I'd start with the basics by testing the fuses and relays. They should always be the first thing to check when there is an electrical problem before starting to throw parts at a problem or even poking around with a multi-meter or volt-meter. A visual check of a fuse is not always accurate. A continuity test with a multi-meter or an inexpensive fuse tester from your local auto parts store is the definitive way to test a fuse. Testing a relay is a little more complicated, but if you can hear it click when you turn the power on, then you know that the relay is probably good. It could still be bad, but fully testing it requires a much much longer explanation and more poking around with a volt-meter. But swapping a suspected bad relay for a known good one is usually a good test. E.g. if the starter isn't working, but the fuel pump is, swap the starter and fuel pump relays. If the starter starts working and the fuel pump stops, you know you have a bad relay.
Anyway, a volt-meter will not read ohms. It will only read volts. An ohm-meter or multi-meter will read ohms. Ohms is a unit of measure for resistance. I know a lot of guys like to check ohms because its easy, but it is not reliable or accurate. The main problem is you have to know the exact resistance specification for the part of the circuit you're trying to measure. All metal has some resistance. Even a 1 inch piece of wire will have an ohm reading (all be it very small). The ohm spec for a part can vary widely between manufacturers and even what batch it was made in. So you can't reliably use 900-1000 ohms as a rule of thumb to indicate a good CKP (crankshaft position sensor). Sorry, I don't mean to pick on you M&P340.
Another problem with doing an ohm test on a specific component is that it assumes that the component is the cause of the problem. A damaged/corroded wire or loose/damaged/corroded connection could just as easily be the problem and ohm test won't tell you much there since the ohm spec for entire circuits are usually not published.
Another problem with an ohm test is the test itself. It sends a very small amount of voltage & amps down the wire. It measures the difference in what it sends out to what it gets back between the two leads. It is effectively doing a voltage drop test, but with a very small amount of power. The problem is that the power it sends down the wire is so small that it can easily be fooled. To truly test a circuit it should have its 'normal' amount of power flowing through it.
A continuity test is also only partially helpful. A large battery cable, for example, is made up of many tiny wires wound together. If all but one of those tiny wires is broken a continuity test & an ohm test might pass, but a voltage drop test (explained below) will not pass.
A better test is a voltage drop test which, as the name suggests, measures voltage and can be tested with a volt-meter or multi-meter. The main advantage to a voltage drop test is you don't need to know about any specs for the circuit your testing other then how many volts are on the circuit which is easy to figure out. A voltage drop test measures the effect of any resistance on the entire circuit, not the amount. The only requirement of a voltage drop test is that the circuit is loaded. Meaning that power is flowing through it. You can't do a voltage drop test with no current (amps). It is also good to know how many volts are on the circuit. Most on our bikes are 12 volts, while some sensors are only 5 volts. A voltage drop of 0.5 volts or less is acceptable for a 12 volt circuit, while a voltage drop of 0.2 volts or less is acceptable for a 5 volt circuit. Anything more then that and there is excessive resistance on the circuit.
If the problem is your crank sensor, and I'm not sure it is, here is how I would test it without a circuit diagram handy. If you have the circuit diagram and know how to read it, skip to doing a voltage drop test on the power feed wire below.
First step is to figure out which wire is which. If memory serves, the CKP is a three wire sensor. One is a power feed that is a constant voltage of 12 or 5 volts. I think its 5 volts. One is a ground wire and the other is the signal wire. The CKP sends pulses of power down the signal wire to the computer to tell the computer the position of the crankshaft.
- Set the ignition to the ON position without starting the bike (this should load the CKP circuit).
- Set your meter to as close to 12 volts as possible without going under
- Unplug the CKP
- Touch negative lead of the meter to the negative battery post
- Touch the positive lead to each wire in the connector in the wiring harness for the CKP
You should get 5 volts on one of the wires. This is your reference voltage and the power feed wire. Remember this voltage & wire. If you get 0 volts on all the wires, then there is a break in the power feed wire or the computer didn't load the CKP circuit when you put the ignition in the ON position. Both of which are hard to track down without the circuit diagram.
- touch the negative lead of the meter to the positive battery post
- touch the positive lead of the meter to the two wires that are not the power feed wire.
You should get a 12 volt reading from one of the wires and zero from the other. The wire that got 12 volts is your ground wire which makes the other wire your signal wire. If you get zero volts on both wires, then there is a break in the ground wire which is a little harder to track down without the circuit diagram.
Assuming all that checks out OK, Now to start the actual test. Do a voltage drop test on the power feed wire:
- Plug the CKP back in
- Take a small safety pin or needle and back-probe the power wire at the connector
- Touch the negative lead of the meter to the negative post on the battery
- Touch the positive lead of the meter to the safety pin
You should get the within 0.2 volts of the reading you got earlier for that wire (between 4.8 & 5 volts). If the voltage drop is more then 0.2 volts, then there is resistance on the circuit between the safety pin & the positive battery post. If you get 0 volts, then the safety pin is not making a good contact in the connector.
Do a voltage drop test on the ground wire:
- Take a small safety pin or needle and back-probe the ground wire at the connector
- Touch the negative lead of the meter to the negative post on the battery
- Touch the positive lead of the meter to the safety pin
You should get between 0.0001 & 0.2 volts. Any more then that and there is resistance on the circuit between the safety pin and the negative battery post. You should get some small amount of voltage reading here. If you get 0 volts, then the safety pin is not making good contact in the connector.
Do a voltage drop test on the signal wire. This is a little more complicated because the signal voltage send down the wire by the sensor to the computer is not a constant voltage. It sends a wave form (pulses of electricity) at a really really fast rate. Even the best volt-meters & multi-meters can't accurately display a wave form like this. Their refresh rate is just too slow. You need an oscilloscope to see this wave form. But you don't care about the wave form here, you care about there being pulses at all.
- Take a small safety pin or needle and back-probe the signal wire at the connector
- Touch the negative lead of the meter to the negative post on the battery
- Touch the positive lead of the meter to the safety pin
With a multi-meter or volt-meter you'll see a bunch of numbers flash around between 0 & 5 volts. You care about the max & min numbers you see. If you don't see it jumping between 0 & 5 volts or it doesn't ever reach 4.8 to 5 volts, and the previous tests were OK, then the sensor is bad. If you do see it jumping between 0 & 5 volts, then the sensor is OK and the problem is somewhere else.........
Here is a really good series of video's explaining voltage drop testing:
But teach a man to fish...... :-)
Linear Mode
