Mighty Mule 502 repair

cappyD · Jun 2, 2012

Thanks to this site I've learned more about my gate opener in the past 30 minutes than I have in the past 2 days blindly searching the internet.
My MM502 was bought by the previous homeowner so I have no idea where or when it was bought. The wiring was already installed as well as all the mounting brackets but the arms were never installed. I dove into this project without knowing the first thing about gate opener. I put both arms on the brackets and double checked all the electrical connections and to my surprise it worked. Both gates opened and closed but there were just a few tweaks to be done. I adjusted the brackets mounted on the posts and gates so everything was perfectly level. After my adjustments I had a hard time putting the arms back on. I managed to get the master arm adjusted perfectly. The slave was a little more difficult because the post wasn't exactly installed like the master. It was a little lower and turned in a bit. Once I got everything as level as possible I tried to install the arm but the holes didn't match up. I (not thinking) pulled on the gate pretty hard to install the arm. I also screwed the arm in possibly too far in order to line up the holes and mount the arm.
Now when I activate the remote the master side works perfectly but the slave side only closes the gate about 1/2 an inch and stops. When the slave stops an alarm sounds. It is 1 beep per second and it doesn't quit unless I turn the board off.
I swapped the wiring on the board master/slave, slave/master. Same problem.
I think I damaged something inside the arm. I want to take the arm apart and replace whatever I need to but the casing seems stuck. I removed the screws thinking the arm would come apart but it seems stuck. I don't want to force it apart and break something else. I have learned that replacement parts are expensive.
Actually I have learned a lot since I broke the daggum thing.
This is the only website I have found discussions in detail about replacing parts. I hope I gave enough details to help someone help me. I will provide any other information needed such as measurements or pictures if anyone needs them.
Thank you in advance to anyone that has advice or even just points me in the right direction.

BeezFun · Jun 2, 2012

Here's the manual. I had one of these many years ago, spent more time fixing it than it was worth. Had problems with control board failing, replaced several times. Had problems with differential contraction of bushings in gate arms when it got real cold, replaced those. Only good thing I can say was the company was very helpful and kept sending me stuff for free because it was relatively new.

cappyD · Jun 3, 2012

I took the arm apart and located the limit switch.
When the arm is completely retracted and I push the remote everything starts okay. When it moves about an inch the limit switch releases. That is when the arm stops. It will not move again until I manually screw it back in and it touches the limit switch. Does that mean the limit switch is bad or does it mean something else?
Any help at all is appreciated.

flyerdan · Jun 3, 2012

When I worked for a fence company I was constantly pulling openers out of the trash and taking them home for parts. Mostly the cheapies with the rectangular sleeve, but I do have a 502 in the pile, I'll take a look at it and see what I can find out.

cappyD · Jun 3, 2012

Thank you for your time. I just wanted to add that it is my slave arm that is not working correctly but when I connect the master arm to the slave connections (on the board) the master arm works perfectly.

cappyD · Jun 7, 2012

I can absolutely verify that it is the limit switch I have to replace. I took it off and took the limit switch apart and found the tiny spring inside was a little corroded or rusted. I wasn't getting any spring action that would allow the arm to get power in it's normally open position.

caricia78 · Oct 1, 2013

my master arm randomly started to over retract and get stuck every time it opens and sound the alarm. any way to fix this?

weldingisfun · Oct 2, 2013

caricia78 said:
my master arm randomly started to over retract and get stuck every time it opens and sound the alarm. any way to fix this?

Have you tried calling the GTO Service Department, 800 543-1236? The number is in the manual Beezfun attached to his #2 post. They are very helpful and have helped me out several times over the years.

Soundguy · Oct 2, 2013

check the stop sensors in the arm if it is out of warranty.

caricia78 · Oct 2, 2013

I think you are right about the stop sensors..its still in warranty but problem is this is the arm that is farthest fromt he box and I have the wiring going under my driveway in a conduit..would be a big pain to remove
And I think the customer service department just reads through the same online help handouts and then if they cant solve it tell you to send it in.
so I ended up fooling around with it and turning the extension arm a little in and a little out, with the system ON, until I heard the box beep, which I assumed was its way of telling me that this was the point it triggers the open limit..so I made sure that in the fully open position the arm was retracted to just past that beep and not a millimeter more, and that seemed to work and now it opens without jamming itself, but that arms still retracts a lot more than the other arm (its a dual gate opener)..must be something with the stop sensor or limit switch (is that the same thing?) inside thats off..as you said. hopefully it works now but this did happen also while I was installing it and then it stopped, so if it happens again under warranty ( I still have 13 months) I guess Ill send it in unless anybody can tell me how to open it up and check the stop sensor???
thanks!