JoeH

Buying a 2003 MR688 with an AI350 and an Allison auto. What are my options on engine brakes? Can't find Mack on PacBrake website, an exhaust pipe brake is by far easiest to install.

How many miles/hours? Head gasket could be blown or cylinder sleeves could be pitted enough from cavitation (if they're wet sleeves, not dry) to bleed combustion air into the coolant system. Either scenario puts air into the coolant, which pushes coolant out the overflow discharge hose. Or you have EGR issues.

Our old trucks work every day, we put Mack dealer parts in them, but some guys like PAI aftermarket Mack parts. Haven't tried them myself. We have a '70 2 valve that dropped a valve once. We got away with one head and one cylinder/liner kit.

I suspect we'll never hear from you again....

Looking at buying a used mr688/690 in the 2001-2003 year range. Allison auto 4560. Application will be a volumetric concrete mixer, about a 15 mile radius from our yard. What should I be looking for? One I'm looking at has 484,000 miles, 19,000 hours. What's the life expectancy on EUP alignment dowels?

Picture? And help with what?

I had a blowout on a rear drive tire last week I couldn't drive on as the alligator was still half attached. Was flapping around doing damage to the back of my truck body. Had to have a pair of tires brought to me from the shop. Way easier to tighten 6 spoke lugnuts than 10 Budd lugnuts with a cross wrench.

To answer my own question, here's dimensions of the MRU cab on top, and LEU on the bottom.

does this picture exist for the MR600 series?

Looking for dimensional drawings for Mack MR600 series cab/chassis. Cab/axle, wheelbase, etc. Ultimately looking for a mid 90's to 2001 triaxle MR with a cab/axle of 168", but don't know what that will translate to in wheelbase because I can't find drawings!

VIN will tell you real easily, assuming it has original engine. Dm685xxxxxx is an endt685, dm686xxxxxx is an endt676. Former is the 237, latter is the 283hp motor.

Probably ENDT675

Are you flipping the headlight switch on the dash up or down? Up is headlights, down is clearance lights, middle is off. The switch up on the left of the dash is a high beam momentary switch. It doesn't activate low beams. If you're flipping your switches properly then start with putting new bulbs in and checking the wiring as mentioned above. You can lose a low beam filament without losing the high beam filament in a bulb.

I've driven oil and propane single axle autos, the exhaust brake will throw you through the windshield. The feedback I heard years ago is that they suck off-road, because the torque converter doesn't lock up at low rpm, so you sit there winding up the engine trying to get enough friction built up in the TC to move the truck through soft ground or over obstacles like curbs. The other day on a job I had to back my right drive tires on my triaxle onto 6 inch blocks to keep from tipping over in a soft spot. Do the older autos lend themselves well to this sort of maneuver?

Is that the new stuff or pretty much all Allison's?

Looking into using an MR688 or 690 (mid '90s fully mechanical hopefully) to mount a concrete mixer (volumetric type, not barrel) on. Most are automatic, but the few that I glanced at look like the auto trans has very little ground clearance. Does anyone have any experience with MR's? We hop curbs once in a while, back into people's yards, the usual things Macks are supposed to do. How do the automatics perform in these conditions? Do they have a good torque converter lockup to be able to hop over curbs or soft ground without much fuss? Chances are an auto is a terrible fit for us. Ideally I'll find a gem e7-350 with an 8LL, but I'm curious about the automatics.

Short wheelbase trucks tend to "buck" like a bull rider more noticably, and with how far off the ground you are you feel it more than you would in different wheelbase cars/pickups.

Citing low back pain, get something with light duty springs and a longer wheelbase, plus an air ride cab is a must. Not sure when they started, but Mack offered an option on the R model where the rear cab mount is air bags. Night and day difference on operator back pain. You can convert any truck to single axle air ride, would likely give you a much better ride quality than traditional springs.

What kinda hobby truck? Hobby dump truck for picking up your own mulch? Hobby rollback to support an existing automotive hobby? 5th wheel for towing some old John Deere tractors? The possibilities are endless. A bit of mechanical know-how is important, and so are some big wrenches! As with all things metal, rust is the biggest enemy to watch out for. Solid bones are important.

The Go Pedal looks unused...

Unfamiliar with the cat cement. Proper procedure is to clean everything down, wipe it dry, position gasket, place Jake spacer on, position next gasket, make sure gaskets are properly positioned, set valve cover on, take the three bolts to the torque setting in the book.

No, you take them to the torque setting in the book, no more.

We probably have 100k hours on the one truck, 1979 R686ST, with the ENDT676 motor. Still working 5-10 hours a day.

We've been using the rubber ones through our mack dealer for a long time, they work fine. The key is don't use anything on them and do not overtorque the bolts. Do not use silicone, do not use high tack. They lubricate the gasket enough so it can shift and slide. Also, overtorquing the bolts distorts the valve cover, making it leak. We have had to straighten ours out a number of times over the years until we figured out the tricks to seal them up nice.

Reason to keep that lift axle: it's an extra set of brakes.