tachometer question

Newbie Joined: 17 Nov 2004 Posts: 13

Hello

I was browsing the net and I found some specs for tachometers.

I thought the tachometer reads a voltage from the computer, but it seems it takes the signal from the negative terminal of the coil.

Does the dash board tachometer receive a train of pulses from the ECU (as if it would come from the - terminal of the coil, and is converting the "train of pulses") or is a simple moving coil that is voltage controlled?

Forum Moderator Joined: 02 Mar 2004 Posts: 19978

its pulsed from the ignition module.

Which is from the negative side of the coils L.T. circuit. Its the negative side thats switched, the positive side is a constant supply.

Newbie Joined: 17 Nov 2004 Posts: 13

I thought the dashboard tachometer is rather "silly". But it seems it is quite complicated. I've seen some IC for freq-to-voltage conversion, but they were quite expensive, so now I understand why a tachometer is regarded as a "optional feature" on many models from different brands, still.........

Forum Moderator Joined: 02 Mar 2004 Posts: 19978

Its not so much complicated as expensive to add.

The electric ones are ten times better than the old mechanical ones. the Jones Chronometric is a good example, it was driven off a handy camshaft/etc and was slow to react so you knew what revs you were doing 20 seconds ago!

Site Subscriber Joined: 02 Oct 2004 Posts: 1575

the feed from the coil to the tacho in my experiance is white lead. one plug has 3 leads the other 1 the one with the 1 lead (brown) goes to the engine management, the other plug has yellow, white and black leads, yellow +ve from the ignition, white to tacho and black -ve. hope this is of use.

Site Subscriber Joined: 10 Jun 2004 Posts: 2317

Does the diesel tach work in the same way? As it takes a feed off the back of the alternator

Forum Moderator Joined: 02 Mar 2004 Posts: 19978

yes leccy pulses but from the alternator instead. Quite funny if the pulleis the wrongs size, the tacho goes off the end etc.