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.

1 Answer 1


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.

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    Hmm, how do they get the RPM from the battery voltage variations? The RPM of the alternator shouldn't necessarily match the engine RPM if the pulleys are sized differently, should it? Commented Apr 8, 2016 at 9:37
  • that's one of the figures as well, I guess
    – Rory Alsop
    Commented Apr 8, 2016 at 10:16
  • @IhavenoideawhatI'mdoing It could be that they have it preprogrammed in their emissions computer just like they have camber / caster etc for doing alignments. Commented Apr 8, 2016 at 10:32
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    If you measure the voltage when the engine is running, you can see a ripple (it's not constant at 14V) and this ripple corresponds to the current draw from the battery to fire the spark plugs, ie one dip per plug firing.
    – Rory Alsop
    Commented Apr 8, 2016 at 10:54
  • 1
    This is one explanation by monitoring spikes from the ignition system. No doubt similar pulses will be produced by the injectors - hackaday.com/2014/06/10/…
    – HandyHowie
    Commented Apr 8, 2016 at 11:04

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