I have a Mack with 2 shifters 1 for hi lo rev and 1 for 1-5. I cant get my back up alarm to work,i jumped it out and it works fine so I think the switch might be bad. Does anybody know specifically were to find it on this trans.

It is most likely on top of the trans behind the shift tower. Corroded wire terminals at the switch are very very common. In some cases it is easier to remove the shifter boot on the cab floor to access the switch.

Thanks Tim Ill check it in the mornining!

I meant to ask this in my previous post. Do the reverse lights work?

Nothing works when its shifted into reverse.(lights or alarm) But I jumped them out with a battery charger and the lights and alarm both worked.They worked last summer and quit working towards the end of my season.Its a 1980 Mack DM Tri axle dump.

Go throught the floor. You have to be a dang twig to get to the tower, from the bottom, on most trucks. I am used to Eaton stuff, and they arent nearly as big as a Mack trans. Good luck.
DS79

Go through the floor as stated above and it should be directly in front of the shifters. I highly doubt yours is one of the very rare ones that it is behind but there are some Macks like that. Whichever it is, it will be a two prong switch with one wire on each and screws holding them on. Tiny little flat head screws that love to get lost when you take them out so be careful. With the key on you should have 12 volts on one posts and with the right shifte in reverse you should have 12 volts on both posts.

Thanks all! I found the switch behind the shifter towards the rear of the trans,on the top.I pulled the floor and did not need to.I was able to access it just under the cab in the back.I was able to now follow the wires back into the frame and I found one wire was broken.Its soldered back together and works great.

Love that trans set up, fifth gear reverse is fun to scare the heck out of your passenger.