1994 Peterbilt Dump Truck Questions

We recently purchased this truck to work on and off our ranch. It’s rated 46000 GWV, 13’ bed, Tandem axles, CAT C7 engine, Air spring suspension, 10speed transmission.

I have some questions about it.

1) When driving down the road empty (never carried a load yet) it makes a “thrumming” sound. The frequency of the sound changes with the speed of the vehicle. The tires are in fair shape, tread depth is around 5/32”. My brother thinks it’s because the tires don’t have all the exact air pressure. The pressure ranges between 85-95 psi in the different tires. Is this the problem, is something else wrong, or it’s just the way these trucks sound?

2) When shifting and I double clutch, it is very hard to get it go into the next gear, particularly downshifting. The road and engine speed has to be perfect before I can get it in gear. I actually find it easier to clutch to neutral and then push it into the next gear (without grinding) when the engine syncs up to the road speed. I help on the ranch on weekends so I am new to double clutching, but all the videos that I have watched on double clutching, this issue doesn’t seem to be an issue. I want to make sure that it’s me and not the transmission that needs work.

Thanks

Can't help you with the sound, is it possibly just tire noise. Do you have traction type tires? And where is the noise coming from, front, back, etc?

As for shifting no matter which way you do it you have to get road speed, engine rpms synchronized. So that means when downshifting tapping on the accelerator to get engine rpms up.

The only times I use the clutch is when starting, stopping, and to break torque when shifting on a steep hill with a heavy load.

And some transmissions are just what I call notchy, a little more difficult to shift. We have a 1989 Peterbuilt dump with Detroit 60 series 475hp and 13 speed, and a 2006 KW dump truck with Cat 475 and 18 speed, and a 1976 International boom truck with 290 Cummins and 10 speed. The Peterbilt tranny was replaced with a rebuilt a few years back and was notchy before and after. But the KW tranny is smooth as butter, have never driven a tranny this smooth and have driven a lot of other heavy trucks in my life even 5 and 4's back in the old days.


Forgot to add:
Do you have a air dump valve in that truck to dump the air out of the air bags?

If it doesn't add one. It is better when you are dumping a load to get the air out of the suspension, the truck will be more stable and won't be as likely to tip over. Also make sure you are fairly level when dumping.

I believe they are traction tires, they are for on/off road work. The front tires are the straight groves designed for long haul. The noise sounds like it's coming from the back.

There is a dump valve for the air suspension.

The noise could be from flat spots on the tires? Have you inspected them all the way around. I drove for several years. Only used the clutch for starting or stopping. I never could shift the truck with a clutch and had driven several trucks. JJ

Check your driveline for play, I had a drone in my 99 turned out being the spline shaft between the drives! Took to a machine shop and welded an new spline in for half the cost!

noise

Ranch Hand said:

We recently purchased this truck to work on and off our ranch. It’s rated 46000 GWV, 13’ bed, Tandem axles, CAT C7 engine, Air spring suspension, 10speed transmission.

I have some questions about it.

1) When driving down the road empty (never carried a load yet) it makes a “thrumming” sound. The frequency of the sound changes with the speed of the vehicle. The tires are in fair shape, tread depth is around 5/32”. My brother thinks it’s because the tires don’t have all the exact air pressure. The pressure ranges between 85-95 psi in the different tires. Is this the problem, is something else wrong, or it’s just the way these trucks sound?

2) When shifting and I double clutch, it is very hard to get it go into the next gear, particularly downshifting. The road and engine speed has to be perfect before I can get it in gear. I actually find it easier to clutch to neutral and then push it into the next gear (without grinding) when the engine syncs up to the road speed. I help on the ranch on weekends so I am new to double clutching, but all the videos that I have watched on double clutching, this issue doesn’t seem to be an issue. I want to make sure that it’s me and not the transmission that needs work.

Thanks

Click to expand...

