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Ironmike
Ironmike, ASE Master Tech, L1
Category: Car
Satisfied Customers: 3385
Experience:  30 years GM certified, Ford certified
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78 Chevy: mph..is only while accelerating, it goes away with my foot

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78 Chevy 1 ton starts shaking/shuddering at 33 mph, though gets less noticeable as speed increases. This is only while accelerating, it goes away with my foot off the gas. Why?

Ironmike :

greetings,

Ironmike :

where do you feel the shudder or shaking the most, the steering wheel or the seat?

Ironmike :

is the truck loaded?

Customer:

seat, i suppose.

Customer:

not steering wheel

Customer:

loaded or unloaded

Ironmike :

I would check the trans mount and the drive shaft

Customer:

turning the wheel sometimes makes it worse

Customer:

what exactly might be the issue?

Ironmike :

I have seen broken mounts cause a shudder and even banging

Customer:

broken transmission mount....other possibilities if this is not the case?

Ironmike :

If the drive shaft has loose u joints under load it can vibrate and if the trans mount is loose it changes the drive shaft angle

Ironmike :

If the rear differential has limited slip or a posi traction design with clutches it can shudder or vibrate

Customer:

and this would only be at a higher rate of speed (33 mph exactly, is when it starts, and then smooths out at higher speeds

Ironmike :

it would also do it on turns when the axles need to turn at different rates

Customer:

and not while coasting/not accelerating?

Ironmike :

tires can be like this, at certain speeds the out of balance can be felt more

Customer:

but that would cause vibration while coasting too, right?

Ironmike :

You should look at the rear tires also

Ironmike :

if tires they would vibrate at the same speed , coasting or acceleration

Customer:

so, transmission mount, drive shaft, rear tires....one of the three or all, anything else?

Ironmike :

that is where I would start

Ironmike :

vibrations felt in the seat are usually from the rear of the vehicle

Customer:

and if all good there, (last question) where to look next?

Ironmike :

trucks do make a lot of torque and with older trucks the drive train usually does not get fixed until something fails

Ironmike :

Loook carefully at the drive shaft yokes. I have seen them split and take a long time to fail

Customer:

alright. Thanks for the info. Not I can feel less stupid when talking to the mechanic.....;-)

Ironmike :

I would first look carefully at the tires, trans mount and the drive shaft and if I did not see anything I would remove the drive shaft and move the u joints and look closer at the yokes

Ironmike :

good luck

Customer:

Thanks again!

Ironmike :

you could even look at the leaf springs for rust

Ironmike :

if the pins break the leafs can move

Ironmike :

you are welcome

Customer:

again, that would cause vibration while coasting too, though, right? this is only under acceleration that I get the vibration

Ironmike :

I would check the ring gear back lash and take a fluid sample to make sure the read differential is not an issue

Customer:

1st, or after checking the other before mentioned areas

Ironmike :

All these test can be done when the truck is on a lift and not take too much time

Ironmike :

I just look around many times something sticks out

Customer:

Great. Anything else?

Ironmike :

Sometimes I run the vehicle on the lift, you would be able to hear the differential or see the drive shaft as it moves

Ironmike :

great for spotting axle bearing noises

Ironmike :

This might have aXXXXXshaft and if so the hanger bearing might be bad

Customer:

k. Answer accepted. Have a good night!

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