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Them analog tachometers

This question has to do with the operation of those things -- maybe a Speedy Jim question.

The Wonderbus has a VDO analog tach on the dash that I find pretty useful. But now and again it decides to malfunction. In various ways. Sometimes it just sits at 2,000 rpm regardless of engine rpms, and at other times it drops to zero without warning. I look out the window and I'm still going and the engine is still running, so I know it is an electrical problem.

The tach has good ground -- this I know because the light bulb in it turns on.

So there are two other wires: the one from the coil, and the other (non lamp) +12.

If the wire from the coil is disconnected I bet the reading goes to zero.

But what about the if the +12 is disconnected? what do these kinds of tachs usually do then?

Reply to
Mike Rocket J. Squirrel Elliot
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You are using a Pertronix, are you not? If not, as my fading memory of things like distributors flickers dimly, it seems to me there was something about a failing capacitor causing similar problems. -BaH

Reply to
Busahaulic

I did once have a bad condenser that caused engine problems and associated tach issues, but I am now running pointless and the tach has just now started acting up with no associated engine problem. It's a tach issue and I'm wondering if anyone know what happens when you yank the +12 wire from a tach. Does it drop to zero, or maybe sit at some midrange position?

Reply to
Mike Rocket J. Squirrel Elliot

"Mike Rocket J. Squirrel Elliott" >

my Sun "supertach" drops dead when you disconnect the power supply...

Reply to
Joey Tribiani

Interesting . . . anyone know the theory of operation of analog tachometers?

Reply to
Mike Rocket J. Squirrel Elliot

....sure do. You hook up all the wires..and "theoretically" the tach works...

...glad I could beof assistance. =-)

...Gareth

Reply to
Gary Tateosian

I'll remember to call on you the next time I need some real brainpower.

Reply to
Mike Rocket J. Squirrel Elliot

If you have a timing gun with a tachometer on it, you could disconect the tachometer on the dashboard and hook up the timing gun. If it reports rpm correctly, then the problem is in the tachometer, otherwise check the wiring.

If you have a soldering iron and have some experience on soldering PCBs, then you might want to dismantle the tachometer and look for connections that might have come loose from vibrations and need re-soldering, or for some electrolytic capacitor (the cylindrical ones with polarity) that might have leaked and replace it with a new one. Look also for cracks on the PCB or any other obvious problems. Other than that, there isn't much you can do to fix it.

Bill, '67 Bug.

Reply to
Bill Spiliotopoulos

Hi Bill,

The engine is fine -- I guarantee that it has not been running at a steady 2,000 rpm with no variation for the past two days. I been driving the bus and I would have noticed, I think. I agree with you that the problem might be in the tach, but my posted question was just to find out what something like a working round face VDO tachs might do when the

+12v wire comes undone. Do they drop to zero, climb high, or (maybe) sit at some inbetween point . . . like 2,000 rpm?

Yah I been too lazy to get under the dash to look, and it sometimes fixes itself. I wuz just wondering if anyone knew.

Reply to
Mike Rocket J. Squirrel Elliot

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