mattd860 162 #1 Posted February 14, 2013 Is it possible to use my digital multimeter to read the rpm on my tractors, particularly on my c-161 k341as and my 418 m18qs??? Share this post Link to post Share on other sites
wp205a 43 #2 Posted February 14, 2013 Search for it under squidoo dot com, I don't have a meter that reads frequency as they say you need. Let us know. Share this post Link to post Share on other sites
Save Old Iron 1,568 #3 Posted February 14, 2013 Matt, the overly broad answer is YES you can. Before I can give a specific answer, we need to discuss which event in the engine will you be monitoring to check the RPM's. Each choice has its special challenges. If you wish to pick off RPM's from the crank or possibly the rotating flywheel screen, you will need a circuit to convert an event (the passing of a trigger on the crank or a reflective dot on the flywheel screen) to a voltage and the voltage will be calibrated to be displayed on the multimeter screen as RPM. I'm guessing this is not what you are after since many inexpensive optical tachometers are available to do the measurement without building circuits. If you wish to monitor the opening and closing of a points based system - no problem. If the meter has a HZ or FREQ function, you can most likely connect the meter to the (-) terminal of the ignition coil and away you go. You must check the specifications on the meter to assure the HZ or FREQ function can handle near 300 volts without damage. You will be measuring in a circuit that has at least that amount of voltage present for brief periods of time during the normal operation off the ignition circuit. If you meter does not have specifications for the HZ or FREQ range, you may be able to build an "antenna" to pickup the electrical emissions from the spark plug wire. I'll draw up a diagram for you and post later this morning. This will mimic the maintenance meters / tachometers you can purchase for $30 online. With a piece of wire and a multimeter you will be able to calculate engine RPM's. Once we get a signal off the spark plug wire, then we have to determine if you are reading from a K engine (1 spark for every 2 crank revolutions) or from a magneto engine (1 spark every crankshaft / flywheel revolution). The numbers you multiply the HZ readings will be different for both those engines, Share this post Link to post Share on other sites
meadowfield 2,710 #4 Posted February 14, 2013 as chuck says it can work well from the coil... ... putting the meter on Hz and then measuring off the coil you should see 60Hz when the engine hits the max governed speed (3600rpm for k series). If it's too high or low then you can adjust as necessary. -------------------------------------------- <edit> I'm probably wrong here - myself and Chuck (SOI) have been discussing offline. Fingers crossed I will hook up a meter/scope at the weekend and share the actual results! mark ----------------------------------------- You can actually buy cheap tachos for sub $30 from that auction site which makes the job a whole lot simpler. mark Share this post Link to post Share on other sites
Save Old Iron 1,568 #5 Posted February 14, 2013 Matt, see post The 418 we will need to discuss as there is a possibility the frequency of the AC pulses coming from the charging stator may be used to determine RPM. Share this post Link to post Share on other sites
mattd860 162 #6 Posted February 14, 2013 Matt, see post The 418 we will need to discuss as there is a possibility the frequency of the AC pulses coming from the charging stator may be used to determine RPM. I tried the method above with my 1000v max multimeter set on hz but I wasn't getting any readings like the ones metioned in the link. When I took a reading off the spark plug wire I was getting readings between 40hz and 170hz. The readings were similar if I took them from the negative side of the coil as well. Maybe the wire I was using wasn't appropriate. I used a 20 gauge wire coiled around the plug. I also tried using the lead that came with the multimeter and touched it to the negative coil, plug wire, and spark plug itself. Share this post Link to post Share on other sites
mattd860 162 #7 Posted February 14, 2013 How can I measure hz from the startor wiring? This should be a simple solution and can be done on both my 418 and c-161. Share this post Link to post Share on other sites
Save Old Iron 1,568 #8 Posted February 15, 2013 Matt, see post The 418 we will need to discuss as there is a possibility the frequency of the AC pulses coming from the charging stator may be used to determine RPM. When I took a reading off the spark plug wire I was getting readings between 40hz and 170hz. . I also tried using the lead that came with the multimeter and touched it to the negative coil, plug wire, and spark plug itself. Matt, with the HZ readings jumping to 170, you may have been overloading the meter with too many wraps of wire and "double triggering" the meter. Try varying the # of loops of wire and put the loops around the spark plug wire. This is the whole trick to getting this type of measurement to work - the Goldie-locks rule - not too many, not too few - just right. Experimentation is the only way to establish the right number of wraps. I'll say it again, this is not the gold standard for how to measure RPM's but it can be made to work accurately and reliably. The meter leads should never touch the spark plug top terminal or the conductor within the spark plug wire. It is possible the HZ function of the meter could be damaged by doing so. To test the meter, insert the probes directly into a home wall socket. The power company regulates the HZ of incoming power very closely to a 60.0 HZ value. If your meter does not show a steady 59.9 to 60.1 HZ reading, the meter is probably compromised. Now to reading off the stator. I have never done that before but it is easily done. Place the meter directly across the AC leads on the rectifier regulator unit and start the engine. The meter should read some HZ from the stator. Remember this is a multi pole generator whose frequency will be determined by engine speed. For now, place the meter across the AC terminals and idle the engine - write down the HZ reading. Run the engine up to full speed and record the HZ reading again. I would love to see what those numbers turn out to be. I'll try to compare a c120 over the weekend. I think Mark (Meadowfield) is looking into this too. Share this post Link to post Share on other sites
rmaynard 16,197 #9 Posted February 15, 2013 ...and when you get tired of playing with your multi-meter and a coil of wire, just go to Amazon.com and pick up one of these for around $10.00 No coils, no wires, just aim and read. Just sayin'... Share this post Link to post Share on other sites
wh500special 2,256 #10 Posted February 15, 2013 Outboard/marine tachometers use the alternator output to determine engine RPM. Manufacturers like Teleflex Marine and Faria Instruments sell mounted tachometers with selectable pole settings for connecting to various applications from Evinrude, Yamaha, Mercury, Tohatsu, etc all who may use different alternator types. I suppose if reading alternator pulses works out and you back out the number of poles, you could probably equip your tractor with a marine/outboard tachometer and hook it up permanently...if there's a reason to have such a thing mounted permanently. Steve Share this post Link to post Share on other sites
Ken B 3,175 #11 Posted February 17, 2013 I bought a small digital Sentac for 35 bucks. I attached it to the hood stand with velcro. Its is 3 yrs old now and it still works perfect. It even has an hrs. display on it to keep track of oil change intervals. Can't beat that. Share this post Link to post Share on other sites