frequency-voltage don't work with electronic ignition.

[Screech]

I built an electronic tacho from a frquency to voltage converter (LM2917),to voltage to bargraph driver(LM3914).

It doesn't work on the car. I think it's because the car has an electronic ignition.

I used the pic provided from the datasheet.
Notice the pic uses a points igniton system.
I wonder if this is the cause of my problem.

To test that the chip works, I place a 555 timer running at 200Hz to simulate 3000 Rpm. The chips worked fine.

The car is ford using je electronic fuel management.
It has standard electronic distributor,with hall sensor on it and a cylidrical shaped coil(old type).

Why wont my tacho work?
thanks guys

 

[Sebi]

If possible, connect the 10k input resistor directly to hall sensor output.

 

[Screech]

Sebi,that just might work, but I don't really want to go there. I might damage something.(unfamiliar territory).
I'll Just keep it in mind for now.
Thanks for your idea.


I also wonder if there is a way that will suit all ignition types?

Still don't know if the above circuit is only suited to breaker points igniton.

 

[Nigel Goodwin]

I also wonder if there is a way that will suit all ignition types?

A common way is to wrap a wire around the HT lead going in to the distributor, amplify the signal picked up, and use that.

 

[Screech]

Nigel, I've used that idea too(wire around a HT Lead).
I've used that idea in the past, and it's not very reliable.

More ideas people?

 

[Screech]

Nigel, I used 2N222A's and TIP31's as you suggested.
I get a lot of misses(maybe a weak coil).
it also picks up interference from the other leads.
but on other cars it seems to works fine.

are these transistors the type to use?
thanks

 

[Nigel Goodwin]

Nigel, I've used that idea too(wire around a HT Lead).
I've used that idea in the past, and it's not very reliable.

More ideas people?

You probably need to experiment, to get a reliable signal - an oscilloscope would be very useful. From what I can remember, professional car analysers usually use a spring clip which goes over the HT lead (rather like a clothes peg).

 

[Johnson777717]

That's correct nigel. Some have in line springs too.