Unstable Unimog

[pat356jones]

I have a Unimog FLU419. It's been a great vehicle but when you lift off the throttle, the vehicle becomes unstable, wanting to swerve back and forth so it doesn't feel safe. When you apply throttle again it stabilizes. The more quickly you lift off the throttle, the more radical the swerving. It used to be very stable and it used to go over 50mph but now it only goes 40mph and has this swerving problem. The only change we've made is to remove the part of the fuel filter which appears to be a bypass of some sort. If anyone knows what is causing this I appreciate your input.

 

[Grenadiers]

Any of the brakes seized? Drive it, and check for warmth.

 

[Sitec]

Hi Pat, and welcome. Usually if things get entertaining on overrun, it's to do with bushes... When under drive load, everything is pushed tight and the wear is not allowed to move, but the moment you back off the accelerator, the axles are then just floating under the vehicle, allowing them to move about producing the effect you describe... I'd be getting under the vehicle with a big pry bar, and flexing every suspension bush and see how much movement you have. Another thing to do is have a friend drive the vehicle slowly in 1st gear low box. get him to accelerate and decelerate... while you walk beside the vehicle (emphasis on beside at a safe distance). Look for movement between the axle and suspension arms. If there's excessive play, you'll see it.

 

[GoldCloverLeaf]

It can happen with a small 4wd too, in my Defender, as soon as a rear trailing arm bush goes, you can get rear wheel steering sensation when on and off the throttle, not the best feeling on the highway. Sometimes a bolt has loosened itself but more often then not you need new bushes.

 

[part time nomad]

Sitec is right! after 40 years working on trucks what you discribe is most definatley suspension/ axle related, also if it`s as bad as you say, I would be looking towards the rear axle, as that would have a move profound effect, also if this has happened only recently, pay particular attension to the U bolts holding the axle to spring, they even apear tight but the spring centre bolt may have snapped.
Look for rust signs arrond the area, Also you can chock the front wheels both sides with big blocks, and then with the brakes off, try gently shonting back and forth while someone watches the rear axle for movement.
I would be interested to know how you get on.

 

[pat356jones]

Hi Pat, and welcome. Usually if things get entertaining on overrun, it's to do with bushes... When under drive load, everything is pushed tight and the wear is not allowed to move, but the moment you back off the accelerator, the axles are then just floating under the vehicle, allowing them to move about producing the effect you describe... I'd be getting under the vehicle with a big pry bar, and flexing every suspension bush and see how much movement you have. Another thing to do is have a friend drive the vehicle slowly in 1st gear low box. get him to accelerate and decelerate... while you walk beside the vehicle (emphasis on beside at a safe distance). Look for movement between the axle and suspension arms. If there's excessive play, you'll see it.

Thanks! We found the problem and it was exactly as you described. The bolts to the suspension arms wiggled loose and fell out. This is why it got progressively worse over time. We put new bolts in and that fixed problem but now I have a new problem. I moved the Unimog several hundred yards and tried to back it up but it would not go. Then I tried to go forward and would not go forward either. Any idea what I should do to troubleshoot this? Thanks!

 

[pat356jones]

Sitec is right! after 40 years working on trucks what you discribe is most definatley suspension/ axle related, also if it`s as bad as you say, I would be looking towards the rear axle, as that would have a move profound effect, also if this has happened only recently, pay particular attension to the U bolts holding the axle to spring, they even apear tight but the spring centre bolt may have snapped.
Look for rust signs arrond the area, Also you can chock the front wheels both sides with big blocks, and then with the brakes off, try gently shonting back and forth while someone watches the rear axle for movement.
I would be interested to know how you get on.

Thanks! We found the problem and it was exactly as you described. The bolts to the suspension arms wiggled loose and fell out. This is why it got progressively worse over time. We put new bolts in and that fixed problem but now I have a new problem. I moved the Unimog several hundred yards and tried to back it up but it would not go. Then I tried to go forward and would not go forward either. Any idea what I should do to troubleshoot this? Thank you!

 

[pat356jones]

It can happen with a small 4wd too, in my Defender, as soon as a rear trailing arm bush goes, you can get rear wheel steering sensation when on and off the throttle, not the best feeling on the highway. Sometimes a bolt has loosened itself but more often then not you need new bushes.

Thanks! We found the problem and it was exactly as you described. The bolts to the suspension arms wiggled loose and fell out. This is why it got progressively worse over time. We put new bolts in and that fixed problem but now I have a new problem. I moved the Unimog several hundred yards and tried to back it up but it would not go. Then I tried to go forward and would not go forward either. Any idea what I should do to troubleshoot this? Thank you!

 

[billiebob]

I'm thinking alignment might be a bit off. Camber, Caster, Toe In..... loose bushings???

 

[billiebob]

I moved the Unimog several hundred yards and tried to back it up but it would not go. Then I tried to go forward and would not go forward either.

And yesterday it was running fine ???
You need to post more info.
I take it the engine runs.

I'm starting to lose interest..... the bolts in the suspension arms FELL out...... really....

 

[pat356jones]

And yesterday it was running fine ???
You need to post more info.
I take it the engine runs.

I'm starting to lose interest..... the bolts in the suspension arms FELL out...... really....

Yes, the engine runs. I took it to a mechanic to troubleshoot the unstable condition. I did not look underneath myself and he felt a bit sheepish when he figured out is was something so simple. Everything seems fine now except it will not move. It starts right up and runs and it shifts through all the gears and the PTO for the backhoe works with the clutch - it just won't go forward or backwards.

 

[Tudor Tanase]

transfer case in neutral? or that Unimog specific forward/reverse lever not engaging? Bolts fell out?

 

[pat356jones]

No, the transfer case is not in neutral and the forward/reverse lever is engaging.

 

[pat356jones]

Dunno specifics of FLU gearbox, or what you mean by ‘suspension arms because the only suspension “arms” it has are torquetubes.
But step back take a breath.
Something is still in neutral or its clutch is stuck open.

OK thanks. This is useful information. I'll share it with my mechanic. Thanks!

 

[part time nomad]

Get someone to take a look underneath (safe distance) hand brake on, put it in gear with the clutch up, ticking over!
1) is rear prop turning? if yes you have rear axle problem.
2) is front prop turning? if yes front axle
3) is prop turning into transferbox (more difficult to see) from gear box if yes transfer problem, if no clutch/gearbox problem
Also if you try to put it in gear with the engine running, does it grate/resist? if not clutch may be stuck down. does the clutch feel ok?

My guess is transfer stuck in nuetral!