Maintenance Q&A – May 2021
Q: The household goods fleet I’m leased to has always run long-nose Kenworths. I started as a helper riding in them, then as a driver. When I got my first truck in 2001, it was a W900L. I’m ready for another truck, and I’m looking at a W990L. The dealer has a few on the lot. I’ve decided on a Paccar 500 horsepower engine with an Eaton automated transmission. I guess I have some decisions to make that I’m not sure about. Some come with an air front suspension and the others with spring suspension. The dealer also wants to know what drive gears I want, how big an alternator I want, and what batteries. I am confused by some of this because I never had to make these choices before.
A: When we spoke, I did my best to answer your questions. Your dealer did well to try to determine your needs instead of just pushing whatever they wanted to move off the lot. Let’s take your questions one at a time.
Air suspension
It’s been a while since I drove a truck with front air suspension, and early models tended to wander going over bumps or ruts. Their geometry was based on swing-arm design that altered geometry as the suspension flexed. The AG130 front air suspension design uses four airbags and two shock absorbers, all mounted vertically. The airbags are equally spaced in front and behind, and close to the axle on each side. The shock absorber is mounted directly above the axle. All loads are transmitted vertically to the chassis. There should be virtually no “bump-steer” effects from the AG130, so you should have as stable a ride as with any other suspension.
Axle ratio
Your axle ratio will depend on where you are driving and what speeds you intend to cruise. Your MX-13 engine develops 510 hp with maximum torque of 1,850 lb-ft, which moves the truck and stays at maximum until 1,400 rpm before starting to diminish. For best performance, you want to keep rpm at the lower end of the torque curve. With your engine, that should be around 1,100 rpm. That will give you ample torque to accelerate when you need it and adequate torque to keep going as you climb hills.
Your rear-axle ratio determines your rpm at any given speed. If you are driving regionally, in the Midwest for example, and will be averaging 65 to 70 mph, you won’t need as long a rear-axle ratio as you would running out West where you can cruise well above 70 mph. Keep in mind that the lower the number of the gear reduction the slower your engine will turn at a given speed. Put another way, you can go faster at a given engine rpm. The tradeoff is that the faster the engine turns, the poorer your fuel economy will be. Your dealer has computer programs to show the performance profile of the truck with each axle at any given engine rpm. The programs also show grade-ability for each axle at every engine speed and gear.
If you will be in hilly or mountainous terrain, you will need greater gear reduction to maintain speed. Work with your dealer on profiling your expected routes, but keep in mind the trade-offs. You can have fantastic hill-climbing ability if you need it but you will lose fuel economy. Or you can have great fuel economy at the cost of hill climbing capabilities. The program will guide you in your decision.
Alternator
The rule of thumb is to get all the amps you can afford. Most trucks today come with 160- to 180-amp alternators, way up from the 85 to 100 when I got into the industry. But today, with so many hotel loads and more electrical and electronic devices coming inboard to make driver’s lives more comfortable, convenient and safer, I would recommend a 220-amp alternator to allow for additional equipment if space and budget allow.
Batteries
There are four considerations when selecting batteries: cold-cranking amps (CCA), reserve capacity (RC), weight and price. Most trucks come with three or four lead-acid batteries with combined CCA well above 2,000 considered necessary to start a truck in the worst conditions. Reserve capacity is the number of minutes a fully charged 12-volt battery will provide 25 amps at 10.5V. It is a relative measure of how deeply the battery can be discharged without the truck running. RC is important to maintain engine-off hotel loads. Weight and price are self-explanatory.
I’ve been a big fan of absorbed glass mat (AGM) battery technology ever since it was introduced several decades ago. They are completely sealed and prevent any possible spillage in the event of crashes. They also have greater reserve capacity than the lead-acid counterparts. The trade-off is that they are more expensive and heavier than the standard batteries they replace, but they last longer and are more reliable. Batteries range up to 1,150 CCA with 205 minutes RC for the most expensive aftermarket batteries. Those batteries can weigh 75 pounds or more each compared with 40 pounds for an 800 CCA 100-minute RC lead-acid battery.
AGM batteries can have their life shortened by overcharging or charging at too high voltage. Special smart chargers are needed for AGMs, but they also can be used with lead-acid batteries.
Work with your dealer to determine your need: how much load you expect to put on the batteries with the engine off, and for how long a period of time. LL