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Discussion Starter #1
I'm having problems with my air trac suspension. I've had problems with a driveline vibration for awhile now.

Here is the first place I read about ride height being a common cause of vibration. All of my driveline was worn out so I had it rebuilt. Both shafts had new slips welded in and all new u joints. It's a day cab with 186" wheelbase, so no carrier bearing.

I replaced the leveling valve and linkage, and set the ride height. It was way too low before. I also replaced the wear pads and roller bushings on the rear axle. Ran out of time and gas for the torch before doing the front. Not perfect, but totally different truck after this. Almost all better. I chalked up the little bit of remaining vibe to worn out suspension bushings. That was probably correct but not sure.

Then the frame cracked badly. I had it professionally repaired and it has held up well. But after that, vibration was worse.

Kept feeling vibe and then my clutch was way grabby and something didn't seem right. I had just had the clutch replaced around 8k miles ago. Did a visual check, trans about to fall out. The rear support was completely loose. Only 4 bolts going from trans to clutch housing. Only 3 on the bottom were tight. 5 have the heads busted off. Put in 2 missing bolts and tightened all the others. Replaced bolts for the rear support and got it tightened up all the way. Noticed that the output bearing in the tranny has some play. Vibe still there after securing trans.

Blew 2 air bags within a couple weeks of each other because the tops rusted and the bolts broke out. Bobtailed home each time to replace. All of the weight rested on the bag on the opposite side of course. This leads to where I'm at now.

The pedestals where the bags mount appear to be pushed down towards the ground. They are uneven. To get even close to the correct ride height, the bags are almost fully extended. There is 2.5" between the highest and the lowest pedestal measuring from the ground to the bottom of the pedestals. Even the one furthest off the ground seems too low. I can't tell exactly where the problem is. Do I need to replace all the u bolts and add shins? The rear axle seems to point upwards a good bit more than the front. It looks like this is intended with the way the brackets are made but not sure?

A couple interesting things about the vibe. I can drive out of it if I drive fast enough. If I go over bumps while coasting it will stop, also.

The truck is a 98 Pete 378 day cab, DDEC III Detroit, Eaton 15, 44k air trac, 46k Rockwell full locking rears, w/ 4.56 gears, 11R24.5's.

I can get pics if that will be helpful. Sorry for the novel, but I wanted to be thorough. Without the proper info, I know you won't be able to help! Hoping 379Longhood and Sledman will chime in. I'm really close to Sled in Lexington, NC.

Thank you in advance!
 

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Sure dont sound like you are measuring your ride hight correctly.
The bags should not be fully extended or even close to that at proper ride hight.
Nor should you be measuring from the ground to the pedestal.
Take it to a axle and frame shop to have it looked at .
Just dont take it back to the same people who put your clutch in because they clearly dont know how to do anything right.
 

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The correct way to measure ride hieght and I don't care what anyone says is from center of axle tube to bottem of frame rail, I have the same problem, all my bags are way off from the frame being bowed and bent real bad, used to be a tow truck and was abused, can't even align the truck, had to kick the *** end 3/8 inch to the ditch to go down the road straight, now it vibrates so bad I can't stand driving it. This is after I rebuilt front end and all rear suspension parts, two sets off front wheels, three sets of Michilen steers, new Michelin drives, merchury balencers, steering box, wheel bearing, ect front to back, get your ride hieght correct is all I can tell you
 

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Discussion Starter #5
Thank you all.

I don't *think* the driveshaft is bent, but I guess it could be? Maybe happened when the tranny was hanging down?

I know it doesn't sound like I'm measuring ride height correctly, but I am. I'm doing like TIZDEEP says to. I can't remember the spec I used but I'll find it. I could be a little off with that.

You are right that the bags shouldn't be anywhere close to fully extended at proper ride height. That is my problem and why I'm here. That's why I'm measuring from the pedestals to the ground, which I know is a strange thing to do.

I definitely agree about not taking it to the same shop that did the clutch work. They did the frame repair also. They have a great reputation, but whoever worked on my truck wasn't doing a good job. When they did the frame they didn't put my battery cables back in the frame rail. They zip tied them to my wetlines and called it good. Also had an audible air leak when I got it and an air line laying on my exhaust elbow in plain view. I could go on and on with other things but won't waste your time.

The output from the front rear going to the rear rear has a little bit of play, but doesn't seem like a lot. The vibe doesn't seem to be as bad until things get warmed up. Maybe I do a rear end going out?

Thanks for the help so far and I'll keep everything updated. If/when I get it fixed, I'll post it up.
 

