Hello everyone - I want to measure the base RPM’s at idle on my 91 Volvo 240. I am fairly sure it is running above 770 RPM’s at idle, so I what to test it so I can adjust the idle to around 770 RPM’s. I bought a handheld tachometer, but was wondering where is the best place on the engine to apply the reflective tape? Crankshaft pulley would be my first thought, but any tips? Thxs Justin

 

i connect my digital volt meter in Hz (frequency) mode to pin 1 of the ignition coil. on a 4 cyl motor, there's two sparks per turn, and 60 Hz per RPM, so multiply the frequency reading by 30 to get RPM... 25 Hz == 750 RPM, the nominal idle speed. 20 Hz == 600 RPM, the approximate no load idle for a fully warmed up engine with the IAC disconnected.

don't have much to suggest about an optical reflector, never messed with that.

 

Thxs Pierce for the reply... I went out and bought a automotive multimeter with a Hz setting, plus it came with a clamp. I believe you were talking about attaching the positive test lead to the coil, but I assume with the clamp that came with the multimeter I bought I can just attach that to the coil wire to the distributor cap correct? If not, can you tell how exactly do you hook up your test leads in the way you were describing? Thxs

 

the coil has two connections on opposite sides, one is pin 15, that gets power from the ignition switch, and the other is pin 1, that gets the timing pulses from the ignition control unit, thats what I was suggesting hooking the meter to (with the other meter lead to ground).

that clamp could be different things, it could be a 'clamp on amp meter' adapter, or it could be for clamping on an ignition high voltage wire. and, a automotive meter might well have an RPM and Dwell function, where you have to tell it how many cylinders your motor is, and clamp on the #1 spark plug wire to get RPM readings... Dwell is only of interest when you have breaker point old school ignition.

 

Thxs! I pretty much figured what you meant the first time, but love the extra clarification. Thxs again.

 

Pierce - an update to testing rpm’s... I hooked up the multimeter to pin 1 and I am only getting a reading of .08 hz and .11 hz with the idle control valve disconnected. The Hz on my meter is HZ 20k by the way. So that would mean I would be running at 330 rpms if I take 11 times 30. I must be doing something wrong as far as taking the reading. I am about to test the idle control valve now to make sure it’s clean and functioning correctly in the meantime. Thanks

 

0.11Hz would be one cycle every 9 seconds, I heavily doubt that, so for some reason the meter isn't reading the signal correctly.