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post #6 of (permalink) Old 18th October 2006
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We had a similar problem on our last Clio, though we didn't get around to sorting it before trading it in on this one. In our case, the car would run fine at certain engine speeds, but once you hit a particular rev band, the ECU light would come on, the rev counter would drop right down the scale, and engine power would be lost temporarily. I mentioned it on here and Lagdti reckoned it was probably TDC . He knows his onions.

In a nutshell, the TDC tells the ECU when the engine reaches 'top dead centre' (on no.1 cylinder, I think.) From this, the ECU can work out when to fire each of the plugs. By measuring the frequency of the pulses, the ECU can calculate the engine speed, and it's that speed that's sent to the rev counter.

If the TDC is faulty, the ECU thinks that the engine has stopped when it hasn't. The ECU will, however, relay this reported speed to the rev counter, which is why it drops down the scale.

Back in the days of the not so good old distributor, rev counters worked by counting pulses on the low tension side of the (single pack) coil . Now we have wasted spark ignition and multiple coils , it's easier to generate the rev counter reading from the TDC via the ECU .
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