2005 Model Commercial Driver’s License Manual  If the lamp stays on after the bulb check, or goes  on once you are under way, you may have lost  ABS control at one or more wheels.  In the case of towed units manufactured before it  was required by the Department of Transportation,  it may be difficult to tell if the unit is equipped with  ABS. Look under the vehicle for the electronic  control unit (ECU) and wheel speed sensor wires  coming from the back of the brakes.  ABS is an addition to your normal brakes. It does  not decrease or increase your normal braking  capability. ABS only activates when wheels are  about to lock up.  ABS does not necessarily shorten your stopping  distance, but it does help you keep the vehicle  under control during hard braking.  Truck tractors with air brakes built on or after  March 1, 1997, and other air brakes vehicles,  (trucks, buses, trailers, and converter dollies) built  on or after March 1, 1998, are required to be  equipped with antilock brakes. Many commercial  vehicles built before these dates have been  voluntarily equipped with ABS. Check the  certification label for the date of manufacture to  determine if your vehicle is equipped with ABS.  ABS is a computerized system that keeps your  wheels from locking up during hard brake  applications.  Vehicles with ABS have yellow malfunction lamps  to tell you if something isn’t working.  Tractors, trucks, and buses will have yellow ABS  malfunction lamps on the instrument panel.  Trailers will have yellow ABS malfunction lamps on  the left side, either on the front or rear corner.  Dollies manufactured on or after March 1, 1998 are  required to have a lamp on the left side.  On newer vehicles, the malfunction lamp comes on  at start-up for a bulb check, and then goes out  quickly. On older systems, the lamp could stay on  until you are driving over five mph. Section 5 – Air Brakes  Page 5-4  Modulating Control Valves. In some vehicles a  control handle on the dash board may be used to  apply the spring brakes gradually. This is called a  modulating valve. It is spring-loaded so you have a  feel for the braking action. The more you move the  control lever, the harder the spring brakes come  on. They work this way so you can control the  spring brakes if the service brakes fail. When  parking a vehicle with a modulating control valve,  move the lever as far as it will go and hold it in  place with the locking device.  Dual Parking Control Valves. When main air  pressure is lost, the spring brakes come on. Some  vehicles, such as buses, have a separate air tank  which can be used to release the spring brakes.  This is so you can move the vehicle in an  emergency. One of the valves is a push-pull type  and is used to put on the spring brakes for parking.  The other valve is spring loaded in the "out"  position. When you push the control in, air from the  separate air tank releases the spring brakes so you  can move. When you release the button, the spring  brakes come on again. There is only enough air in  the separate tank to do this a few times. Therefore,  plan carefully when moving. Otherwise, you may  be stopped in a dangerous location when the  separate air supply runs out. See Figure 5.3.  5.1.16 – Antilock Braking Systems (ABS)  Figure 5.3 