engine rpm

[woodmills1]

I have a multi meter that can do RPM  can I use this on the lt70 and edger to find rpm?

[John Bartley]

As long as you know how it's calibrated it should work. The earlier tach/dwell meters had settings for 4-6-8 cylinders and were based on the cam turning the distributor once for every two turns of the crank. How to rethink the calibration will depend on what motor you have and how your particular multimeter is configured

cheers

John

ps : we're talking gas motors with spark here eh?  ???

[DanG]

Actually John, I think he's talking diesel on the LT-70, but not sure on the edger.  But since you bring up the dwell/tach, I have a question.  I just got my newly rebuilt engine on my mill, and I'm having issues with getting the rpm right.  I've borrowed an old dwell/tach to set it up.  I'm wondering just what the number of cylinders would have to do with it.  Whether it is a single cylinder or a V-16, each cylinder fires once in every two revs on a 4-cycle, and once in every rev for a 2-cycle.  Since the meter is triggered by the spark on one cylinder, why would it matter how many cylinders there are?

[bandmiller2]

Dan,I'am not John but the tach/dwell works on spark pulses to give you RPM,the more cylinders the more pulses per revolution.To anser James origional question I don't think the multimeter will work on a small diesel you need an electrical impulse.Best solution is a hand held tach,Stewart/warner makes a dandy it works like a speedometer with a rubber nub you put on the end of a shaft or pulley.Most diesels have a place to drive a mechanical tach,you know the old flex cable,but that could get expensive.Cheapest way is to find an old machinests RPM counter it counts rev's as you time one minute on your watch.Frank C.

[woodmills1]

actually I wanted to use it on the 4 cyl deisel and the 2 cyl gas on the edger.  I undersatand that with a coil I could tap a signal, but does the 62 cat have a tach signal in its circuitry?

[bandmiller2]

James, I don't know on your Cat.probibly best to ask a dealer.The newer large diesels are computer controled and get the tach signal from it.Older and smaller diesels tend to be mechanically controled and require a mechanical tach.Their are aftermarket tachs that use magnets for pick up and some that use strobe lights but a hand held tack would do everything and cheaper.Frank C.

[shinnlinger]

Hi,

Thinking out loud here, but what if you went to your nearest bicycle shop and bought an electric bike speedometer?  The units have a chip that clips on the spokes and another that sticks to the fork.  Some units give RPMS and I am thinking stick one unit on the outside edge of your  crank pulley and rig a holder to hold the other piece and see what happens.

THese units are cheap and sportbike (11,000+RPM)guys use them so I know they can handle the RPM's your diesel can dish out.

[Magic Smoke]

The easiest and cheapest way to get RPM's on your diesel is from the tach output of the alternator ("P" terminal). Although it is not 100% accurate due to belt slippage, it is generally considered to be close enough.

[Captain]

...or a pezio electric pickup attached to an injector fuel line...

[John Bartley]

Since the meter is triggered by the spark on one cylinder, why would it matter how many cylinders there are?

Hi DanG,

Bandmiller2 nailed it already, and others have made great suggestions, but just to expand a bit....if you were using an inductive pickup clamped around the plug wire, then yes, you're correct and there'd be no problem regardless of how many cylinders there were. Most multimeters tend to tap the primary of the ignition (we're talking old style here - with a distributor) and so the number of pulses changes with the number of times the ignition actually fires (think "points opening and closing for each cylinder"). Another way to do it is a cheap frequency counter with an inductive pick up on a plug wire.

cheers

John

[Hilltop366]

I have a multi meter that can do RPM  can I use this on the lt70 and edger to find rpm?

Are you looking for a once and a while thing to check engine speed or a permanent tach to mount one equipment?

if only the "once and a while thing" for the diesel could a person use a preset strobe speed (example 1800 strobes per min.) with a line on flywheel, when the line stops as you adjust eng speed you would be at the preset strobe speed?

[woodmills1]

I only want to check it from time to time.  I like the alternator idea

[Hilltop366]

the alt one is new to me I would't mind knowing more about it (guessing your talking about the diesel)

one thing to watch out for on the 2 cyl gas eng is that a lot have a single coil with two plug wires (one for each cyl) so each plug sparks every time the piston gets to the top so there is a wasted spark on the exaust stroke this can throw off your reading if your tach is not set right. 2000 rpm would read as 4000 on a car tach ( I think I got that right, it's getting late )

[Magic Smoke]

The alternator provides a nice half-wave rectified waveform that can be used as an input to a tach. Not all alternators have this output but the 190A Delco on the LT70 does. If you just want to occasionally check the RPM's, you can take a digital multimeter that can read frequency (Hz) and probe the "P" terminal of the alternator (you'll have to stick the probe through the silicone sealant) to obtain the frequency in Hertz (Hz.), then you can convert the Hz. into RPM.
The following are approximate values for the given RPM (or see attachment for graph):

RPM's   Hz.
1260   395
1500   480
1800   575
2000   640
2900   930
3018   963

Keep in mind that if you are adjusting your engine idle that you need to keep the RPM's at least 1500-1600 to get near the full output of the alternator. I'm sure most people return the engine to idle when they use the hydraulics and this is precisely when you need the alternator the most.