macattack

does this have air controlled throttle? mine has a auto trans with air throttle control. my torque converter locked up on me and i couldnt move at all. bad modulator valve. replaced and she is fine.

thanks for all the info gang. i actually never got to drive it with the old tires, but i know they were alot narrower tires on the front.

i am gonna swap out those hercules and have the toe in checked. see what gives.

thanks again gang.

thanks guys.

would an out of round tire smooth out after 45 mph? mine shakes the truck between 40-45 mph after 45 its fine. i would think an out of round tire would always be shaking??

or is my thinking wrong.

just wanting to know if any other mr688s owners know if they have one or two spring wedges. one per front side?

mine only has one and i am thinking the original owner had some spring work done on the pass side and didnt replace the wedge on that side. should the wedge be installed thick to back axle or to front bumper?

my question is if my caster was out of adjustment because of one wedge would this cause my truck to wobble from 40-45 and after 45 she rides nice. feels like a flat spot on my front tire. but there brand new and checked out fine. i have beads installed and that helped but not a 100%. dont know if it s the cheap made in china HERCULES 425 65r 22.5 fronts or the one wedge misssing.

also if i needed a caster adj would my steering wheel shake. because its smooth. happens when coasting also.

thanks

i have much the same problem on my mr688s. around 40-45 mph. starting to think i either need an alignment or maybe replace my front steers with a higher quality tire. i have hercules 425 65r 22.5 fronts. maybe i should try a goodyear or michelin?

did alot of work to try and solve this. replaced springs, pads, bushings, torque and transverse rods, tie rods hubs etc....... all new tires fronts and rears. all with balance beads.

dont know if its common with the mr models as well. as stated before by other poster . no shake in the steering wheel at all. i can remove my hands and she will drive straight. just a wobble feel at 40-45 then goes away. does the same coasting.

did notice that i have only one spring wedge on the driver side front and none on the pass side front. maybe an alignment would be my best bet,

what do you guys think??

thanks i did not realize this.

Well I was doing some work on my dm and notice the front wheel seal was leaking so I jacked it up to check the wheel bearing for movement when I stuck a bar under the wheel to move it it moved a lot, so after looking at it while a old friend of mine moved the bar I found the movement was not in the wheel it was the king pins, so at this time I was throwing my hands up and thanking I was going to have to spend 3 or 4 hundred dollars on king pins and bushings when a older guy I know who has truck for years told me that macks have taperd king pins and they could be adjusted to take the slack out of them, he told me how to do it and also said that you had to whacth it because you could get them to tight. In about a hour it was good as new, I think all trucks should be this way it is a good idea

how did you tightened up the king pins? also could you explain how you would tighten up the wheel bearing.

thanks. good to know you have it fixed.

those step by step pics would be GREAT!!!

Thank you all for your input. after reading thomastractorsvc's reply over and over and over i looked into it(the driveshaft). I did some research about driveshafts and found there are different types of setting. So after reading page after page of info i went back to look at the truck again and,THERE IT WAS! The front rear was set back about half to three quaters of an inch on the drivers side. off setting too imaginary parallel lines from the trans and the front rear and off setting the driveshaft. when i put the new pads in under and over the spring they sat in differently than any of the others they looked as if they didnt go all the way in but i still managed to get it back together easy but the upper looked as if it was going to squeez itself out and i didnt think anything of it. So after reading all about drivshafts and having had the problem in the past with another truck i had but a different situation it seemed familiar. so i measured from the spring at the tee to the spring seat WOOO WHOO! so we took it apart the torque arm, driveshaft, and the spring seat. With the puller we moved it foward using the lift axle as an anchor. so we put it back together and so far so good... Thank you guys for all your advice, especially Thomastractorsvc!

so your saying it was your insulator pads in the front rear axle. did you replace all the rubber pads?

thanks

thanks guys, for all your ideas i will look at some of them in the a.m. at least i have a place to start

i have been fighting a similar problem. replaced and retorqued a bunch of things. let us know what you find out. i think there are a few out there with similar problems.

good luck.

thanks. just want to make sure.

i got a e7 300 mack mr688s with a allison 740rs transmisson.

