Thoughts? Mack RD688 - The Combine Forum
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post #1 of 26 (permalink) Old 11-08-2019, 12:04 PM Thread Starter
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Thoughts? Mack RD688

Been keeping my eye open for a truck for a few years now to put the box onto from our old Bruin. It needs to have a long frame as it's a 22 foot box. Came across a '94 Mack RD688 I believe it is the other day. Has the E7 @ 350 horse, 4.64 rears, 18,000 front axle and 44,000 rears. Looks to be in decent shape, few little things but nothing I can see too bad. No oil leaks anywhere. Biggest question I guess is about the transmission. It has a 12 speed air shift. What are they like? Just has the one shift lever, not 2 like some did. Never drove one like this before, might give it a test drive next week. It's no Cadillac by any means, rugged work horse, completely mechanical. Just going to be for a second truck/backup/yard truck. Anyone have one or used one before? Not sure the actual hours or miles, but sounds like it had a somewhat easy life as a potable water hauler for a camp out west then a construction company in Manitoba bought it thinking it was for spreading water on jobs sites so never used it. Whether that's the truth, I can't confirm.


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post #2 of 26 (permalink) Old 11-08-2019, 12:25 PM
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Cousin bought one new and loved it, i understand its a five speed with a deep low and every gear splits. Also have reverse in every gear.

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post #3 of 26 (permalink) Old 11-08-2019, 12:53 PM
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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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post #4 of 26 (permalink) Old 11-08-2019, 01:47 PM
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Like albertabuck said, how tall are you?
If just for a yard truck, shouldn't be a huge issue.
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post #5 of 26 (permalink) Old 11-08-2019, 02:39 PM Thread Starter
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I'm 6'2". When I sat in it there seemed to be a fair bit of room. If you have ever sat in a GMC Bruin with a Detroit, anything would seem to have lots of room! Whoever designed the Bruins must have had skinny feet, only one leg, and no neck.
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It's easy enough to be pleasant, when life goes by like a song. But the man worth while, is the man with a smile, when everything goes dead wrong!
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post #6 of 26 (permalink) Old 11-08-2019, 02:42 PM
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Originally Posted by kevlar View Post
I'm 6'2". When I sat in it there seemed to be a fair bit of room. If you have ever sat in a GMC Bruin with a Detroit, anything would seem to have lots of room! Whoever designed the Bruins must have had skinny feet, only one leg, and no neck.
We have a brigadier.....they were likely only 5’ 2” also. I have drove a few old Mack’s and they are like a Cadillac compared to them.
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post #7 of 26 (permalink) Old 11-08-2019, 02:51 PM Thread Starter
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We have a brigadier.....they were likely only 5’ 2” also. I have drove a few old Mack’s and they are like a Cadillac compared to them.

Perfect! So you are probably looking for a good, used parts truck!!!! '81 GMC Bruin 8v71 Detroit, 8 of the 10 tires hold air, no rust what so ever anywhere near the motor as it has a healthy coat of oil, comes with a box of old style fuel cartridges and box of ear plugs. No box.

It's easy enough to be pleasant, when life goes by like a song. But the man worth while, is the man with a smile, when everything goes dead wrong!
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post #8 of 26 (permalink) Old 11-08-2019, 03:00 PM
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Ours has the 6V92 and it only gets about 20 hours and about 200km a year on it hauling silage
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post #9 of 26 (permalink) Old 11-08-2019, 05:26 PM
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-, I’ve got the Big dog , silver 8v92 in a Kenworth... it’s the 2nd best screaming known too Man !!!
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post #10 of 26 (permalink) Old 11-08-2019, 05:29 PM
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But a old R Mack like that is bullet proof , that E7 350 is a great engine , easy on fuel as well ..

U can’t get a Mack like that anymore ..

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