How I set idle RPM without a tach

[shotty]

As you may know, the alternators on our trucks (AC Delco 27SI units) have an RPM output signal, which on GEN 1 is hooked up to a buffer circuit and then output to the STE/ICE diagnostic connector under the dash. So far I've been unable to get a useable signal from pin c on the diagnostic connector even with two different tach buffer boards. I'm going to try a few more boards before I hook a scope up to it to see what's going on with that signal. In the meantime...

The R terminal on the alternators is the AC output of a single phase from the alternator windings. Using the formula provide from this AC Delco document ( RPM * Ratio / Generator Factor = R terminal frequency) we can work out the RPM of the engine from the frequency on the R terminal. But first, what is the Generator Factor? There's some fancy math that goes along with the number of poles in the alternator and the rotating field, blah blah blah. For our trucks, the Generator Factor is 7.5. That's all you need to know, lets just leave it at that.

Alright, but what's the pulley ratio?

I tried searching high and low for the pulley ratio information, to no avail. So I did the next best thing: I hit the road with a GPS and my frequency counter. At 53 MPH (55 indicated on the speedometer) with my 32.8" tires, 4.56 gears, and a 1.00 transmission ratio my engine was spinning at 2476 RPM. I logged 1084 Hz on my meter. Rearranging the formula and solving for Ratio gave me 3.287. At 55 MPH on the GPS, i got 3.270 for a ratio. If it wasn't the middle of the night I could take some readings at different speeds, but I'm almost positive that the ratio is about 3.275, so that's what I'm going with.

Now that that's out of the way... Everything but the M1010 should idle at 625 RPM when warm, and 800 RPM when cold (or when the cold advance and fast idle solenoid are engaged.) This works out to 274 Hz on the R terminal when warm and 349 Hz when cold.

Just hook up your meter to the R terminal, which is the single post terminal where the connector just presses on and the rubber boot holds it in place. On GEN 1 there's a brown wire going to it. On GEN 2 it's not connected, so it will just be a bare post. Then you just adjust your idle speed until you get the right reading on your frequency meter. Easy peasy!

RPM = R hz * 7.5 / 3.275
R hz = RPM * 3.275 / 7.25
Pulley Ratio = R Hz * 7.5 / RPM

 

[KansasBobcat]

I am amazed at the knowledge and expertise of the members here. Thank you!

 

[tim292stro]

For a bit more accurate output:

Measuring the crank pulley outer diameter, and dividing it by the alternator pulley outer diameter will give you your pulley ratio (use a metric tape/ruler, easier to do the math ).

The tack output of an alternator is pulled form one of the stator phases of the alternator (a coil that does not move it "stays" so it's a STA-tor). The amount of pulses you will get on the output of that stator coil depends on how many rotor poles pass it (the rotor is the part that "rotates" os it's a ROT-or). This is a rotor removed from an alternator:


Those triangles are "poles" of the rotating electromagnet powered by the field coil (the rotor winding). The triangles form one side of the shaft will be North magnetic polarity, the triangles form the other side of the shaft will be South magnetic polarity. Count one sides' worth of poles and you have how many poles will pass the stator coil in a single revolution - a full AC cycle is both an positive and negative swing. Common for Delco is 8 North poles - in fact the rotor pictured above is a 27SI replacement part...

Keep in mind that under loads and out of adjustment, belts will slip adding error. Make sure you do your RPM measurement at the end of a day-time drive (batteries nice and charged) with a tight belt and nothing turned on in the truck. Also using GPS to engine calculations does not account for transmission slip, using a calibrated tach to the GPS speed is a good way to detect transmission issues.

(  Measured Frequency  )  /(    Crank Pulley OD     )
(  ------------------  ) / (  --------------------  ) = Engine RPM
(   Rotor Pole Count   )/  (  Alternator Pulley OD  )

Also, if one has spent or is thinking about spending the money for a meter with frequency measurement, there are cheap RC-car photo tachometers that can be had - this one is commonly found for $20 (quick eplace check showed them for <$15 - just bought one ):
https://www.youtube.com/watch?v=rLBCzZuPscs

 

[dmetalmiki]

I am amazed at the knowledge and expertise of the members here. Thank you!

I definitely second that. some (only) just short of genius in fact.

 

[Winston Wolf]

http://t.harborfreight.com/digital-photo-sensor-tachometer-66632.html

or you could just use a cheap tach.

 

[shotty]

http://t.harborfreight.com/digital-photo-sensor-tachometer-66632.html

or you could just use a cheap tach.

Now where's the fun in that!?

Also, you could get two and a half of these for the price of that: http://www.amazon.com/AGPtek®-Professional-Digital-Tachometer-Contact/dp/B004Q8L894

 

[Chaski]

Strobeoscope is another option.
Most decent modern timing lights have a strobeoscope feature. I have a 15 year old Ferret timing light that can be ramped up and down to measure the speed of speed of anything that rotates at least 300rpm.

 

[85CUCVtom]

Also, if one has spent or is thinking about spending the money for a meter with frequency measurement, there are cheap RC-car photo tachometers that can be had - this one is commonly found for $20 (quick eplace check showed them for <$15 - just bought one ):
https://www.youtube.com/watch?v=rLBCzZuPscs

Sorry for bringing up an old thread but I have something to add/ask

I want to try that laser tool because I checked all three of my multi meters, two Fluke brand and one Fieldpiece HVAC meter and none of them do frequency. I guess the big question I have is where on the motor do you measure from, the harmonic balancer? Alternator? Water Pump? Thanks fellas.

 

[tim292stro]

The crankshaft/harmonic balancer - any other point may introduce ratios and possible belt slip.

 

[85CUCVtom]

The crankshaft/harmonic balancer - any other point may introduce ratios and possible belt slip.

Perfect. Just to be clear, I should be able to stick the reflective tape on the balancer itself and get a accurate RPM reading?

 

[tim292stro]

Correct. Anything which spins at the same speed as the crank shaft, and is accessible, will do.

The operation behind the optical tach is very simple: a laser hits the reflector on the rotating assembly, and bounces back to a photodetector. Each time the photodetector sees a reflected laser pulse a counter is incremented. The tach has a one second timer inside, and every time the one second timer expires it multiplies the counter by 60 (to convert revolutions per second to revolutions per minute). Then it displays the result, clears the counter, and starts the one second timer again (and resumes counting).