Lots of good advise, I agree with all of them. If that info doesnt work, jack up both rear axles and put it in gear and see if you hear the noise in the rear axle, then lock the front diff in and see if you hear it then. This would most likely prove whether the bearings are OK in the rears. You can also check the driveline this way.
Could be a bad ujoint

Shifting, on some trucks, it really takes a lot of practice, with the truck. Most trucks I have run across, that had bad/hard shifting problems, had bad synchronizes, or it took a shift kit (installed with tranny installed) is there a lot of slop in the shifter?

I would check the truck out good and find that noise, it may not be nothing but a bad tire, but it could be more serious than that.

Keep us posted

Thanks for all the info. I'll keep you posted to what I find out, might be a while posting again, but I won't forget.

Willis Bushogin said:

Lots of good advise, I agree with all of them. If that info doesnt work, jack up both rear axles and put it in gear and see if you hear the noise in the rear axle, then lock the front diff in and see if you hear it then. This would most likely prove whether the bearings are OK in the rears. You can also check the driveline this way.
Could be a bad ujoint.

Click to expand...

Be sure that your wheels have stopped turning before you engage the lock or you could do some damage.Ron G

Is the humm your hearing with a little vibration ? if it is you might have your air spring adjusted a little too high ,this makes the noise and vibration at the transmission ,is reflected through the drive shaft .

Can you post what kind of truck and model ,if it is dump, flat or what else , if it was a road tractor converted to a dump ,this could be the culprit ,if this is the case is easy to fix , just adjust the ride hight valve, you could do it your self in a few minutes.

I hope this is it. Good luck.

you might have a bad u-joint. i dont think it will be the tire psi. One tire could be seperating or one of your brake drums could be egg shaped causeing it to rub.
jack it up and spin the tires and check them good

as for shifting it is just you. dont dubble clutch it unless you are pulling a hill loaded
push the clutch in, put it in neutral let the clutch out then put it in gear. do this untill you get better then dont use the clutch at all

I find shifting those small cats soooooo much easier with no clutch.

2109 Stang – there is some vibration with the noise. It is a dump truck conversion, from what I don’t remember what the seller told me. I’ll check the air springs. I have the manual from Peterbilt (from their website) that describes how to adjust the air springs.

I got the truck (almost) stuck on the ranch this weekend after the rain (central Texas). I can confirm that the differential lock does work, both axles where spinning through the grass while my brother watched. The tires have almost no tread on them so driving through grass makes it very slick. I have some new retreads that came with the truck so I’m going to swap those out. If it is a separating tread then this should resolve it.

:update
I did notice one bad thing on the truck, one of the dump rails is cracked through the center of the rail for about 5 feet. It looks pretty bad. The question is (without anyone seeing it) can the rail be welded or does it have to be replaced?
Thanks.

I would replace it, and maybe Double frame the thing, i bet it was a raod tractor before it was a dumptruck.

Are you talking about the truck frame rail? From your post I am thinking it is a frame rail on the dump box.

Yes it is the frame rail on the dump box. This truck is not used on a daily basis and not for comercial use. Do you think replacing the rail is still necessary? How would welding hold up? What do you think the cost would be to replace the rail?

Can we get some pics of the rail, maybe fish plate it and you would be good to go.

Here is a picture of the rail, 3 pictures down the rail.

I've never seen anything like that to that extreme. How old is the body, any idea? Is the manufacturer ID tag still on the body? If so, I think I'd contact them and let them know of your problem that sure looks like a design flaw to me.

What is on the back side of that rail? From the way the cracks are formed I'm thinking something back there contributed to your problem.

I've welded frame rails successfully in the past. Replacing the rail would cost more than the trucks probably worth.

This is the left rail. The right rail has only a small seperation at one joint. I don't know how old the bed is. The biggest crack is where the support post is. You can see this in picture 2, with the post at the bottom of the picture. What is the odds of the dump bed sitting on the post (bed being up) and them them trying to load it. It seems that someone would be out of their mind to do this, but is it possible?

Also how dangerous is it to leave the truck in this shape and how soon should it be fixed?
The rail looks a little twisted also. Sounds like what this best approach is weld a backing plate on the back side of the rail.

I'm planning to take the truck into the Peterbilt shop, is this the best place to take it for this kind of work?