I've Got a 93 Mack and I'm a little confused about how the rear ends work as far as traction. The local Mack dealer says they work automatic but last winter I was on a job and I was spinning and nothing locked up. I was told to put a slight Johnson bar brake pressure and that would help. All that did was take the little power my truck makes away. Also which axel do I put chains on?

Do you have a switch labeled 'Power Divider'? This locks your 2 differentials together, it will cause power to be transmitted to both the front and rear diffs. However, it does not lock the axles, so you may still have one side or the other spinning on one or both differentials. This is just the nature of the Mack drivetrain. The Camelback suspension is pretty much the best in the industry for maintaining equal loading on all the drive tires, so that is a small saving grace. My '86 Mack didn't have a Power Divider switch, but it seemed to have some method of keeping things locked up, perhaps the power divider was set up to be permanently locked?

Oh, and as a rule, the rear axle is the one to install the chains on.

pumps right. should be a single stage switch somewhere of the dash to lock the differential. If wired correctly, a buzzer and or light will go off. My mack's switch is towards the rear of the center console.

i have a older mack and there is no lockout on the rearends the handbrake doesnt really work unless you are in reverse.

No power divider switch. I guess the older trucks don't have em. Thanks for the help tho.

power div. switch is an option that some trucks don't come with... i don't know why anyone would order a pump without one, they can't be that expensive.

we have a couple Mack rigs without it, pick an axle off the ground, you are pretty much screwed! Definitely a must have option.