This article is a synopsis covering the driving techniques that are necessary to prevent engine and transmission overheating going up the hill, and to prevent running away with smoking hot brakes on the way down the other side. 

It is intended to be educational for the newbie Bus Nuts. Those of you without the commercial driving experience, who find themselves at the wheel of a newly acquired twenty ton vehicle with air brakes who haven’t driven anything bigger than the family SUV. Following are some answers to questions you were afraid to ask.

Driving Uphill

Some uphill grades are several miles long, and can be steep. Six to eight percent doesn’t sound like much in a car, but can be taxing for a bus cooling system, especially in the summer heat.

The most common drivetrain in our buses are still the 2-stroke Detroit Diesel that was ubiquitous across most brands until the 1980’s, when modern electronically controlled 4-stroke engines became the norm. These old 2-stroke mills, such as the 6-71, 8V71, and the 6V92 and 8V92 Detroit’s are still great reliable engines. But they produce twice as much heat as a new 4-stroke engines, because there is compression and combustion on every stroke, compared to every second stroke on the latter. Add the fact that the engine and radiators are at the back of the bus, without the benefit of ram air such as in a truck, and the challenge of shedding that heat is evident. 

It is most important to ensure that your cooling system is in good working order. Clean radiators inside and out is a must. Properly fitted and intact radiator shrouds, so the air goes through the radiator, not around it. The fan, or squirrel cages in an MCI, have to be in good shape, clean, and spin as fast as intended, without the belt slipping, and the fan clutch in good order, as the case may be. 

Another thing that affect cooling is the air intake. Make sure it is not obstructed, and has a clean filter. The exhaust has to be as free flowing as possible. You don’t need back pressure in those old diesels. The less the better. Some Bus Nuts have removed the stock muffler and replaced it with a resonator, which doesn’t have baffles. It is noisier, but gives a little more power and better cooling.

Facilitating air flow through the engine compartment can also help in cooling. On a hot day, the rear doors can be latched open for maximum air flow. Louvers in the back and side doors can be permanently fitted with good results as well. Some people install air scoops on the sides of the radiators on MCI cars to force ram air into the radiators at speed. I don’t think they do very much, other than make the bus look like a tired old clapped out church bus. 

With everything in good shape and well maintained, the bus should run cool at around 180F in most conditions. I love driving my bus in ambient temperatures less than 90F. I can enjoy using all the power the 6V92 has to offer, without seeing the temperature gauge go over 190F. But, in the summer heat in the 90’s and above, good driving techniques become paramount. Even before going uphill, a bus will heat up when pushing a hot head wind in the summer. 

Lugging the engine makes the most heat. If, with the pedal to the floor, the bus is not accelerating, or slowing down, with black smoke out the exhaust, it is lugging, or struggling, and will overheat fast. You need to do it a favor, and down shift into the next lower gear. 

These two strokes need to be driven at high RPMS to spin the fan fast, and to circulate the coolant a lot to keep the temperature in check. The power band with usable torque is approximately between 1600 and 1900 RPM, so driving it in that range is the best. 

A lot of these buses are equipped with an Allison HT740 four speed, or an HT730 three speed in the GM cars. They need to be shifted manually to keep the engine spinning in the desired RPM range. They are not smart like modern transmissions, and up shift too soon. They also down shift too late when going uphill, and lug the engine. 

Get to know the shift points, either by speed on the speedometer, or RPM on the tachometer, and make a habit of shifting manually. That is usually around 1500 rpm at the low end, and governed speed at the high end (2100 RPMish). 

The best is to be climbing a hill with about 3/4 throttle on the pedal, at about 1900 RPM, in a gear where the bus is happy, without lugging. In my bus (MC5 with 6V92/Allison HT740), it is third gear up a 6% grade, second gear up an 8% grade. Turn on the 4-way flashers at 40 MPH or slower, and enjoy the view... And occasionally you can pass a loaded semi and feel like you are the king of the road. 

The most important gauge is the coolant temperature. Do not let it go above 200F. Down shift as much as necessary. Pull over if needed and idle for a few minutes until the temp goes down. A blown engine is very costly and inconvenient to replace, especially on the side of the road in another state.

The later model 4-strokes will climb hills a lot faster and cooler, but the same driving techniques apply. Their torque power band is lower in the RPM range than in the 2-strokes, but they need to be driven with care as well.  Also the modern transmissions, such as the Allison B500 that are in a lot of late model buses since the 1990’s, are very good at shifting in the proper gear, at the right time and RPM, by themselves.  You can put them in drive, and go without worrying about shifting uphill or down.

Automatic transmission fluid gets hot, more so than engine oil, partly because of the torque converter that is slipping when not in lock-up mode. That hot oil is typically routed to a shell oil cooler that also cools the engine oil. So, the transmission cooling is added to the engine cooling for the radiators to dissipate. That is a big challenge. Some buses are fitted with an auxiliary transmission oil cooler upstream of the engine mounted oil cooler, and that helps considerably. 

A manual transmission does not generate nearly as much heat, and does not need external cooling. But most people nowadays prefer an automatic over a stick shift as they are easier to drive for most people, especially younger people that never learned to drive a car with a stick shift.  

