I have been working with electronics for many years but I am totally baffled by this one. I am repairing a little Christmas village ski lift for a neighbor. I think she left it on for way too long and the little motor gave up and measured open. I ordered a new motor and installed it, soldered in the wires and switched on the power. Nothing. I measured across the terminals and got zero volts. Should be 4.7 VDC. Then I unsoldered the red wire from the motor and clipped on a short clip lead. Touched the other end alligator clip to the motor terminal and it runs!! Soldered the wire back on and nothing! What is wrong with this picture? I even cut back the red wire, re-tinned it, soldered it on to the motor and the same thing happens. It is so crazy that I made of video of it but it is 16 MB so too big to put here.

 

Did you test both voltage and current at the supply source when these conditions occur?

 

I have no good way to check the current but when the motor runs it runs normally. It either runs or does not run. The wire is fresh tinned copper and the motor terminal is also tinned copper and has been soldered and unsoldered 3 times now but will not run when soldered, only when touched with the clip lead.

 

How about the resistance at the motor soldered terminals?
Also back feed the motor with power off and measure the voltage it should generate.
This ensures there is some kind of connection with the motor.

 

I should add that I unsoldered the wire and then touched it to the motor terminal with no effect. Even when pushing it hard on the terminal. But connecting the wire to the clip lead and touching the terminal in any spot, even lightly, causes the motor to run.

 

Obviously some kind of dodgy connection there somehow.
The meter tests should uncover something I would think?

 

The motor can't be used as a generator because it drives a worm gear train which does not transmit motion backwards. The motor definitaly works but only when the terminal is touched by the alligator clip. Touching the red copper wire to it will not make it run.
The voltage across the black wire and red wire when touching to the motor is zero. When soldered it is zero. But using the clip lead and just touching breifly it is 4.5 V. and it runs.

 

When you are using the clip lead, I assume that the position of the red wire is hanging in space somewhere. Perhaps there is a break in the wire that closes when it is in such a position.

Disconnect power and the red wire. Connect a DMM to both ends of the red wire. Check continuity / resistance. Wiggle the red wire around while watching the meter. Does the meter always show continuity or 0 resistance? Or not?

 

That would be a good test but the other end of the red wire is buried somewhere in the device and hot glued in place. But I have wiggled, twisted and bent the wire with no success in making it run while soldered to the terminal.

 

Touching the red copper wire to it will not make it run.
The voltage across the black wire and red wire when touching to the motor is zero. When soldered it is zero. But using the clip lead and just touching breifly it is 4.5 V. and it runs.

This can either be that the voltage is missing/not making contact somehow, or there is a short circuit. this why the current reading suggestion.

 

To me a well soldered connection has to be making contact. This is why this is so frustrating. I have been working at this for hours now. I am about to make the magic clip lead a permanent part of this circuit and glue it in place!

 

To me a well soldered connection has to be making contact. This is why this is so frustrating. I have been working at this for hours now. I am about to make the magic clip lead a permanent part of this circuit and glue it in place!

 

Clearly the problem is not at that end of the red wire so get a length of suitable gage stranded wire a few inches longer than the distance to that motor terminal, and solder the one end of that new wire to the motor terminal Then at first just touch the end of the new wire to the red wire. If the motor runs then you are set. Solder them and insulate the splice.
My thought is that it is a mechanical action hidden from view causing the problem. Not at all the first time it has happened.

 

I skimmed so forgive me if I missed something, but check the continuity of the wire itself, end to end. If in doubt, run a new wire. It may have broken internally.

 

Update with new info: This morning I have tried a number of different things. The little lift "haus" has a yellow LED inside it which is somehow tied directly or indirectly to the motor. I removed the motor from the gearbox and now when I power up the motor using the clip lead with alligator clips, the LED first blinks off and then comes back on. If I connect without the magic clip lead such as soldering, the LED remains off , the motor does not run, but while holding the motor I can FEEL it pulsing at about 2 Hz. So there is some kind of overload-sensing circuit driving it. When I first powered up this thing last week it started smoking. I found what looks like a small ceramic disc capacitor soldered across the motor terminals and it was burnt and black. I have tried various small capacitors across the motor but it made no difference in the problem of not running the motor. I also tried a different clip lead and to complicate things further, the new clip lead will NOT run the motor! This is a whacky problem. I measured both leads and they are nearly identical showing about an Ohm of resistance. I tried putting various resistors in series with the positive lead of the motor 0.1, 1 Ohm and 10 Ohms. Bingo! With 1 Ohm in series the motor runs! The older clip lead must have just a bit of resistance and this allowed the motor to run but with a slightly greater load the electronics would sense an overload and cut it off.
I will solder in the 1 Ohm resistor and be done with it.

 

Seems to me the whole thing requires dismantling and complete examination in order to find out exactly what is going on!
The capacitor is just for com arc suppression usually.
There should be no need for added resistors etc !!
Is there a fault allowing AC to reach the motor, in place of the required DC maybe??

 

I guessed that the cap was just for RF noise suppression from the brush-type motor. Since this is really just a display item I am not going to dive into it any further. If the motor turns, it works. The DC is supplied by an external wall-wart device. I can hear no evidence of an AC hum and the meter shows nothing on the AC scale.
Thanks to everyone for their suggestions and emotional support! Thought I was going crazy.

 

Pictures and a schematic will do wonders for us lending help troubleshooting the problem.

My thought is that it is a mechanical action hidden from view causing the problem.

I was thinking the same thing - touching the terminal may be causing something inside to make contact. However, when there is no physical pressure applied to the terminals there is no contact and the motor does not run. This could be an issue from overheating plastic parts with either a soldering iron that is too hot or an iron has been held on for too long a time.

HOWEVER: With the additional information of the 2Hz thing - - - that's why we need pictures and a schematic.

 

Sorry but this site will not allow my short 16 MB video of what is happening and there is no schematic available for this. It is essentially a "toy" for collectors. After soldering in the 1 Ohm resistor the motor runs for about 3 minutes and slows down and stops. I need to replace this with a real gear-motor that has some power to it. The other thing I will try first is to wire the 4.5V power supply directly to the motor. Perhaps something went wrong with the current sensing circuit.

 

Take a picture of the PCB.
Upload your video to YouTube, then link us to it.