how many mounts are there for the engine alone. i noticed there is on the front of the engine and a mount on each side of the tranny.

should there not be mounts on each side of the engine??

thanks

ok. i understand. i will see what i can do.

thanks

thanks again. i will put more air in the rears and see what happens. i may be able to get the rear end lifted up this weekend. i am gonna try to run it at the issue speed with the rears up to see what the heck is going on. being in a smaller town a guy does not always have quick access to the right equipment.

thanks

thanks 84superdog. this wobble thing has got me. i will explain it a little better.

accelerating up to 35 mph everything is fine. between 35-40 a wobble begins and lasts for only a very short time. it does seem to be worse rite before the final shift from 3rd to 4th. once in 4th everything is fine. almost feels like the driveshaft is whipping. if that makes sense.

driving the truck my steering wheel does not shake or vibrate. you can steer like its new.

when i go downhill in gear and let the truck coast the wobble shows up and then dissappears once i get to 35mph. wobble lasts for about 5mph.

it actually feels like someone is rocking the cab between 35-40mph. if i stay hard on the throttle wobble is still there but not as bad.

i thought it may be my transverse rod (one from the rear axle to the frame), but it seems fine. i had much the same wobble issue on my other truck (international) and my transverse rod was shot, but this one seems to be fine.

the truck has the large 425 in front and 12r22.5 in the rears. i have only 90lbs in the rears maybe i should up them to 110 or so??

i should also mention that this truck is equipped with air throttle. found this out when my modulator valve screwed up.

the throttle is controlled by a small motor connected to the linkage on the pump. also since it is air operated, the mod valve is a pneumatic type. anyway may have something to do with the way it shifts?? just a thought.

thanks for your expertise.

more air = stiffer ride??

thanks guys. yah still trying to isolate that wobble. happens at around 35-40 and after that fine. the truck has an ht740rs allison trans. it seems that just before it shifts into 4th is where the wobble begins. after it shifts everything seems fine. i checked the trans filters for debris and found nothing. oil looks brand new. the wobble also shows up when coasting back from hiway speeds. if i let it coast downhill with no acceleration it begins to wobble from 35-40?? once its passed it rides great.

changed and checked alot of things still hunting.

thanks david.

just wanting to know if these lug nuts look correct.

should they not be nuts with washers?

thanks

i believe the sidewall says 120 psi. i think my tireman put in 100.

got some balance beads put in my front 425 65r 22.5 steers. definately helped out my bounce wobble, but its still there.

the fellow at the tire shop said they put in 12oz in each,

i think maybe the4y need more. i read should be around 16 to even as hi as 20.

anybody with the same tires run the cbb beads.

also what do you think the tire pressure should be on this big tires?

thanks

had this happen on my old cummins 240. oil cooler was shot. suck all my antifreeze into the motor oil. replaced the oil cooler and about 6 months later BANG. spun a bearing.

no i wish there was. i guess i could jack up both rears and take it up to the speed and see from there. just a little chicken shit to try you know. anyways if i remember the wobble still seemed to be there when i put the truck in neutral and coasted. if that says anything.

thanks for your help.

if the brake drum is out of round would i not see it when the tire is turning?

do you mean front drive axle?

checked my tires today. they all seem pretty good. jacked up the back and rotated each by hand. no issues there.

yah this has been keeping me busy. dont really know what to check next.

my wobble happens at around 35- 40 mph and after that fine. when i slow back down to 35-40 its there again.

the steering wheel does not shake, its like the whole truck rocks side to side between 35-40.

now at times its seems if i creep up to 35-40 and hold it for a while just before the wobble it does not seems as bad. weight doesnt seem to make any difference.

i also noticed that it seems to happen just before the last shift on my trans. its an allison 740rs automatic. it feels like maybe my driveshaft may be whipping or something. all new u joints and the center bearing seems fine. some movement in the rubber but no more than my other units. i also installed a set of timbrens on the back camelback suspension. it helps for road shock, but did nothing for the wobble?

what now? i guess i will keep checking.

does anybody think this may be a power divider issue? i also checked my tranny and motor mounts. they seem good.

any info on what i can check next would be appreciated.

thanks