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What are you using for a height measurement? On problem trucks I prefer to use the angles of the diffs for correct ride height. Eaton Driveline Angle Analyzer is a great help to determine if the angles are too harsh. If you give me the last six of your s/n I can probably get the factory diff angles.
How about some pics of the air bag mounts? Frame repair straight and true?
 

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I used to have an 83 359 that always had a touch of vibration. I think that pushbroom may have the answer, as on mine there was wear where the leaf wore on the saddle (suspension mount on frame. I never did solve the problem as I traded that truck off, but always wondered if that wear (which would change the axle housing angle) was causing the vibration.
 

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Air trac ride height is 11.00 inches centerline of the axle to the bottom of the frame, average between axles should be within 1/2 in. Check to make sure that your tires are inflated equally and that you axle cross locks are fully disengaging. also make sure that your driveshafts are in phase, ie yolks on the same plane.
 
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Discussion Starter #9
The frame repair appears to be straight and true. I'll put a straight edge on it to be sure.

Last 6 of the VIN:454270

I'm using 11.375 inches for my spec on the ride height. I did it as outlined in this video: https://youtu.be/be4YKaPp7gA





 

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Discussion Starter #10 (Edited)
Seeing 379Longhood's response, I've got it set 3/8" too high. I'll bring it down and recheck it. Is this the unladen spec?

Also, are you telling me to check the power divider or full lockers to make sure they're unlocked?

The thing about it being a little better till it gets warmed up may have been in my head.

How is the Eaton analyzer different from a normal angle finder? If you're able to find the spec, will a normal angle finder be able to get me close enough?

The best I can tell, my shafts are all in phase. For some reason I had never heard of that before reading on here. But I did check it when I found that info.
 

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The height shouldn't change laden or unladen, I also don't like the top of the tire to ground method, too many variables. I like to measure from the axle tube to the bottom of the frame near the axle. Compared to my tractor yours appears to be too high from the pictures, but pictures can be deceiving

Peterbilt seems to have changed the height spec lately, maybe different thickness of components??
 
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Discussion Starter #12
Yes, my bags are extended too far. That's one of my concerns. Bringing it down 3/8" will help, but I don't think it will be enough. Look at how different the left and right bags are on the front axle. It's wacky.

Here is a coworkers '95 379 w/air trac. All of his are nice and even. The rear are a little taller than the front, but I think that's correct.



 

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The truck left the factory with 4 degrees on the front diff and 11 on the rear. Pete has changed the air ride requirements recently to add laden and unladen I suppose to take up the delay in the leveling valve. If you cannot get the angles correct without having a crazy high or low ride height, the rear axles will need to be shimmed to get the proper angles at the correct height. Eaton DAA is just a program you input the driveline angles of the truck and it will tell you if it will vibrate or not.
 

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Discussion Starter #14
Thank you pushbroom!

I will set the ride height to the 11" spec and check the angles tomorrow. I'm looking for both of the front shaft angles to be 4 degrees and both of the rear to be 11 degrees, correct? I know that the front and rear angles on a driveshaft are normally the same so they cancel each other out. I just want to be sure I will be doing this right.
 

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Alright, started out today with popping by Harbor Freight to grab a digital angle finder. I had the dial type, but the paper backing with the numbers had started to peel up and the needle would stick in those places.

Got back to the shop and adjusted the ride height to the 11" spec. I realized that I hadn't measured right in between the tandems before so to get it right I brought it down a half inch.

Then I used the angle finder on the diffs. Rear is at 12 degrees, front is 4.1 degrees. The front still needs wear pads and roller bushings and I think this will make my angles a little further from spec.

Took it out for a test drive and it was MUCH better. Still not perfect, but I'm going to continue to rebuild the rear suspension.

So, basically ride height strikes again. Thank you everyone for the help. I plan to keep this thread going as I continue to work through the rest of my vibration.
 

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Wow, amazing that a half inch would make that much difference.
Glad to hear that it's much better. :thumbsup
 

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Actually, looks like I spoke too soon. I tested it bobtail yesterday. Under load today, it seems about the same. Maybe a little better. I'll continue to work through it.
 

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the rear diff angle is going to have more effect, due to the lateral miss alignment factor. you would think that the axle manufacturers could have done a better job of aligning the input with the output.
 
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So when I rebuilt my rear suspension I replaced the crossmember that the torque arms bolt to, on mine it's an ag 100 suspension, vin is 574594 92 Kenworth, could you look my angle up, it did not vibrate this bad before messing with it, on pretty shur it's not right I just threw the spacers that seemed to go in the front/ back
 
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