Electric Stabilator Trim Not Working

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?

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.

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.

What I remember from 6 years ago -

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.

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

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

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.

Try adjusting the pot from one extreme to the other.

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.

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

There were some discussions on VansAirforce about 5 years ago pertaining to problems adjusting the trim speed. I can not remember which way to turn the pot to increase or decrease the speed. I know that when I slowed my trim to the recommended speed, that it was borderline between going slightly too fast or not working at all. If I turned the pot to slow it down just a little bit, it would quit working. If your trim was adjusted at the borderline between working and not working and then you turned the pot in the wrong direction, that might have made sure that the motor does not run. I believe that Van's made improvements to the trim speed circuit since then.
I suggest that you disconnect the trim servo wires from Van's control board pins 31 and 32 and connect a small automotive light bulb (15 watt or less) in their place, perhaps a running light or smaller. Then watch the lamp as the trim pot is turned while the trim switch is held in. The lamp should illuminate and change brightness as the pot is turned. If the lamp does not light up, you could try a voltmeter. If there is no output from the trim speed circuit, then suspect a fault inside of the control board.
Yes, the 2 amp fuse is in the one that you checked.

Mich48041 said:

If there is no output from the trim speed circuit, then suspect a fault inside of the control board.

Click to expand...

The D-180 control module has a PWM circuit for controlling the pitch trim servo speed. The adjusting potentiometer is a one turn pot. that does make the adjustment rather sensetive (as Joe mentioned, the circuit in the Skyview systems uses a 25 turn pot which makes it much easier to adjust, but sometimes messes people up because even a full turn doesn't yield much of a speed change).

If it was working but quit, I would look for anything that could have added some resistance or binding to mechanicals that the servo attaches too. It is possible that a full voltage (or 9 volts) could overcome the binding but the pulsed PWM output used to slow the servo motor would not be able to drive it.

If I remember correctly turning the adjusting pot CCW increases the speed

Electronics again ##$%@

I have learned - when trouble shooting anything electric, start with the simplest and most likely solution and work forward to the less likely solutions - so, are you sure that your fuses are good. Did you check the fuse that you used to replace the original one? Just sayin-----

Per Scott's advice about checking mechanical binding and Ed's advice about looking for simple problems first, check the AST hinge for smooth operation. I lubricate hinge pins by rubbing with a carpenter's pencil (graphite).

Hi Joe,
Is there a concern over potential galvanic corrosion when using graphite as a lubricant on the aluminum hinges? I always thought that it was a no-no.
Alex

Is there a concern over potential galvanic corrosion when using graphite as a lubricant on the aluminum hinges? I always thought that it was a no-no. Alex

Click to expand...

Gee, I don't know. I have not had a problem with corrosion. A quick search of the internet turned up differing opinions, but I could not find an authoritative document about graphite. Although one authority says that carbon fiber will react with aluminum. What lubricant do you recommend?

I have been using LPS1. I figure its fairly safe around aluminum.
Alex

Per Joe Gores? & E.D. Elliot?s posts, we decided to confirm that the 2A fuse really was good. We found that it was blown. We put in the original fuse and all worked. Remember, one of the first things we did was to swap the fuse when we had the original problem. SO, we had the problem of the trim not working with the original fuse and now it does work with the original fuse. Whether the replaced fuse was bad to begin with, we do not know. The only change we made was to the POT. The rest was just testing of the wires and the servo.

To briefly recap and try and explain what our problem was: Trim wasn?t working. Early on, we swapped the original fuse with a spare. We didn?t check either fuse for continuity. After numerous checks, including slight adjustment of the POT, the trim wasn?t working. We lastly found that the fuse (spare) that we had put in was bad. Whether the spare fuse was bad to begin with, we don?t know absolutely. But, I don?t think we ever shorted things out during our testing. In any case, we think that the problem all along may have been caused by the POT. We notice that when we set the POT back to or near its original position to try and get the 23 ? 25 secs. of servo actuation from stop to stop, the servo seems to hesitate during one direction of actuation and only yields 31 secs. We?re thinking that either the original setting of the POT is just too much resistance at times for the power to break through or the place where the POT was set has developed a burn/corroded spot at the position that it?s been kept at for years. Later, we spoke with a mech. at our airport and he told us of a similar experience with a jet that they worked on and the tech. rep. flew in, made a ?wipe? with a POT and it solved the problem.

Part of our troubleshooting frustration, though some might think comical, was caused by our misconception that all of the panel fuses light up when blown. After all, the RV-12 ?BEFORE TAKEOFF? checklist in the ?RV-12 PILOT OPERATING HANDBOOK? reads, ?Fuses ? CHECK?. So, all along we?ve had a false sense of confidence that probably all systems are ?go? prior to takeoff when we did our check for any lit fuses. We now realize that all of the low amp. fuses are not the ?glow when blow? type. Since we bought this ELSA already built, we don?t know for sure if the previous owner replaced all of the ?no-glow? low amp. fuses or if only ?no-glow? low amp. fuses came with the kit. If the latter is true, perhaps Vans considers all of the low amp. systems as non-critical? We would sure like to have all ?glow when blow? type fuses. Can 2 amp. and less ?glow when blow? type fuses be purchased?

Does my theory make sense? Any insight is appreciated in case the POT is not the problem. Thanks to all who have contributed to our plight.

My local stores do not carry all sizes of "glow-blow" fuses. You can order them from Van's however and that is where I get them from.

ES-00202 FUSE ATC 2 AMP
ES-00203 FUSE ATC 3 AMP LIT
ES-00205 FUSE ATC 5 AMP LIT
ES-00207 FUSE ATC 7.5 AMP LIT
ES-00210 FUSE ATC 10 AMP LIT
ES-00230 FUSE ATC 30 AMP LIT

Above is a list of fuses that Van's sells. Notice that the description ends with "LIT" except for the 2 amp fuse. I checked several online vendors and none of them sell glow fuses less than 3 amps.