Servicing your hydraulic system for long-lasting performance

A maintenance item often overlooked is servicing the hydraulic system on car carriers and tow trucks of all sizes, makes and models. This blog is going to be a very general view of hydraulic system service and the importance of performing this procedure.

Always please refer to your equipment owner’s manual and follow those prescribed procedures and recommendations. Please follow all recommended shop safety procedures and recommendations as well as recycling your oil and filters per your shop and area guidelines.

Why should you change the hydraulic oil in your wrecker and carrier? This procedure is just as important as changing your truck’s engine oil. Your hydraulic system has hydraulic seals, motors, cylinders, counterbalance valves, control valves and pressure reliefs, just to name a few items. It is of the utmost importance to service as per recommendations to flush out system contaminations, such as metal particles (from the wearing of components), rust and dirt from the inside of the reservoir tank, and moisture.

Most manufacturers recommend changing oil every 500 to 1,000 hours of machine time, and at a minimum of one a year. How do you know what 1,000 hours of machine time looks like? Most, if not all, of the newer chassis with a factory-installed PTO switch record hours of PTO operation for every hour the switch is on. This is something that should be recorded and watched as the unit comes into the shop for regular maintenance. Large wrecker units might also have an operator control station that records operation times. Keep in mind that PTO hours are machine hours as the oil is pumping through the valves and lines and still breaking down in your system and pumping those contaminants around.

Plan ahead

This is a project that in most cases you just don’t pull your truck in the shop and decide to change hydraulic oil unless you have a few things in place. Primarily, do you have the correct oil filter(s) on hand and enough hydraulic oil to do the job correctly? This is where your owner’s manual comes into play for the correct filter and fluid requirements. If your filter has a manufacturer tag and part number on it, you may be able to source it at your local tow equipment dealer or auto parts store. In most cases, your local auto parts store will not have the recommended hydraulic oil on hand nor the quantity of oil your truck requires. Note that most small wreckers and carriers will need about 10 to 15 gallons of new oil, and a large carrier or wrecker may require 40 to 50 gallons of oil. Keep this in mind when you pull the plug on the system as you need to be ready to catch the waste oil.

What type of oil should you buy? Again refer to your manual, but most will suggest a synthetic or synthetic hydraulic oil blend with names such as Chevron Clarity AW 46, Exxon Univis N46, Mobil Mobilfluid 424, Texaco Rando HD 46, Kendall AW 46, Citgo Mystik AW 46, Amoco AW 46, Conoco Super, Mobil DTE, Excel 32 (for cold weather use below 20°F) or Conoco AW Hydraulic Fluid MV 3.

Getting started

Pull the reservoir cap off and peer into the top of the tank. The assumption here is your tank does not have a site gauge nor dip stick. If by looking at the oil, in any fashion, the oil looks milky, you have a water contamination problem. We should probably figure out where the moisture is coming from. Light moisture contamination will have come from condensation due to the heating and cooling cycles of the oil due to operation and/or running the truck in and out of a heated building. Condensation is normal, and a regular system service will help keep your system performing. If your hydraulic oil looks like an undrinkable glass of chocolate milk or somebody mixed grape juice and milk together, STOP! We probably have a bigger issue or your truck has never been serviced.

Let's look at a couple of things before we proceed. The obvious first place is the fill cap. Is the gasket good and does the cap lock on tight? If not, repair as needed. I have also seen older wrecker units where the oil tank is under the body floor and water was designed to drain off the floor past the fill cap and over the top of the tank. Due to an accumulation of dirt between the underside of the body floor and the top of the oil tank, the oil could not run off the floor. This is causing the floor area to fill full of water and run into the oil tank fill.

Other things to look at include a missing tank vent or maybe someone drove the unit through water a little higher than they first thought. On older units, where the oil tank is bolted under the body where it doesn’t get washed off or road salts can accumulate, we have seen the tops of the oil tank become rusted through.

Draining the oil

I would suggest operating the truck and body to warm the oil up as it will do a better job of draining contaminants out when you pull the plug. This might be a good time to rewrap and inspect the winch cable!

CAUTION: Please exercise proper lock-out/tag-out procedures, as you do not want anyone to drive off while your technician is under the truck, nor do you want them to drive out to a call to find out the unit is without oil and does not operate.

Draining the oil is not a ‘one size fits all’ approach. Some units have drain plugs on the oil reservoir(s). On some units, you will need to pull the suction hose off the hydraulic pump, and yet on others, you may need to split a hose connection. This is the appropriate time to mention to look twice before you pull a hose as you are about to make a mess. Que up the floor dri. If your unit has a shut-off valve on the suction hose, I would suggest shutting it off before you split a hose connection as you can then meter out the waste oil and swap drain pans as needed.

If your oil was milky, I would suggest a flush procedure. Install 3 to 4 gallons of new oil, operate the boom functions, drain the oil, refill with 3 to 4 gallons of new oil and operate the wheel lift or underlift, drain the oil and refill with 3 to 4 gallons of new oil and run through it again until your oil looks as new as you wish.

This is the point that you would change the filters and refill the reservoir. We have had situations where we have added a quart of denatured alcohol to 5 gallons of oil and ran through the flush procedure to grab the water and flush it out of the system. Similar to what a gas line anti-freeze would do.

Once you have drained the oil and changed the filter, it’s just a reversal of reattaching hoses or putting in the plug and refilling the oil to the sight gauge or dip stick. If the unit does not have a gauge or stick, use a ruler and refill to 1.5 to 2” below the top of the tank or per the manual. If your unit had a shut-off valve(s), be sure they are locked open.

Inspect hoses and valves

When servicing the hydraulic system it is also a good time to look at your hoses and hydraulic valves for leaks. If you have a PTO/pump system are there any signs of a transmission-to-PTO leak? Any signs of a pump leak? Also take a look at the oil fill cap and tank vent system. Is the cap gasket intact and not dried out? Is the tank vent clean or covered in mud? Follow your company’s procedures for recording, reporting and repairing said issues.

Zip’s Tip

Record the oil filter part number in your owner’s manual or on the unit maintenance sheet to speed up the process the next time. Also, record the completed procedures on the truck records to alert others that it was performed and record it on the calendar for a year from now to be sure it gets done regularly.