Tachometer Error...To check, connect a tachometer of known accuracy to the engine. Run the engine and make a comparison of the readings of the vehicle and test tachometers. If vehicle tachometer is defective, make repairs as necessary or install a new tachometer.
Engine Operated at High Altitude...Less oxygen at higher altitudes causes the engine horsepower to go down approximately three percent for each 1000 ft. above sea level. *
Brakes Do Not Completely Release... Check the brakes by feeling all the brake drums. If the brakes do not completely release, the brake drum for that wheel will be hotter than the brake drums for the other wheels. With the truck jacked up the wheels must rotate freely when turned by hand.
Vehicle Operated in Too High a Gear... If the operator does not shift the truck correctly or operates the truck in a "lug" condition (using the truck in too high a gear for engine rpm to go up as accelerator pedal is pushed farther down, or using the truck in a gear where engine rpm goes down with accelerator pedal at maximum travel), poor vehicle performance is the result. For best engine performance do not lug the engine more than 600 rpm below its full load speed.
Extra Engine Driven Equipment...Air compressors, hydraulic pumps, generators, and other engine driven equipment that has damage, or that was not installed correctly, or that is not in correct adjustment, can take more horsepower to drive than expected. If necessary, disconnect the equipment and test the engine.
Speedometer Error...A damaged speedometer does not give the correct speed or the correct indication of fuel consumption. An indication of low speed can cause the operator to feel that he has a power problem.
Speeds Too High...The need for more horsepower is easy to see as the speed of the vehicle is increased. This is especially true if the front of the vehicle has a large surface area. Your authorized dealer can give you the horsepower needs for a vehicle, and the horsepower necessary for different vehicle designs at different speeds.
Overload on Vehicle *
High Moving Resistance...Soft ground conditions cause a need for more horsepower. To see if the problem is the engine, test the vehicle on a surface known to be good, or test on a chassis dynamometer.
High Wind Resistance...The horsepower needs for a truck can be divided into two parts. Part of the horsepower is used to move the vehicle and part is used to get through wind resistance. The horsepower necessary to get through the wind resistance will increase as the vehicle is used at higher speeds. Vehicles with a large front area have a higher wind resistance and take more horsepower than those with a small front area. Some types of trucks, for example those used for the transportation of automobiles and/or boats have high wind resistance even if the front area is small. Moving against the wind has the same effect on wind resistance as does higher vehicle speed.
Power Loss in Drive Gears...It is possible for a transmission or rear axle to use extra horsepower because of damage, not being in correct adjustment, having the wrong type of fluid or not enough fluid in them, or an inside mechanical problem. If a part of the drive train unit operates at a higher temperature than normal, it can be the problem. Check this part of the drive train unit before working on any other part of the drive train unit. Power shift or automatic transmissions can cause the vehicle performance to be low if they are out of adjustment or not working correctly. *
Wrong Gear Ratios...The tire size, rear axle ratio, and transmission gear ratios must be correct to get maximum engine performance. If the transmission gear ratios are wrong, they can cause the engine rpm to go low enough during shifting that the engine can not have correct acceleration. A rear axle gear ratio which supplies too high a vehicle speed with the engine at a low rpm during normal vehicle operation, will cause the engine to be "lugging" (when the truck is used in a gear too high for engine rpm to go up as accelerator pedal is pushed farther down, or when the truck is used in a gear where engine rpm goes down with accelerator pedal at maximum travel). Your authorized dealer can give you the correct tire sizes and gear ratio for your operation.
Chassis Dynamometer Error...Chassis dynamometers can be a great help in testing a vehicle for engine performance if they are in good condition and used correctly. When the dynamometer is not in good condition and incorrect operating procedures are used the results will be inaccurate. For good comparison of horsepower readings from different vehicles use the same dynamometer with the same operator.
Trailers That are Difficult to Pull...Some trailers are more difficult to pull than others because of one or more of several factors. Some of these factors are: brakes not releasing completely, high wind resistance (because of a large front area and/or the design of the trailer), axles not in alignment, extra axles, and low tire pressure.
High Inlet Air Temperature...Air coming into the engine must be cool for the engine to have full horsepower. If the air inlet system is not of correct design or is not in good mechanical condition, hot air can come into the engine causing a loss of horsepower. Check the inlet air temperature.* With the engine running under load the inlet air temperature must not be more than 135°F (57°C).
*Authorized dealers are equipped with the necessary tools and personnel familiar with disassembly and assembly procedures to perform these services.