Fire Fighting in Canada

Truck Checks: August 2009

Get your creeper and trouble light and put your coveralls on. Make sure the truck’s wheels are chocked and the brakes set. If it is not possible to disable the truck then make sure you place the truck out of service and, if possible, have another member of your department make sure no one starts the truck with you under it. Let’s look at rear-axle lubrication.

July 27, 2009 
By Don Henry

Get your creeper and trouble light and put your coveralls on. Make sure the truck’s wheels are chocked and the brakes set. If it is not possible to disable the truck then make sure you place the truck out of service and, if possible, have another member of your department make sure no one starts the truck with you under it. Let’s look at rear-axle lubrication.

The rear axle, often called the differential, uses a SAE 80W-90 lubricant of the APL–GL-5 designation. The W stands for winter operation and while it may not be a requirement for your fire truck to start outside at -20 C, it is most likely too hard to find a straight weight oil.

Photo 1: Rear axle
Photo 2: Brake shoe contaminated with axle oil  
Photo 3: Check here for input pinion seal leak.  
Photo 4: Diesel fuel oil cooler: note how clean this truck is.


Your axle most likely came with synthetic oil and you should use only synthetic oil as a replacement. Remove the oil fill plug. The oil level should be just up to the bottom of the threads of the fill plug. The level should be checked with the truck level and the axle at a temperature between 20 C and 25 C.


If the truck has been recently driven, or the oil is hot, then the oil will appear to be over full and will flow out when the fill plug is removed. This is for two reasons: oil expands when heated; and the oil contains air bubbles that tend to bulk up the oil.

On the axle housing there may be more than one plug: the bottom plug is for draining; the plug about half way up the housing is the fill plug. There may be a third plug lower down than the fill plug; this plug is often where you will find an oil-temperature sensor. This sensor is not often used on fire apparatus so most times the hole will just be plugged. It is not necessary to remove this plug.

If you are going to take an oil sample to have it analyzed then you should not let the oil stand for more than 30 minutes or the heavier particles will sink to the bottom of the axle housing and the sample will not be accurate.

I recommend that oil be sampled once a year and changed as per the oil analysis. Unlike a gravel truck that spends less than 50 per cent of the time fully loaded, a fire truck spends almost all of its service life fully loaded – and often overloaded.

I am a firm believer in oil analysis. One very common pollutant in axle oil is water. Even a very small amount of water – less than 0.1 per cent (1,000 ppm) – will shorten the life of a bearing by 70 per cent. Where does the water come from? It most likely comes from the breather on the axle. As the oil cools after a run, the oil volume shrinks; this draws air into the axle housing. Because the air in most fire halls is often very moist, the axle oil will, over time, become polluted with water. This does not happen to over-the-road highway trucks but the oil in fire trucks never gets hot enough to effectively boil off the water as it does in a normal long-haul truck. By the way, the oil will not appear milky white from the water until after the 0.1 per cent level. In other words, by the time you can see the water it’s too late and the damage has been done.

■ Draining the oil
To conduct an effective drain, drive the truck to warm up and circulate the oil. Remove the bottom oil drain plug and catch the oil in a container. Be careful. This oil is hot and could burn you. Wear your safety glasses and protect your eyes.

The bottom plug should have a magnet as part of the plug. It is normal for the plug to have a small amount of metal dust on. If the magnet has particles the size of a pin head or larger, or if the oil has a brassy or golden colour to it, then you have problem. These brass particles will not be attracted to the magnet. I suggest that you refill the unit, drive and have the oil sampled again for analysis. While it is not common – some axles do have their own filters on the axle housing – remove and replace with new ones at each oil change.

When replacing the plug – because national tapered pipe thread is used – use a small amount of a sealer on the threads of the plug and do not over tighten.

My recommendation is that the axle be inspected daily for leaks (weekly for rural departments). Check the oil level at six months, sample yearly and change as per the analysis report. If no leaks are found during the daily inspections then I believe that checking the oil level each month is a waste of time.

It is important to make sure the axle breather is functioning at each inspection. This can be done by simply twisting the top of the breather with your fingers and spinning the top. If the breather becomes plugged then the axle housing will over pressure. This will cause axle oil to leak from either the axle input pinion or at the axle wheel ends. If this oil leaks at the wheel ends then oil will contaminate the brake drums and brake shoes. This could also happen if the axle wheel seals were to fail and leak and is an excellent reason for a daily inspection. If the axle oil has gotten on the brake shoes –no matter how small an amount – the brake shoes must be replaced. Don’t even think about washing the brake shoes in any type of cleaner or solvent to remove the oil, no matter who told you it was OK to do so. The brake shoes will have absorbed the oil and you will never be completely remove the contamination. It is permissible to use a high-pressure steamer to wash the inside of the cast-iron brake drum to remove the oil but it’s almost always cheaper to replace the old drum with a new drum.
The repair industry has classified leaks into three categories.

A class-one leak is just a discoloration around the area caused by oil. This is also called an oil weep. It is best to keep an eye on this but it is not a reason to take a unit out of service. Wipe off the oil and dirt with a clean rag and note it on your daily inspection report sheet.

Next is a class-two leak. With this kind of leak, you can count the drops as the oil drips. You could say that the pump packing on a fire truck is meant to have a class-two leak of water.

The last is a class-three leak, in which the oil does not drip but forms a steady stream.

Both class-two and class-three oil leaks will put a unit out of
service. Check around the driveshaft input pinion seal on the axle housing for leaks.

Fire apparatus often have problems with overheating; on some units an engine diesel fuel oil cooler is used to keep the diesel fuel oil cool. Hot diesel fuel does not have the power that cooler diesel fuel has. It is often located above the rear axle. Check this cooler for leaks; these will often leak onto the rear axle and could be mistaken for an axle oil leak. It is a good idea to gently wash this cooler.

While we are on the subject of washing, the photos on page 14 came from trucks that were so clean on the outside you could eat off them. It is possible, and highly encouraged to wash the other side of the truck – the bottom side. Just be careful that you do not spray high-pressure water at any seals, breathers or loose electrical connections. A clean gear box, transmission or rear axle will run cooler and it will be easier to spot leaks if these components are clean.

Best of luck to all the technicians who took the EVTCC exams on June 6.

And, finally, a quick note about an organization that is close to my heart and has been very supportive of my work. Fire-etc (Alberta Fire Training School for you older people like me) holds a 50th anniversary celebration Sept. 25-26. Check out the website at or e-mail for more information. Hope to see you there.

Don Henry teaches in the Automotive Services Technician and Heavy Equipment Technician programs at Lakeland College in Vermilion, Alta. He can be reached at

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