how to test for bad starter

I had the same problem, turn key get a click. I would get out and rotate the propeller a couple of time and it would engage-start. Turns out my problem was not the starter itself. I had a loose ground strap going to the starter. I tightened it and have not had a problem with it since. The ground wire can be tight and have corrosion at the connecting points and it will produce the same problem. Good luck with your search.

To expand a bit on Ronnie's post, if you have a loose or corroded connection between the starter and the battery (it doesn't matter whether it's in the positive or ground half of the circuit), that joint can have significantly higher resistance than the rest of the circuit.  That resistance creates a large voltage drop when a high current flows to the starter, which in turn reduces the motor's speed and torque.

When you measure voltage with a multimeter, you're using a device with a very high input resistance (generally around one million ohms), so only a very tiny current flows into your meter.  That tiny current, across even a high resistance, produces almost no voltage drop, so the problem isn't evident by measurement.  However, when you engage the starter -- a device with a relatively low resistance, on the order of 0.05 ohms -- a very large current flows, which creates a large voltage drop across the bad, high resistance connection.

If you've been measuring voltage at the starter with the positive cable removed from the starter, this may be your problem.  Put the positive cable back on the starter, make sure the aircraft is VERY securely chocked and tied down, turn off the fuel supply so the engine can't start, then measure starter voltage (positive probe on the starter, negative on the battery) while it's actually turning over the engine.  If you get a low reading (below ~10V), you're on the right track.  You can use crocodile clips and extension wires to your meter without affecting the readings.

Next, determine which side of the circuit has the problem (positive or ground) by taking a measurement between the positive posts of the battery and starter, and between the negative posts of the starter and the battery (again, both measurements done while cranking).  With this measurement, you shouldn't expect to see battery voltage; instead, you'll be indirectly measuring resistance. If the larger measured voltage is in the positive side, you should suspect any of the cable connections or contactors from the battery to the starter.  If it's in the ground side, you should suspect any connection, strap or cable between the negative posts of the starter, engine block, airframe and battery.

To test battery and starter contactors, measure from input to output, again while cranking (positive probe closest to the battery).  This will tell you if the contacts inside are making a poor connection.  There should be almost no voltage drop across a contactor (no more than about 0.2 volts).

In this way -- measuring the voltage drop across small portions of the circuit during cranking -- you can narrow the problem down until you have only a single element that must contain the bad connection.  Test every cable, strap, crimped connector, connection post, etc.

My starter does not have a ground wire connection.  It must be grounded where it mounts to engine?  Also, my starter will not turn at all.  If i move the  prop and get it in a "no compression" place, it will start to move it but barely.  then nothing.  I am leaning toward a bad ground or pos wire to the starter.  will keep looking

Eric Page wrote:

To expand a bit on Ronnie's post, if you have a loose or corroded connection between the starter and the battery (it doesn't matter whether it's in the positive or ground half of the circuit), that joint can have significantly higher resistance than the rest of the circuit.  That resistance creates a large voltage drop when a high current flows to the starter, which in turn reduces the motor's speed and torque.

When you measure voltage with a multimeter, you're using a device with a very high input resistance (generally around one million ohms), so only a very tiny current flows into your meter.  That tiny current, across even a high resistance, produces almost no voltage drop, so the problem isn't evident by measurement.  However, when you engage the starter -- a device with a relatively low resistance, on the order of 0.05 ohms -- a very large current flows, which creates a large voltage drop across the bad, high resistance connection.

If you've been measuring voltage at the starter with the positive cable removed from the starter, this may be your problem.  Put the positive cable back on the starter, make sure the aircraft is VERY securely chocked and tied down, turn off the fuel supply so the engine can't start, then measure starter voltage (positive probe on the starter, negative on the battery) while it's actually turning over the engine.  If you get a low reading (below ~10V), you're on the right track.  You can use crocodile clips and extension wires to your meter without affecting the readings.

Next, determine which side of the circuit has the problem (positive or ground) by taking a measurement between the positive posts of the battery and starter, and between the negative posts of the starter and the battery (again, both measurements done while cranking).  With this measurement, you shouldn't expect to see battery voltage; instead, you'll be indirectly measuring resistance. If the larger measured voltage is in the positive side, you should suspect any of the cable connections or contactors from the battery to the starter.  If it's in the ground side, you should suspect any connection, strap or cable between the negative posts of the starter, engine block, airframe and battery.

To test battery and starter contactors, measure from input to output, again while cranking (positive probe closest to the battery).  This will tell you if the contacts inside are making a poor connection.  There should be almost no voltage drop across a contactor (no more than about 0.2 volts).

In this way -- measuring the voltage drop across small portions of the circuit during cranking -- you can narrow the problem down until you have only a single element that must contain the bad connection.  Test every cable, strap, crimped connector, connection post, etc.

Eric, at this time, my starter does not turn over at all.  my starter solenoid does make a clicking sound when i turn the key.. but no starter activity.   My batt is in the tail of the rans s7.  very hard to get to and run jumper cables.  

Jeff,

As a low tech person, I lean towards the low tech diagnosis/solutions - why not try disconnecting your battery start leads at the engine (first securing your aircraft & have a helper in the cockpit). Use a fresh battery & using temporary leads (jump if you have sufficient access), see if engine cranks/turns over (actual start not required), if yes the problem is the start battery/leads, if no the likely cause is a defective start solenoid, or start motor, or Spragg Clutch

From the proceeding commentary it would seem that pilots/maintainers still don't understand that:

 (1) 12volt electrical systems MUST have good (low resistant) contacts throughout (2) Effective earth/ground returns are, if not essential, highly desirable ie a dedicated earth return circuit (not solely an airframe return).

Remote start batteries ("My batt is in the tail of the rans s7") require extra large (low resistance) cables and a  higher energy/power battery,  all of which adds weight & complexity (yes I understand that there may be W&B issues)