Electric Stabilator Trim Not Working

[jrich]

RV-12, S/N 120147 with D-180, 360 hours, flying 5 years:
Depression of trim switch up or down does not actuate the servo and does not yield a voltage signal on either the white or white/red wires at the spade connectors at the servo. A continuity check on these individual wires from the spade connectors to pins 31 & 32 at the D-sub (WH-00046, Fuselage Wiring Harness) show zero resistance (i.e., continuity on each wire). Turned trim potentiometer slightly counter clockwise and no effect. I think that the next step is to find out what signal is coming out of the ?control panel?. I?m not sure how or if I want to tackle that. What do I check next or whom do I contact?
More history & troubleshooting details:
Trim has worked flawlessly for years until recently when during pre-take-off check the trim down would not work. Then, tried it again and it did work. Thought that perhaps I had not depressed the switch all the way initially. I doubt that this is associated, but for added info.: Prior to take-off charging system appeared normal. During the flight, the amps were more in the negative than the positive and the voltage was dropping gradually. Continued to monitor the volts and amps for say approx. 10 mins. Then, the amps. began going positive and the voltage increased. Performed the forum prescribed voltage regulator diagnosis and found nothing conclusive. But, decided to replace the voltage regulator anyway. Immediately after the v.r. replacement, and during pre-take-off check found that the trim was not working. Troubleshooting included checking the battery voltage after its overnight rest: 12.6v. Tried the trim while charging the battery: No effect on trim. Bypassed everything with (2) separate wires hooked directly to the plane?s battery and to the spade connectors white and white/red wires at the servo and the servo does work and the indicator appears to be indicating the correct position.
During troubleshooting, found when trim switch is pressed, there?s 5v to ground at one or more of wires that are not the white only or white/red wires. I didn?t take note exactly the colors of these wires since we?re supposed to get the voltage on the white and white/red wires (power wires) and we?re not getting voltage there. Also, I was surprised to find continuity between the wht./grn., wht./blu., wht./org. wires pins 4, 22, & 25 respectively. I don?t know what signal or cross connection is to be expected on the non-power wires. I only mention my findings if this might be a clue as to what?s going on.
Side note: Tried actuating the servo with a 9v transistor radio battery and it didn?t actuate the servo.

Again, what do I do next or who do I contact for assistance?

[Mich48041]

From what you posted, it seems that the servo motor is the problem. I suggest that it be tested on your workbench. I have never taken one apart, but there must be two limit switches inside to shut off the motor when it reaches full up or full down. A limit switch could be bad. Or there could be a bad electrical connection. The motor could be bad, or there could be a mechanical problem. If the servo is completely disconnected from the aircraft, then the servo motor should run if a 9 volt battery is connected to the two power wires. Try reversing the battery polarity in case the servo is at its limit. If the motor does not run with a battery connected directly to it, then there is a problem inside of the servo.
The 3 position wires connect to a pot inside of the servo. It is a completely separate circuit, independent of the motor. The wht/org wire that connects to pin 25 is grounded. The wht/blu wire that connects to pin 22 has 5 volts on it. The wht/grn wire is the signal wire and should have a voltage on it somewhere between 0 and 5 volts. Do not connect a battery to the pot wires.

[Ueli N]

From what I understand that you have continuity of the wires in question, does not guarantee that they aren't shorted to ground.
I would test that first. That would be my first guess of your problem. Especially since you mentioned that the motor works with separate wires run to it.

[JBPILOT]

The early wiring cable had TWO white wires ( NO TRACERS ). They run the actual trim motor. You had to be careful initially to be sure motor was turning the correct way to start with.

[Raymo]

Test the servo with a 9v battery. Swap polarity. If it does not move in either direction, you'll need a new servo.

[Dave12]

If it is determined to be in the panel, I have a good used one available.

[Mich48041]

Oops, I misread your post and did not think the servo motor worked. After reading Ueli N's post, I realize that the servo motor does work. Have you checked the 2 amp fuse?
Here is link to a PDF Schematic of the trim circuit.
An option is to replace the circuit with a PWM 12V computer fan speed control.

[Mich48041]

Try adjusting the pot from one extreme to the other.

[jrich]

Thanks to all who have responded thus far to my problem.

In the initial troubleshooting, I did so many checks that I had forgotten what I had checked. Note to all: Write this stuff down. So, after Ueli N's post about checking wiring faults to ground, I second guessed myself about that so, I did some more checking/confirming last night. I double checked that all wires of the Fuselage Wiring Harness (WH-00046) were not shorting to ground or to each other. I found that all of these wires have no fault to each other or to ground.

Since it was indicated by Mich48041's & Raymo's response to this thread that a 9v battery should have moved the servo, it bothered me that the 9v battery didn't run the servo motor. I tried another 9v battery and it worked at the servo. Then, I tried that 9v battery as the power source located at the D-sub pins (D-sub disconnected from the panel and 9v source connected to pins 31 & 32) and the servo worked (both directions of travel). So, I'm confident that my wires, crimped connections, and servo are all O.K. It appears that my problem is somewhere beginning where the D-Sub connects to the panel board and upstream from there.

As I indicated previously, I did turn the trim potentiometer slightly ccw and was wondering whether to try to turn it further. Thanks to Mich48041 (Joe Gores) for answering that question. I will try turning the POT as you have suggested. Also, a big thanks for the schematic diagram of the trim circuit that you provided. Very well done. The schematic will come in handy for someone, if not myself. BTW: I assume that the 2 amp. fuse you wanted me to check is the panel fuse. I had checked that and swapped it out with the spare to confirm that it wasn't blown.

Knowing the above, any other suggestions on how to proceed at this point would be appreciated because I don't know how to approach this.

[Hotscam]

I had some water entering the electronics bay some time ago during heavy rain. Same results. After opening the box and drying the pcb all worked again.
I elevated the box by stand offs after this so it will not happen again