Need assistance in Swanton VT

Trim motor failure

Picking up my -12 in Swanton VT after a month in the paint shop. The trim didn't work on initial preflight. It worked intermittently when I opened up the tail and wiggled the connector but it sounded labored. I broke one lead (orange/white) and after splicing, the motor doesn't work at all. To proceed with debug I need help in identifying the wires at the motor. I want to drive the motor from a power supply to see if the problem is in the motor or wiring.

Is it a standard MAC servo?

if so, the motor wires are the two white ones

the three striped wires are for the position indication, so the orange/white wire should have no effect on the function of the motor, just on the availability of position info.

Yes the white wires are the motor wires. To check the trim motor you can connect a 12V battery to the white wires. Polarity determines the direction of travel.

See:
Ray Allen Servo
Chapter 11 page 9

Ok. So I've confirmed that the trim switch sends +/-12 V to the white wires, meaning that the switch is working. Applying 9V from an alkaline battery directly to the two white motor leads results in nothing. I have a bad servo don't i? Anything else to check before replacing it?

Did you disconnect the servo hardware - could be something binding?

If the servo won't operate on a good 9v battery then there is a break in the wire from the 9v to the servo, or the servo is bad. You mentioned it was labouring, seems likely something in the servo itself has gone wrong.

Hi Ken

I am in Burlington (VT) this weekend, heading home tomorrow morning. I live in Bolton, MA. If you have to abandon your plane, you are welcome to a ride. If I can help, please call my cell - 978.549.0043

Regards,

As it turns out, there was a broken motor wire inside the servo, where it passes thru the housing. Another wire was close to failing in the same spot. I think I have it fixed. Thanks for the offer Dean! I'll call if I need to take you up on it.

Thanks all.

update

I thought I'd post an image of what I found, as I found it to be quite alarming. When I opened up the servo, i found that the wire bundle exiting the servo housing had what appeared to be a clean cut through the outer shrink wrap. This cut extended into or through a few of the wires inside.

I had to cut away the shrink wrap to investigate, but in the photo you can see a white wire (pointing up and to the left in the photo) that is broken cleanly, and a white/blue wire that is hanging by a few strands. There is also, though it's harder to see, an orange/white wire with a cut/crack forming in the insulation.

I assume all of this was caused by vibration and exacerbated by the way that I secured the wires. But I'm surprised that the bushing in the servo tray didn't better protect these wires.



I soldered the wires as a short term fix, and that got me home. Now I will replace the wires, the molex connector, and/or the servo itself.

In happier news, here's my newly painted RV-12, after about 15 months of flying!! It's starting to feel like this project is as close to "done" as it's ever going to get! Thanks to everyone here for all the help and guidance along the way. To mark the occasion, a check is on its way to support this site!

Nice job of troubleshooting, Ken, and a very nice looking paint job.

kritsher said:

I thought I'd post an image of what I found, as I found it to be quite alarming. When I opened up the servo, i found that the wire bundle exiting the servo housing had what appeared to be a clean cut through the outer shrink wrap. This cut extended into or through a few of the wires inside.

I had to cut away the shrink wrap to investigate, but in the photo you can see a white wire (pointing up and to the left in the photo) that is broken cleanly, and a white/blue wire that is hanging by a few strands. There is also, though it's harder to see, an orange/white wire with a cut/crack forming in the insulation.

I assume all of this was caused by vibration and exacerbated by the way that I secured the wires. But I'm surprised that the bushing in the servo tray didn't better protect these wires.

I soldered the wires as a short term fix, and that got me home. Now I will replace the wires, the molex connector, and/or the servo itself.

Click to expand...

Based on failure of wires in aircraft I have seen in the past (vibration fatigue, etc.) my guess is that it more likely was induced by heavy strain on the wires. Possibly by the disassembly / reassembly process that would have occurred during the painting process?