and same thing. The Honda has 3 gray,one white and one white green strip, the only two that could get mixed is the two top gray with male fittings.
RPM's worked fine until I replaced the Rectifyer (Old one put out 17.5 volts).
Have you run into this?
Dave