On Tue, 01 May 2018 00:35:31 +0100, "Dave Plowman (News)"
wrote:

In article ,
Bill wrote:
What I wish I could find again was the online lecture by some American
who had delved into his Jeep's ECU and demonstrated that, for example,
the rev counter was not driven directly from sensors, but via lookup
tables whose inputs combined info from more than one source. If I
remember correctly, he deduced that the rev counter was more of a driver
comfort display rather than direct reality. So you can't trust any of
this stuff.

Can't think of a reason for the rev counter not to be in effect directly
driven.

Directly triggered rev counters from an ignition coil are a remnant of the
1960's and 70's with either a contact breaker or a contactless system but still
using a distributor. Last car produced with contact breaker? 30 years ago?
Last car with a distributor 20 years ago?

The commonly used means of acquiring an engine revs signal requires signal
processing for measuring the passage of the toothed wheel and the position of
the missing tooth on the crank or cam. It then uses this to synchronise the
spark and fuel injection elsewhere in the ECU.

By the mid to late 1980's the crank pulse detector fed the ECU, that then
triggered an (often external) external ignition coil pack and that unit often
fed the moving coil revcounter with a suitable loop current or a PWM signal.

But when everything is in the digital domain it's often easier, cheaper and more
reliable to send everything to the dash over a thin twisted pair to operate a
dozen warning lamps and half a dozen gauges than use a huge bunch of discrete
wires.

Rev counters and indeed most dash gauges including speedos nowadays are either
driven by stepper motor or are a visual widget on an LCD display.

--