JoeH

Clogged fuel filter symptoms are exactly that. Your truck acts like the governor is kicking in at a lower RPM than you remember.

Change your fuel filters. Primary and secondary.

Few more odds and ends, including painting the cab then this truck will be driving to Iowa! Hope the top speed isn't 50 mph, we've never had this thing on the road, lol!

That frame doesn't look very happy. Do you plan on straightening it?

Engine picture is a 1981 ENDT676, quietly waiting for the next time our 1979 needs another engine component. So far it's donated it's camshaft and a lifter tappet and the air to air intake housing. And some valve lash adjusters. Dealer wanted $80 each and only had 3, couldn't get more.

Matt's Heavy Duty Mobile Diagnostics in Sellersville, PA came out this afternoon. Tech (Cole) did a good job sifting through. Scanner immediately showed no power to the EECU. He managed to find the EECU power relay in the dash with no fuse/relay diagram, and found spread connectors for the related fuse. Truck is up and running! I think all we have left is to mount and plumb up air tanks and move the battery box as it interferes with the mixer peripherals.

Could be your pump/pressure relief is failing. We have an old mack truck here with no power steering. You can't turn the wheel for shit unless you're moving. If you hit a bump it rips the wheel out of your hands.

Yea, he's probably tight up on money. Sounds like piston rings or worn valves. If you give him 5k down and he uses it to "fix" the truck and you decide you don't want it: good luck getting your 5k back.

I have 2001 in my head as the ideal year for a Mack motor. Rebuilt engine in the last three years? If the frame is clean I'd take it if the wiring harnesses look clean. 2001 is the first year they moved the engine computer to the left side, away from heat and possible pinch against frame rail. Emissions BS with the ASET engines started sneaking into the trucks in 2003 and was in all 2004 trucks.

I'd talk to someone in your state about axle weight configurations. In PA we get away with 73,280 on steer, lift, and tandem. Other states require 4 or 5 rear axles to hit that much weight. Don't know what Arkansas allows. Pull up and ask a state trooper next time you see one! As far as being caught overweight on one axle, DOT enforcement has portable scales they'll use to weigh each wheel. If you're moving dirt on site you don't have to worry about "legal" weights, Mack rates their 44k rears for more if kept below a certain speed. If you're going down the road it's another story.

Lift axle bolted on. Have the trans power sorted out, but now we have a blink code 6-4, cranks but won't start. Mobile diagnostic guy comes out on Wednesday to try and figure it out. Yea, we're planning on driving this sucker out to Iowa (from Philadelphia area) by the end of this month... Call us crazy. I'm not looking forward to a probable breakdown on the 20 hour drive...

If I had a 1908 I don't think I'd be driving at night, I'd be too afraid of hitting a deer!

Axle Ratio is 5.55, so that will be nice with that trans/engine. Frame and underside of the bed look immaculate short of being new (bear in mind a halfway decent used truck up in the northeast has the frame rails spreading up to 1/2 inch from rust...) Hard to really tell frame rust from the pictures, but it looks good. Just needs a lift axle to take it to PA's 73,280 maximum legal weight for triaxles and it'd sell quick up here!

Not sure how familiar you are with Mack model numbers, but a 688 is a 350 hp standard fuel setup with about a 400rpm powerband. A 690 is a 300 hp Maxidyne that makes 90%+ of it's peak torque from about 900-1100 rpms all the way to the governor. Hence the 690 gets away with a 5 to 7 speed trans. You simply don't need in between gears because you're engine pulls so far down. An engine brake is perfect on the 690 for shifting up hills, cuts the shift time down to 1/3.

It's never cost prohibitive due to rears and trans. You can have a cutoff from a junk yard for 2k with good differentials. Rebuilt 8LL is 3-5k I think plus install. What you save on truck cost will far outweigh those "incidental" costs you might incur in 5 years. If you can pick up a mid 90's mechanical e7 then you are into about the best Mack possible. No aging computer and related wiring harness. I have a 1995 RD688S with 272k miles and 22,000 hours. In the 7-8k hours we've owned it we had the valve covers off once just to check adjustment. And replaced the oil pan as the old one had fatigue cracks. And the truck got a clutch. Engine brake is a joke but I wouldnt trade the truck for anything.

Mileage never really bothers me. Frame condition does. I've seen a couple otherwise perfect trucks have horizontal flex cracks from where the dump body rests at the very front. Rust Jacking is the other big problem. There's always spare engines and transmissions to be had. But you have to have good bones to put it in.

Fuse wires at the battery box never got hooked up after frame reinstall. Now I have a Crank No Start with blink code 6-4. Need to delve into voltage fluctuations under start conditions for the EECU and VECU... Apparently it's an issue, and there's a service procedure to add supplemental power from the starter relay to the 3 EECU 12volt inputs.

Great info right here; I'm chasing a crank/no start code 6-4 problem on a new to us truck. Can you explain what the intersection letters mean exactly?

As with all things mechanical, parts can fail. If you have a noise coming from the rear you could have a bearing failing. We had one go about 10 years ago. Ate a few teeth off the ring gear so we dropped a pair of differentials that gave us about 10mph more speed. (Previous top speed was 48mph floored. You could slide into first gear from a stop with no clutch.)

Clogged fuel filter? New problem? Symptoms surrounding the problem's start?

Truck should have 3 Torque Arms, one from the top of each axle tying to the trunnion stand cross member, and the third Torque Arm goes from the rear drive axle to the right frame rail. The joints of the torque arms wear out, expect to replace one or two of them.

Camelback suspension is about the best you can have for offroading. Provides perfect weight distribution to all 4 wheels at all points of articulation, regardless of how fast you're going over rough terrain. Air suspension articulation requires time for air to vent from one bag and the truck to fill the other bags. By the time it's done that you're three bumps ahead of where you were or you're stuck.

I'll have to sift through all the wiring to find it... Currently I'm looking for the "VIM" as Allison calls it: "Vehicle Interface Module", a series of ~6 relays, a couple 10a fuses, and a couple other odds and ends. Probably need to just grab a 10a fuse and start swapping it for 10a circuit breakers to see if I get a positive result....

After a day of beating my head against the wall, I have determined that Pins 1 and 16 on the "grey" "V" plug on the Allison ECU don't have power, when they are supposed to. Allison wiring schematic shows a 10a fuse, but it's on the Vehicle side and I don't have a diagram showing me where this fuse is located.... I'd love a fuse location diagram for a 2003 MR688S if anyone has access to one... Or could provide any insight as to where fuses/breakers are for the auto trans...

Thumb is pointing at cab gauge oil signal. Index finger is pointing at computer oil pressure sensor. So yes, 2 separate oil signals. (This is a 2003 ai350. Forget what yours is.)