Radiator misters can also help cooling when all else is not enough. A simple system is piped from the fresh water tank. So, with the water pump on, a solenoid valve operated by a switch on the dash is opened, and water sprays the on radiator fins. Some people prefer a mist, others use more water out of 1/16” or even 1/8” holes to douse the radiators. That is a very good way to bring the temperature down, either as a band aid, if the cooling system is not as good as it could be, or as an additional measure for a soup-ed up motor with more power than stock.

Driving Downhill

Driving a ten-ton bus, and as much as a 140,000-pound B train semi down a long mountain grade requires skills that are normally taught by a driving school for obtaining a Commercial Driver’s License, by an employer for new drivers, and through the seat of the pants experience. Reputable bus and trucking companies will not turn an inexperienced driver loose on mountain roads without some training, or two to three years’ experience. The skills required in mountain driving are beyond entry level.

Yet, anyone in North America can buy an old bus conversion and drive it out with an ordinary driver’s license, without knowing anything about air brakes, or having any experience driving a heavy vehicle. At least in every Canadian province, an air brakes endorsement is mandatory to drive an air brake equipped vehicle. It is usually a Friday night and all day Saturday and Sunday course. 

I know you Americans don’t have to do that, but I recommend studying your states CDL (commercial driver’s license) manual to learn the basics of air brake systems. These manuals are available on-line on the states DOT (Department of Transportation) sites. Also, do an internet search for air brakes and all kinds of helpful information will come up, including Bendix manuals on the subject. 

As the operator of your vehicle, you have a moral and legal obligation, or “duty of care” to drive any vehicle safely. Sooner or later, you will eventually find yourself in British Columbia, Colorado, Western Pennsylvania or West Virginia, and you better have a thorough understanding of how to manage long downhill grades in a heavy bus. 

The first thing to do is to know and make sure the bus is in good working order. You have performed a pre-trip safety inspection before leaving, so that is covered. You should double-check your brakes at the top of the hill like the truckers do, by stopping at the brake check pull-off area and verifying that:

1. The air compressor is maintaining full reservoir pressure.

2. The brake slack adjusters are set within proper limitations. I know you can’t crawl under the bus to do that, so that must be done at home on a regular basis.

3. There are no audible air leaks.

4. The drums, bearings, and tires are not overheating. Just feel with your hand, or use an infrared thermometer.

Overusing the brakes will quickly overheat them. The normal operating temperature for a properly functioning brake is about 250C (500F). A hot, smelly, smoking brake is about 425C (800F). Dangerously overheated brakes can reach 1000C (1800F). They glow red, and may lead to tire fire. The heat expands the diameter of the drum so much that the shoes cannot effectively press against them to brake. Also, the brake lining materials lose some friction capability at high temperatures. And this is when you have a runaway situation, with the “check underwear” light flashing, and desperately looking for a runaway lane, if there is one.So, you have to descend the hill in a gear that does not require the use of the service brakes. If the bus is not equipped with an engine brake such as Jakes, that gear might very well be one or two lower than it would take to go up this same hill. Jake brakes allow going down a little faster, like one or two gears higher, depending on the kind of transmission. 

You must use engine braking only. Save the service brakes for emergency stops, that is your backup plan. What if the engine brake fails? What if you come around the corner, and traffic is stopped? Or, there is an animal standing in the lane? That is what you are saving cool brakes for. 

If you find yourself in too high a gear, brake now while the brakes are still cool to slow down enough to downshift into a lower gear. If the engine is slowing you down too much, do not upshift. Press the accelerator pedal to speed up a little, and to disengage the Jakes. If the bus is equipped with a two speed (or more) Jake, you can toggle between the one and two position as required. The hill might get steeper around the corner, so you’ll be glad you haven’t upshifted. Jakes are most efficient at high RPM, like between 1800 and 2000 rpm.

An engine brake is nice to have, but not essential. Just use as low a gear as necessary, without the need to apply the service brakes to slow the bus. Turn on the 4-way flashers at less than 40 mph, and don’t worry about the traffic behind you. You can go down a hill a hundred times too slowly, but only once too fast.

Back in the 70’s and 80’s, we were taught to use a steady, light brake application if necessary, going downhill. It was later found that the system does not always apply equal brake pressure on all the brakes, especially on multiple axle vehicles where there are variations in push rod adjustment between wheel ends, and/or axles. 

Nowadays, the stab method is preferred and taught at driving schools. You apply the brakes hard enough to slow down, then let go, and repeat if necessary. So all brakes are effective, and they have a chance to cool off somewhat between applications.

With miles you will build up some experience, and get to know your bus. For instance, I know that I can go down an 8% grade in second gear with the Jakes on, a 6% grade in third, and 5% and less in fourth gear (6V92/4-speed Allison). Your bus will vary, depending on engine/transmission combination, Jakes or not, etc. 

Remember, if a loaded B train at 140,000 lbs. can be controlled downhill without using the service brakes, so can a 25,000 lb. bus conversion. 

So, venture forth, up and down the hills of our beautiful countries. Drive defensively and conservatively, and never park your bus in front of a runway lane! Some other Bus Nut might need it.