When an Engine Problem Isn't an Engine Problem

It won't crank normally. It acts locked up. The starter struggles. Maybe the engine can barely turn over at all.

The natural conclusion is simple: the engine is bad.

But on hydrostatic equipment, that conclusion can get expensive in a hurry if nobody checks what the hydraulic system is doing first.

We recently dealt with a Genie machine that needed to be moved and evaluated. The current direction was to replace the engine, but before assuming the engine itself is the problem, there is another possibility that needs to be considered: the hydrostatic drive system may be loading the engine.

The Engine and Hydrostatic Pump Are Connected

Many pieces of mobile equipment use an axial-piston hydraulic pump for propulsion.

Inside that pump is a swash plate that changes the angle of the pistons. Move the swash plate one direction and the machine travels forward. Move it the opposite direction and it travels backward.

When everything is working correctly, the pump returns to essentially neutral when no travel command is being given.

But what happens if it doesn't?

If the pump remains stroked toward forward or reverse, the engine may be trying to turn a hydraulic system that is already under a tremendous load.

Instead of simply spinning the engine during cranking, the starter is also trying to rotate a pump that is attempting to move the machine.

The result can look surprisingly similar to a seized or damaged engine.

Why It Can Fool You

Imagine trying to start a truck while simultaneously asking the drivetrain to pull against an immovable load.

That is essentially what can happen hydraulically.

If an axial-piston pump is stuck on stroke because of an internal failure, control issue, stuck servo, damaged linkage, contamination, or another hydraulic problem, the engine can be heavily loaded before it ever starts.

You may see:

• Extremely slow cranking
• A starter that sounds like it is struggling
• An engine that barely rotates
• Excessive starter current draw
• The appearance of a locked engine
• A machine that will not roll normally

At that point, replacing the engine without isolating the hydraulic load could create an expensive surprise.

The new engine might do exactly the same thing.

Separate the Systems Before Condemning the Engine

That is why good diagnostics are about isolating systems.

Before condemning an engine, we want to know:

Will the engine rotate normally when the hydraulic load is removed?

If the answer is yes, the investigation changes completely.

Now we may be looking at the hydrostatic pump, its controls, the drive system, or another component creating the load.

That doesn't automatically mean the hydraulic pump is bad either. It means we have gathered another piece of information instead of guessing.

That distinction matters when an engine replacement can cost thousands, or tens of thousands, of dollars.

Moving a Dead Hydrostatic Machine Creates Another Problem

There is also the matter of simply getting the machine transported.

A hydrostatic machine with drive motors and planetary hubs often cannot just be dragged onto a trailer.

For this machine, the plan is to release the planetary drives so the wheels can turn freely and use a winch-equipped trailer to load it.

That allows the machine to be moved without trying to force the hydrostatic drivetrain to turn.

Once it is in a location where proper testing can be performed, the engine and hydraulic systems can be evaluated independently.

Parts Replacing and Troubleshooting Are Not the Same Thing

There are times when an engine absolutely needs to be replaced.

There are also times when the engine is simply the component being blamed because it is the most obvious thing that appears not to work.

Heavy equipment is a collection of interconnected systems. Engines drive pumps. Pumps drive motors. Electronics control hydraulics. Hydraulics can load engines.

A failure in one system can easily make another system look bad.

That is why replacing the component showing the symptom isn't always the same thing as repairing the actual problem.

At Mondak Equipment & Hose, we believe the diagnosis should come before the parts bill.

Whether the problem is an engine, hydrostatic drive, hydraulic pump, electrical control, or something else entirely, our job is to determine why the machine is doing what it is doing before expensive components start getting replaced.

Because sometimes the best engine repair is discovering there was never anything wrong with the engine in the first place.