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GMC TopKick 7000 with a hop


vtfireman85
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as I have posted on here we recently put a 1989 GMC TopKick 7000 on the road with a dump body on it, retired firetruck, moved the axle forward, new springs and hangers, took out the pump in the center and had a custom driveshaft section made to replace it. has 1 carrier bearing in the middle. total of 3 U joints in the shaft. had a hop when driving before the dump body went on, more under load than not, but some still while coasting, and even going down hill, hops while braking, hops in neutral, but it is worse under load. figured it was rear tires, so we replaced them with new, and new rims, no change, had them balanced, no change. put new fronts on, with new rims, no change. tried taking the driveshaft apart and aligning the joints, no change, tried opposing the joints, no change. it is possible it is in drums I suppose but they look fine and it has no unusual behavior while braking. any suggestions would be appreciated, we are kind of at a loss here. 

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Seriously doubt drums but maybe. Are the ujoints straight? I have seen driveline builds that they didn’t get them aligned. Is the hop in the seat or steering wheel?

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3 minutes ago, Lazy WP said:

Seriously doubt drums but maybe. Are the ujoints straight? I have seen driveline builds that they didn’t get them aligned. Is the hop in the seat or steering wheel?

tried u joints both ways, I would say the hop is in the seat, though some does transmit through the steering wheel it shakes everything in the cab, drive it for an hour and your nipples are sore. driveshafts are also in alignment, that is to say there is no bend in the middle. 

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26 minutes ago, vtfireman85 said:

as I have posted on here we recently put a 1989 GMC TopKick 7000 on the road with a dump body on it, retired firetruck, moved the axle forward, new springs and hangers, took out the pump in the center and had a custom driveshaft section made to replace it. has 1 carrier bearing in the middle. total of 3 U joints in the shaft. had a hop when driving before the dump body went on, more under load than not, but some still while coasting, and even going down hill, hops while braking, hops in neutral, but it is worse under load. figured it was rear tires, so we replaced them with new, and new rims, no change, had them balanced, no change. put new fronts on, with new rims, no change. tried taking the driveshaft apart and aligning the joints, no change, tried opposing the joints, no change. it is possible it is in drums I suppose but they look fine and it has no unusual behavior while braking. any suggestions would be appreciated, we are kind of at a loss here. 

4020310A-9BC5-4BE4-9BF3-35AE69F77DED.jpeg

1EF4D504-5FE6-4638-BFC4-7862602C158F.jpeg

Check the rear motor mounts. If back of engine is down or offset it will make big ,weird things happen. More than one Chevy truck fixed with motor mount replacement. Should ask what engine trans combo also

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5 minutes ago, dale560 said:

Check the rear motor mounts. If back of engine is down or offset it will make big ,weird things happen. More than one Chevy truck fixed with motor mount replacement. Should ask what engine trans combo also

3208 cat, 643 Allison 

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What kind of pump was removed? Is there a possibility that there was something was counterbalanced to compensate for something in the pump? 

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1 minute ago, TractormanMike.mb said:

What kind of pump was removed? Is there a possibility that there was something was counterbalanced to compensate for something in the pump? 

big center mount hale, I suppose it is possible, I don't know where I would have the driveshaft checked for balance, those leave the transmission and the pump serves as sort of a transfer case, it either allows the rear driveshaft to spin and move the truck or it takes input form the transmission to run the pump. 

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Jack the back wheels off the ground, first one then the other. Block well, use stands, etc. try to see which side makes it jump.

 if it jumps each time, the drive line is out of balance.

 obviously, if it wont jump, the front end is at fault.

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I'm a loooong way from an expert, but how do you "time" 3 U-joints in a drive shaft?

I imagine it can be done, but I would guess it isn't as easy as 2 joints.

