How to measure rpm with multimeter

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richtbiscuits
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Joined: 28th February 2015 - 11:25pm

How to measure rpm with multimeter

Post by richtbiscuits » 5th February 2018 - 6:48pm

Hello all.
On Friday I set the valve gaps, points gap and the ignition timing on my 412 LE and wanted to measure the idle as I think the engine sounds a little fast. Even but a little fast. I put my meter onto rpm(x10) for 4 cylinders and at idle it read 336, fluctuating +/- 5 but the engine is certainly not running at 3360 rpm. I don't know if I'm missing something here so I wondered if anyone can give me an idiot's guide to measuring rpm with a multimeter. I wanted to measure because I suspect my throttle control switch isn't working properly as I can't set it according to the methods in the various manuals (currently it's set so the accerator cable just closes the butterfly at idle and if I close it any more to reduce the rpm the engine hunts). The idle speed screw is turned right the way down to give as slow an idle as possible. I've read about measuring using frequency on the multimeter but I don't have that function. Any advice would be warmly welcomed. Thank you.
'72 412 LE Variant
'73 412 LE Variant
'83 1200 Beetle

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purplepeter
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Location: Bath, Avon

Re: How to measure rpm with multimeter

Post by purplepeter » 5th February 2018 - 7:05pm

Have you got it set for 4 cylinder engines?
I'd refer to the manufacturers guide to be honest, in case theres something you've missed

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monkeywrench
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Re: How to measure rpm with multimeter

Post by monkeywrench » 21st February 2018 - 1:54am

reset idle to spec and adjust the mixture. To smooth out the idle

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937carrera
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Joined: 7th June 2013 - 11:54pm
Location: North Yorkshire

Re: How to measure rpm with multimeter

Post by 937carrera » 21st February 2018 - 2:10am

Do you think you are getting more than one spark affecting the inductive pickup?

336 x 10 = 3360, but then divide by 4 and you get 840 rpm

Just an idea, 840 rpm would seem a bit slow rather than fast to me
David
1974 412LS Variant
1973 412LE 4 door Fastback / Saloon

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richtbiscuits
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Joined: 28th February 2015 - 11:25pm

Re: How to measure rpm with multimeter

Post by richtbiscuits » 21st February 2018 - 8:02pm

Having read around a bit more I think measuring with the engine running from cold was not a good idea as it would be running a faster idle. I'll measure it again when it's properly warm. 3360 rpm is not right but it does seem fast, for example, when driving from cold and I depress the clutch I can hear the idle increase. I'm wondering if my auxiliary air regulator is sticking a bit and perhaps the throttle control switch isn't quite right. I have a spare throttle switch which I've set with the multimeter and I'll give the air regulator a wash through with WD40 and see if that helps. The idle does slow as it warms up and the car drives fairly well so I'm not panicking - I'm just a bit obsessed with getting the engine smack on!
'72 412 LE Variant
'73 412 LE Variant
'83 1200 Beetle

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937carrera
Posts: 1190
Joined: 7th June 2013 - 11:54pm
Location: North Yorkshire

Re: How to measure rpm with multimeter

Post by 937carrera » 21st February 2018 - 8:30pm

You can measure at whatever speed you like, but I wouldn't make any adjustments until the car is properly warm. Once warm idle is smack on, them just check that cold idle is somewhere near. 1000 rpm would seek quite OK to me.

By the way, I have recently taken delivery of a tachometer for fitting to a T25. Chinese ebay special for around £13.
David
1974 412LS Variant
1973 412LE 4 door Fastback / Saloon

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