Measuring idle rpm with multimeter and IG terminal?

**abently** · 11-09-2010, 06:02 PM

Hi guys,

After some reading here and there, I assumed this was going to be a straight forward process...

Put the multimeter leads on IG (In the diagnostic box) and Gnd, and flick it onto Hz, then multiply the answer by I would assume 60 (and divide by 4 for 4 cyl) to get RPM.

Now that would be all well and good, if I got a steady answer on my multimeter, but I don't... the answer I get doesn't even look to be right, as it seems the same when I remove the gnd connection.

Out of curiosity I switched it to duty cycle, and I potentially seem to be getting a number that looks usable, but I have my doubts since duty cycle is suppose to be a % representation of the on/off time over some period, rather than how many spark signals have been sent over that period..

Has anyone used a basic multimeter to check idle RPM and if so how?

**oldcorollas** · 11-09-2010, 07:46 PM

which system? is that IG the reluctor pickup? or igniter input or?
multi may not be good at reading square wave signals as frequency, as it is probably looking for alternating signal..

**MWP** · 11-09-2010, 08:02 PM

... and you divide by 2, not 4.

**woggin** · 12-09-2010, 03:46 AM

you could look @ the harmonic balancer and see how many times it goes round

how about putting the leads on the coil pack connector (ifs its got coil packs, since you havent stated wat car/motor you have) and seeing the frequency there then multiplying it by 4 to give your end rpm??

you could get a tacho, plug it into your ig terminal, ground the blackie and it should give you your rpms!

**abently** · 12-09-2010, 01:56 PM

Originally Posted by oldcorollas

which system? is that IG the reluctor pickup? or igniter input or?
multi may not be good at reading square wave signals as frequency, as it is probably looking for alternating signal..

IG is the igniter input.

Coil is located in the dizzy, remind me to thank (not) the designer of that system... lol

I think I may just end up getting a tacho, or a multimeter that can read rpm.

We'll see.

Thanks for the replies.

**thechuckster** · 12-09-2010, 05:06 PM

probbaly not a good idea putting a low-impeidance device on the IG signal.

doesn't the tacho out signal from the igniter appear at the diag port?

**oldcorollas** · 12-09-2010, 06:10 PM

both would be square wave tho?
can multimeter read "digital" type frequency?

**abently** · 12-09-2010, 07:03 PM

Originally Posted by thechuckster

doesn't the tacho out signal from the igniter appear at the diag port?

Correct, labeled as IG.

OC, not this multimeter, just looked through the manual, it states RMS so I assume AC only.

Part No: QM1535 for those who want to know what Multimeter NOT to buy.

**thechuckster** · 12-09-2010, 08:04 PM

ah ... my 3SGTE engine wiring diagram is unreadable (or my eyes are fucked) so I couldn't work out which igniter signal appears at the diag port.

in that case... it should be a fairly hefty AC signal with freq=rpm.