Mike

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Is it only a certain speed that causes the shake?

my father had a log truck we chased a shake in a few years ago, changed all the parts you did and then some 

it would shake extremely bad between 35 and 45 mph

final thing we did was have the drive shaft balanced, vibration was completely gone

hope this info helps

 

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I’ll throw out 3 possibilities 

Are the spline on the transmission yoke all the way around or is there a gap with splines missing? Did u joints on my Duramax and didn’t mark where it came off and it was a bad vibration on reinstalling, some positions you could hardly slide it back on and even harder to pull it back off, finally got it back in the right position 

Is there any bend in the 2 piece driveshaft or is it basically straight in? 

Is there any vibration damper on the rear diff? Is it good?

Maybe shortening the wheelbase has caused some bad harmonics?

Guess that was 4...

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24 minutes ago, hardtail said:

I’ll throw out 3 possibilities 

Are the spline on the transmission yoke all the way around or is there a gap with splines missing? Did u joints on my Duramax and didn’t mark where it came off and it was a bad vibration on reinstalling, some positions you could hardly slide it back on and even harder to pull it back off, finally got it back in the right position 

Is there any bend in the 2 piece driveshaft or is it basically straight in? 

Is there any vibration damper on the rear diff? Is it good?

Maybe shortening the wheelbase has caused some bad harmonics?

Guess that was 4...

it did it before the wheelbase was shortened, driveshafts are straight, no bend in the middle, as far as I know the yoke has not been removed from either end since we got it. I did do U joints in it, thinking perhaps they were brinnled (sp?)no vibration dampener that I know of, effectively the driveshaft portion of the driveshaft hasn't really changed much, we took out the pump and shortened it by about that much (19" give or take) had to have a new one made for it to work, but it is not excessively short, actually slightly longer than the one on the truck it replaced ( a C65).

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14 minutes ago, yellowrosefarm said:

Any chance it could be the same as your Heep?  I see you moved the axle forward. Could there be too much angle from the center bearing to the diff input yoke? 

 

13 minutes ago, edwardporter1 said:

sounds like a driveshaft balance problem. make sure all the yokes are lined up. if they are off one spline you will have a problem too

the angle is not excessive, the wheel base is not unusually short, it was actually an unusually long truck, ill measure tomorrow, that bed is 14' long and pivots at 11 to give you an idea. 

presumably yoke alignment is best checked with a magnetic protractor? 

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Looks like a nice clean truck otherwise. I always thought the headlights looked out of place on those, kind of like they got the truck all drawn up and suddenly somebody noticed they forgot them so they just stuck them anywhere. They could have added more but then it would have resembled the family truckster.

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By any chance does it have a parking brake drum on the back of the transmission?  Company I worked for had a truck like yours with the same engine and transmission, and the same problem--turned out the parking brake drum was off balance.

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Just from symptoms it is more than likely driveline related. Have had a bunch of them have like a wind up then hop. It is usually almost always back of transmission yoke angle giving troubles. Either from loose mounting bolts or motor mounts. The truck looks low mileage though but I would thoroughly check angle of trans yoke and pinion yoke.

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27 minutes ago, Eric V Bielke said:

By any chance does it have a parking brake drum on the back of the transmission?  Company I worked for had a truck like yours with the same engine and transmission, and the same problem--turned out the parking brake drum was off balance.

Nah, air brakes 

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Would an alignment issue with the moved axle do it? 
 

 

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Fwiw.   Seth. 

1. Take the drivelines all out and bring to a professional shop to have them checked an balanced.  From small experience its worth it.

2. Get same shop to look at pinion angle.  My lowered chev did that.  Took several shims to get that right.

3. Trans and motor mounts tight?  Like jack up and see for movement.

4. Does the yokes slide easily on all points?  Even the one out of trans?  Friend stoooole a 07 Fliner from a sale with 65k on it.  7000 with wet kit and clean as can be.  Could not figure out what happened.  He was sure he got had.  ECM confirmed miles. No issues in weeks of use but a random and significant vib.  Fiddled with all things.  No help.  Driveline shop told him to check the trans yolk even though it really dont move move.  Voila it was siezed good.  Took alot of work to beat it out.  Got new yolk and lubricantion and truck is like a new rig he says.  

5. Get full coverage an forget the brakes near a body of water.

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