Kato RPM Guage Reads Oddly at Idle

Have you cleaned the potentiometer on the tach?

I fixed mine that way, but it was showing 1/2 the correct RPM not double, but the potentiometer is used to properly calibrate the tach at the factory so it may be a possibility.

See post #7 on in this thread: (lots of good tach info there too)
http://www.ferrarichat.com/forum/mondial/262520-88-mondial-tachometer.html
Alden

 

I'd wager the probability is that you'll have to do something with the tach itself as Alden indicated (although your symptom seems more "noise/crosstalk"-related as you aren't having trouble all the time), but a couple of easy things to try before that are:

1. make sure the grounding at the DigiPlexes is in good shape, and

2. swap the DigiPlex connectors (only one of them creates the tach signal IIRC, and this also gives a "wipe" to all the connections).

Just a thought...

 

I had some ghost tach behavior myself - I ended up taking it to Palo Alto Speedometer, they fixed it for about $170.

Here are instructions for the (t) not sure if it applies to the others but I would hope they are similar

Steps to remove tachometer

1) Using a flathead screwdriver, remove black plastic panel face (snaps off)
2) There will be four screws holding the tach into the windshield, remove those. Be careful not to remove the 4 screws that connect the silver face plate holding the tach into the casing: you want to remove the 4 screws on top of the white plastic.
3) The tach will come right out after you remove the screws.
4) VERY IMPORTANT: once the tach is out label each of the wires and terminals with some colored tape or other marker. All the connections/connectors like similar so you’ll want to make sure you know how to connect back.
5) There are 5 bulbs, 3 terminals, and two clip connectors.
6) use a tiny flathead screwdriver to carefully pry the bulbs out of their sockets
7). Use a needle nose pliers to remove nuts/washer from the 3 terminals.
8) the two clip connectors come right off
9) the lighting stalk knob comes right off.

I see you can also buy some off ebay for around $100 so that might be more economical...

If nothing is wrong with the tach, they should be also able to tell you - not sure how much a diagnostic would cost -- but i would venture to say probably not too much!

Cheers
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The rev counter is in fact a frequency counter. If it shows at certain revs a double reading than the rev counter could be ok. I would suspect in the first place the signal to the rev counter is distorted for example due to a bad ground or a bad connection. In that case a single period of a block wave can trigger the input of the rev counter twice instead of once, leading to a double reading.

 

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Brock