Was one of three that maintained and kept a 32 truck fleet of 1980-1998 vintage 600 series Macks running in the Alberta patch for seven years...can tell you pretty much anything about them you want to know.
They are tough, kind of crude in some aspects, but will take more **** and abuse than pretty much anything else out there. I'd buy one no question except I have issues sitting in them for extended periods due to leg room, the cab is not really made for tall people. Relocating the seat so you can jam it up against the back of the cab helps.
Not sure if its still there or not, but we actually donated a twisted input shaft that Nait used to have on display, thats how tough that 12 speed is.
Many driver issues we run into regarding power complaints and more were direct result of the truck not being driven "as a Mack". They need to be wound out and shifted properly to produce power. Once you learn how that tranny shifts, its actually quite simple. The two sticks was an option for low range, normally everything was all done with the one. Reverse is shifted into by selecting it with the knob. PTO is run by selecting the neutral position and then selecting a gear suitable for whatever speed you require. Some had the PTO run off the rear of the counter shaft for high HP.
IMO biggest area of concern with the driveline is the peanuts in the power divider, its normally what fails when they let go, but usually it is due to wear from high usage.
Different suspensions were available, standard was camel back, if components are wore, some axle alignment issues can result and you will get a lot of clunking and banging while driving especially when empty, caused by slop in the spring caps where they bolt to the housings or the trunnion bushings also. Torque arm bushings are another high wear point.
Check the frame rails for a few feet behind cab for cracks along withe ends of the mid frame crossmembers. Also in the suspension area where torque arms attach. Also a sign of how hard its been rode, check the lower front of the cab behind the engine in the middle for cracks in the sheet metal, also check if the dash frame is all tight. Hood hinges are another telltale area.
If you want to run in in winter, you need an auxiliary heater, but most have had that already installed by this time.
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