Hot Diagnosis

When one of our drivers reports that the "brakes need adjustment," how important is it that I get the brakes up to operating temperature before I try to diagnose the problem?—Reader

Brake systems at the wheel end are simply a heat conversion device. Energy in motion, the vehicle and the cargo, are converted to the energy of heat at the wheel ends to slow the vehicle and bring it to a stop. The temperatures at each wheel end are extremely important in diagnosis and service needs determination.

Since all of the wheel ends on a vehicle are equipped with brakes, they all must do their fair share of the braking. When the system is working properly, all of the temperatures should be even during operation. Therefore, from the ambient temperature outside to the operating temperature, all of the wheel end locations should be within 10 percent of each other at the maximum.

These temperatures can be measured by the technician upon driving the vehicle simply by checking those wheel end temperatures with a heat gun and comparing them around the vehicle. However, there are some requirements when testing that must be remembered and adhered to. This will ensure proper and accurate results.

Rather than climbing under the vehicle and checking the brake drums directly, it is easier to measure the lugs on the lug-nut assembly at each wheel. Using the highest lug-nut assembly on each wheel end position, aim the heat gun end one inch from that lug nut and read the temperature and record it. Do this at each wheel end location on the maintenance record. Then take the temperature recorded and multiply it by two and record this after the initial temperature. Properly operating brakes should never exceed 400-450 degrees F.

For example: 195 X 2 = 390. Therefore, it should be written as 195/390 for each position; record this and compare the temperatures. They should all be within 10 percent of each other if the brake system is operating properly. Each wheel that is colder is not contributing to the vehicle’s braking. Each wheel that is much higher is doing all of the work.

Without this diagnosis procedure it is impossible to repair the vehicle braking system with any accuracy. After a repair is completed, the same temperature testing should be done to ensure that the vehicle’s braking system is ready to work properly.

When diagnosing brake problems, there are basic fundamentals that must be considered that are extremely important:

  1. Brake spyder alignment and bushing. The spyder must have proper bushings and clearances to mount all of the brake components in their proper position and hold them there when the brake are operating.
  2. Brake attaching hardware and clips. The attaching hardware and clips must be replaced at each friction reline to ensure that they have the correct tension and operation balance to hold, retain, and allow all of the brake wheel end components to function. Never reuse old brake hardware.
  3. Brake shoe steel and friction material combination. Seventy to 80 percent of each brake shoe should contact the brake drum to allow for the proper amount of friction material to be heated and to absorb the heat from the stopping effort. Uneven contact from shoe to shoe and side to side can cause many problems.
  4. Brake drum assembly. If the brake drum diameter or conditions are different than required, they will cause excessive heat to be generated and brake failure is possible.
  5. S-cam and bushing assembly. If any movement is allowed at the bushing end of the S-Cam, it will move the cam and change the amount of contact to the brake drum surface and reduce the footprint of the friction material, causing overheating and poor performance.

All of these conditions must be measured and recorded to ensure that the brake system operates within the designed temperatures during operation of the vehicle. Failure to do so will cause problems. That is why it is so important to do what is never done to a brake service. Once brake service is accomplished, the brakes must be burnished and temperatures recorded to ensure that the brakes are functioning properly.

Tom Golden has been in the repair business since he was 18 years old. He started his own automotive and truck repair business, then became Director of Training for the Echlin Group. After a stint as an independent crash accident investigator and trainer, Tom became the Brake Pro, Ltd. Director of Technical Services and Training. He has also worked closely with ASE in conjunction with automotive and heavy-duty truck brake testing.