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No RPM on the Tach

E

e3fe01

New member
New guy here. I have a 2000 Mercury 200 EFI on a 2000 Champion 193DC. Problem: No RPM on the tachometer. Where should I start troubleshooting? Thanks in advance.
 
Make sure you have power going to the tach (usually fed by the keyswitch - purple wire).

Make sure the "signal" wire is connected (grey wire).

Make sure the Tach has a connected ground wire (black wire).

Make sure the Tach is set to the correct number of poles (if it's an aftermarket/multi-model tach).

If everything looks good under the dash and you confirm it has power and ground then you have to start tracing the tach wire back through the harness to the motor and make sure nothing is broken, chaffed, pinched etc. The easy way would be to simply check for continuity between the engine and the wire at the gauge (you can with an ohm meter if you have leads that will stretch far enough - just looking for no or near no resistance).

If all that checks out you probably have a bad gauge.
 
If it has been working and just stopped check some of the above posted tests....It to falls to zero when key is turned on swapped the grey wire on top rectifier to bottom one...
 
galamb said:
Make sure you have power going to the tach (usually fed by the keyswitch - purple wire).

Make sure the "signal" wire is connected (grey wire).

Make sure the Tach has a connected ground wire (black wire).

Make sure the Tach is set to the correct number of poles (if it's an aftermarket/multi-model tach).

If everything looks good under the dash and you confirm it has power and ground then you have to start tracing the tach wire back through the harness to the motor and make sure nothing is broken, chaffed, pinched etc. The easy way would be to simply check for continuity between the engine and the wire at the gauge (you can with an ohm meter if you have leads that will stretch far enough - just looking for no or near no resistance).

If all that checks out you probably have a bad gauge.
Click to expand...

Thank you. Looks like it could be the gage.
What component(s) are feeding the electrical signal from the engine to the gage? How s the RPM read at the engine? I can't find much info in the overhaul manual or on line.
 
The gauge is measuring how many times per minute that magnets on the flywheel pass the poles (windings) on the stator.

As the magnets on the flywheel pass the stator (bobbins) that feed the rectifier (which converts the AC produced by the stator to DC to charge your battery) you can think of the transition from one bobbin to the other as a "click" (best way I can think to explain it).

The signal wire that feeds the tach is connected to one of the posts on the rectifier. Those "clicks" are transmitted to the gauge. The gauge counts them. So if there is eight (poles) there will be 8 clicks for each full rotation of the flywheel (1 revolution). So for every 8 clicks the gauge (counts) "per minute" the gauge displays it as a RPM. (That is a rather simplistic explanation which while not exact should give you an idea of what is happening).

So your tach is really just a "counter" - counts clicks per minute, divides them by your pole setting on the gauge and displays them via the needle. No special sensors etc on this type of ignition.
 
Excellent explanation, Professor G. Now even a dunce like me understands.

Jeff

PS: I added a voltage regulator to my triple. The tach worked fine with it--for a while. It then started dive bombing above 5,000 rpm. coming back to work okay after throttling back down. Removed the regulator and the tach works fine again. Wish I knew why.
 
Jeff the reason why is .....the tach actually works of the unfiltered AC pulses . A tach is nothing but a AC volt meter with a dividing/multiplying feature.(pole switch) If you measure the grey wire at idle you will see 1-3 VAC and it increases with RPM. The reason the tach "dive bombs" is the regulator is bleeding the excessive AC voltage off that leg of stator to ground so you lose the AC current to drive tach. That's how the Merc did it with their early 16 amp system to prevent over charging. Hook it back up and try the opposite stator leg.. Below is the Merc one that was tied into the 3 wire rectifier...

reg.jpg
 
Way ahead of you, for once. Tried the other stator wire and no difference.

I'm running an electric fuel pump which, to my way of thinking, keeps enough load on the alternator to keep from over charging the battery. Got away with this for years before adding the voltage regulator, so... Also, my battery has refillable cells and rarely needs more electrolyte.


Jeff
 
Then the regulator is extra sensitive and cutting out. Hook it back up and watch you battery voltage on a meter when it happens...
 
Did this ever get resolved? My tach just stopped working one day. I can Turn the key on, and i see the tach needle move a bit, but it doesn't move once the motor is running. I'm just wondering if your situation was/is similar and if u figured it out. Thanx.
 
Removing the voltage regulator cured mine completely.

In your case, I suspect the alternator is either not putting out, or you have an open circuit on the way to the tach. Put a volt meter on the yellows wires coming from the stator (to ground) and see what AC volts you have.

Jeff
 
galamb said:
Make sure you have power going to the tach (usually fed by the keyswitch - purple wire).

Make sure the "signal" wire is connected (grey wire).

Make sure the Tach has a connected ground wire (black wire).

Make sure the Tach is set to the correct number of poles (if it's an aftermarket/multi-model tach).

If everything looks good under the dash and you confirm it has power and ground then you have to start tracing the tach wire back through the harness to the motor and make sure nothing is broken, chaffed, pinched etc. The easy way would be to simply check for continuity between the engine and the wire at the gauge (you can with an ohm meter if you have leads that will stretch far enough - just looking for no or near no resistance).

If all that checks out you probably have a bad gauge.

that happened to me too haven’t fixed it yet
Click to expand...
 
fastjeff said:
Me too!

Jeff
Click to expand...

Hi! New member here. I was searching the interwebs for this same problem I am having with my Ford Lehman powered Groverbuilt. Tach was working fine and then it cut out. Around same time I noticed the volt meter is only reading 10 amps. I could swear it always read near 14 while running. Does this mean it is most likely caused by a bad alternator? Good thing being diesel, it doesn't use as much juice while running compared to a gasser. Any input is greatly appreciated. Thanks in advance!
Kyle
 
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