**Before I begin, I want to state that I am completely aware that in older Athearn locomotives the motor is not isolated from the frame and it needs to be to do decoder installations.
Anyways to my actual post, I am trying to get familiar with my multimeter as I want to check if the motor is isolated in older non-DCC locomotives. Since I know that older Athearns have motors that are not isolated, I am using them to test my multimeter.
I put my multimeter in the lowest OHMs and one probe to either the motor terminal or the motor brush and the other probe on the frame, however I am getting no continuity, but I know I should. What am I doing wrong?
I want to make sure I use my multimeter properly before I install decoders in older non-dcc locos (not just Athearns).
Not a stupid question at all. I don’t get exactly a reading of zero, I always get something like 00.4, which I’ve read online, that it is close enough to zero.
You should not get continuity to the top motor terminal from the frame, only the bottom terminal/brush. Place the locomotive on a scrap piece of track. Check continuity from the track to the frame.
If there is no wire going to the bottom of the motor, you have a hot frame, and its probably connected.
The weirdest thing, as suggested I tried connecting one probe to the frame and the other to the rails of a seperate piece of track and nothing, no continuity
Then I tried putting both probes on the frame and again, no continuity, but there should be.
So I thought maybe the problem is the paint, maybe the black paint that Athearn uses does not conduct. So I grabbed my Dremel and I sanded off the paint on a small part of the frame. Then I tested that part of the frame with the rails, bottom motor terminal and motor brush and voila…I got a reading of conductivity, proving that the motor is not isolated from the frame. (even though I knew this already).
Does this conclusion make sense?
Valueable information moving forward into future decoder installs.
Just because there is coontinuity from the rail to the frame does NOT mean the motor is not isolated. Is this loco an older Blue Box oone, or is it one of the newer RTR models that has a circuit board clipped on top of the motor instead of those big metal clips? The newer RTR models fo have the lower motor brush isolated fromt he frame - it’s not a bad idea to verify this, but the ones with a PC board clipped on top of the motor are designed for plug-in decoder installs with no rewiring.
Frame to rail but no frame to motor = perfectly fine, just possible coupler issue depending on how the coupler is mounted.
Frame to motor but no frame to rail = not good, a derailment could push a wheel into the frame resulting in rail to motor conduction and a blown decoder.
Part of the reason analog meters need a zero-adj for the ohms scale is because the battery inside is directly part of the circuit. As the battery depletes, the meter indication will change. Most digital meters, the battery feeds a voltage regulator circuit so the voltage applied for the ohms range is constant right until the battery gets too low to work, better ones will have a low battery indicator so you know when it is time to replace the battery. With an analog meter, it was battery replacement time when you shorted the probes and even with the zero adj at the extreme as far as it would turn, you still couldn’t get a 0 reading.
Probably both. That Atlas S2 (newer one, not the old Roco one) has the same problem, additionally compounded by having the metal frame of the motor connected to a brush, requiring a new nylon motor mount screw. On my P2K S1 the lower brush holder is in no way connected to the motor frame, so I just added a wire, cut off the tab that rubbed the frame, and put a piece of Kapton tape over it.
Would be interesting to check the Gold ones. Those are like the third series, the originals were the Roco ones, then there is the one that internally looks almost exactly like the P2K - purple motor endcap and all. Those are the ones that need isolation.
Sorry to be pedantic BMMECNYC, but you will get continuity to both motor terminals if either one is connected to the frame. This is because the motor itself is a low DC resistance, especially older motors. One motor lead will be closer to 0 ohms than the other, but it may not be by much. Test each motor wire to the frame and the lowest resistance one will be the one you need to disconnect from the frame. As the OP discovered you always need a shiny metal part of the frame to test as most paints do not conduct electricty.
I agree with your point, once I sanded the paint on the frame, I had continuity from both the top and bottom motor terminals to the frame. BTW, the locos that I’m testing are older BB Athearns.
