ail lift jerky when lowering — hydraulic issue?

[bwtom]

My truck tail lift has started behaving strangely when lowering. It’s no longer smooth and now feels jerky, sometimes pausing mid-way before continuing down. It almost feels like the hydraulic pressure is unstable.

Hydraulic oil level looks normal and I don’t see any obvious leaks, but the issue is getting more noticeable over time.

Could this be low hydraulic pump pressure, air in the system, or maybe a faulty solenoid valve? Has anyone dealt with something similar before?

 

[Lnewqban]

Welcome!
It could be any of the mentioned causes, plus some unusual resistance or friction in the mechanism of the lift.
Does yours look like the one in the picture?

 

[Baluncore]

Welcome to PF.

Start by checking the oil level by dipping the tank. Sight gauges are often marked by varnish from the oil, so can make you think the level is OK when it is not. Air can enter the system if the tank level is low.

Examine the hydraulic oil filter for blocks or an internal burst, then simply replace it anyway. The filter may be blocked sufficiently to reduce the descent rate while not influencing the lift rate. Some filters, when blocked, can shed fibrous material into the hydraulic system. That material can block any hydraulic valve or orifice. If it lifts OK, then the oil pressure regulator is probably OK.

There may be a pilot operated check valve at the lift cylinder. It is a block with four ports that connects between the cylinder and the spool valve. It is a safety feature that protects against the platform falling next time there is a burst hose. Oil is prevented from leaving unless there is some pressure on the other line, to pushing it down. A simple system may only have an adjustable orifice to regulate the maximum rate of fall. Any of those valves can be contaminated with debris that will give the observed symptom.

The lift cylinder(s) can have failed piston seals that jam the motion, or a broken rod. If there are two lift cylinders one may be mechanically faulty.

Lastly examine the mechanical linkage. That may be worn, or not lubricated, which can give the jerky behaviour.

 

[bwtom]

Welcome!
It could be any of the mentioned causes, plus some unusual resistance or friction in the mechanism of the lift.
Does yours look like the one in the picture?

View attachment 371560

Thanks!

Mine is actually a cantilever tail lift, not the vertical type shown in the picture. The platform folds and unfolds from the rear and is driven by hydraulic cylinders on each side.

The strange behavior only happens when lowering — instead of coming down smoothly, it moves in small jerks and sometimes pauses briefly before continuing. Lifting up still seems normal.

 

[bwtom]

Thanks for the detailed explanation, I appreciate it.

I’m pretty new to working on these tail lifts, so this is really helpful. I’ll start by checking the oil level more carefully and then look at the filter and the linkage like you mentioned.

Welcome to PF.

Start by checking the oil level by dipping the tank. Sight gauges are often marked by varnish from the oil, so can make you think the level is OK when it is not. Air can enter the system if the tank level is low.

Examine the hydraulic oil filter for blocks or an internal burst, then simply replace it anyway. The filter may be blocked sufficiently to reduce the descent rate while not influencing the lift rate. Some filters, when blocked, can shed fibrous material into the hydraulic system. That material can block any hydraulic valve or orifice. If it lifts OK, then the oil pressure regulator is probably OK.

There may be a pilot operated check valve at the lift cylinder. It is a block with four ports that connects between the cylinder and the spool valve. It is a safety feature that protects against the platform falling next time there is a burst hose. Oil is prevented from leaving unless there is some pressure on the other line, to pushing it down. A simple system may only have an adjustable orifice to regulate the maximum rate of fall. Any of those valves can be contaminated with debris that will give the observed symptom.

The lift cylinder(s) can have failed piston seals that jam the motion, or a broken rod. If there are two lift cylinders one may be mechanically faulty.

Lastly examine the mechanical linkage. That may be worn, or not lubricated, which can give the jerky behaviour.

Thanks for the detailed explanation, I appreciate it.

I’m pretty new to working on these tail lifts, so this is really helpful. I’ll start by checking the oil level more carefully and then look at the filter and the linkage like you mentioned.