Diesel Rev Couters. How....

from the alternator.....

The car alternator is effectively a pulse generator which varies with engine speed. Take a tap off of that and use a pulse counting instrument to give engine speed. The same principle, really, as electronic speedos.

john ( snipped-for-privacy@yahoo.com) gurgled happily, sounding much like they were saying :

Used to be. Or, sometimes, mecahnical.

Now they'll be from the injection ECU. Diesel injection is FAR more precise than petrol injection.

Or better still, take a look at the flywheel where you are likely to see the entire circumference will be covered in little 'lumps', with one missing.

On the bellhousing will be a little bolt-on box with one wire exiting, disappearing into the morass of your car's wiring loom.

When the car's engine turns, the electrical pulses generated by the passing 'bumps' on the rotating flywheel tell the bolted-on electromagnetic sensor that there is activity, when the one that is missing passes, there will be a change in activity. This will result in one missed electromagnetic pulse, every revolution of the engine.

The tachometer in the car's dashboard will interpret the frequency of missed 'pulses' and turn that into a reading on the instrument. Good stuff eh? And no breakable moving parts, except for the instrument on the dashboard.

Hey presto! How many revs per minute?

Artie (I'll be here all week!)

It's how Peugot and Citroen basic diesel engines do it , flywheel sensor, BX, Xantia 306, etc etc, I assume HDI has a signal in the ecu, though probably derives a signal from the flywheel sensor as well.

Des

Only the ECU can see that signal. Most modern cars use a signal from the engine ECU, to the dashboard ECU, including petrols and diesels.

To use a crank sensor just for a rev counter is an awfully expensive way to do it, especially when there's already a perfectly good signal available at the alternator.

Probably just from a crank angle sensor used by the ECU.

I had a Marina like that.

They didn't use electronic systems like that on Marina's

Marinas did go 'Whoosh' though.

Ours didn't. But we did have a Mk II Escort that did. That meant you had to pull over gently (not that it gave you much option of braking abruptly) and perch the air filter back on top of the carburettor before the fan got it.

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember "M Cuthill" saying something like:

True, but 25 years ago I built a closely-governed diesel genset using just such a specially made unit. It was for outside filming for a TV production company and they were getting pissed off at the poor speed regulation of standard sets affecting the lighting.

This unit was able to keep the output frequency within 0.1Hz, iirc. Cost and arm and a leg, though.

Whoosh! Starter motor ring?

Forgot about them. But then again, french designers never do anything the simple way.