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When doing emissions testing, they connect to the battery, either directly, or using the cigarette lighter socket.

I'm assuming that this is to measure the RPMs, as the only other thing connected to the car is a gas analyzer in the tail pipe.

I'm wondering exactly the method by which this gives an RPM reading, and if problems with the charging system, like for example low voltage, can change the results of the emissions test and how? This question came up because I noticed that the RPM reading they were getting was 2700 while on my tach it says 3000, and my voltage is weak at 13.75v.

Robert S. Barnes
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1 Answers1

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The voltage being a little low (13.75 is not low enough to be considered a problem in any case) is not going to affect their RPM reading - as it's the frequency of voltage variations that is measured to get your RPMs.

I'd be tempted to say your tach is out slightly. You could ask them about that.

There is a smartphone app that you can use to gauge RPMs - it does it by sound (you have to input number of cylinders and some other info and it works out RPMs) - I'm trying to find a link to it.

Rory Alsop
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