Air Brake Job Using Machining Tools

Machining School Bus Brake Shoes from John Whelan on Vimeo.
The lathe in the video makes a complete cut on the brake shoe allowing 100% contact. This is necessary when a chatter or brake pulsation is occurring with brakes applied. We used to machine all of our brake shoes on all brake jobs but found it to be time consuming. Now we replace the drum and shoes plus hardware.

There have been some cases where the contact is not favorable and machining was necessary. Our buses use all sizes of brake shoes from 6 inch on the steering axle to 8.5 inches on the drive axle. All air brakes come standard with automatic slack adjusters. I talked to a sales rep from Canadian Brake and he says there are still customers who purchase manual slack adjusters. I haven't seen one of those for years.

Since we run a fleet of school buses it's policy to replace the auto slacks every brake job. There is only one drive axle so 200.00 for a couple of slacks is nothing. Spring brake pots get replace every 5 years. Our choice is to use MGM piggy back pots because of their superior quality. For an extra 50.00 that's great insurance.

A few months ago I attended a trade show in town and talked to the Haldex slack rep and he informed me to not grease them too often. This will disable the mechanism with the hydraulic effects of the grease. We have auto grease systems installed on our S cam tubes and slack adjusters. That has caused grease leakage on to the brake shoes from over greasing which is picked up by the government vehicle inspectors.

If it's bad enough the wheel has to be removed and shoes cleaned up. This is a procedure we want to avoid so now we advise the auto grease system installers to avoid hooking up to those areas including the steering box bearing. That's another spot the DOT makes notes on during their fleet inspection thinking it's a seal leak.

Getting back to the brake lathe it does wonders on pesky brake problems that the operator experiences. There was one time when we just kept machining down the shoe material until there was a total 100% contact to fix a pulsation. The drums had to be machined as well. There was a lot of material removed to accomplish this but the front brakes last twice as long as the drive axle brake so it was worth the labor to do so.

How To Troubleshoot Air Brake System ABS Code .

This ABS system is by Meritor Wabco found on most highway transport vehicles. An ABS light on the dash indicates a problem that has to be repaired since an ABS fault is a fail according to Motor Vehicle Inspection criteria. The ABS system does not operate normally when there is an active code.

The code description reads "Right Front Wheel Sensor - Open"

When looking at the Right Front wheel the sensor looks like it's intact. The first thing to check is to make sure the sensor is pushed all the way into the housing. The sensor reads a tone wheel on the hub and sends a signal to the ABS Control Module.

According to the troubleshooting specs the resistance of the sensor should be between 900 and 2000 ohms. When I checked it out there was no continuity causing an obvious open circuit as indicated by the fault description. The first step is to check the wiring itself for any damage or the possibility of a faulty sensor. Most times exposed wiring to the environment is at fault when troubleshooting a circuit problem.

Further inspection shows a spot on the sensor wiring that has rubbed through on the brake pot assembly. One of 2 wires has been damaged and caused the open circuit condition. I use heat shrink butt splices since they seal very nicely from the environment.

Once the sensor was replaced I moved the bus in a forward direction & the ABS warning light went out. This was an easy fix and it was nice to stumble across the bad wire in an easy to reach location. This conventional school bus is a only a year old but you never know what can happen to wiring and components when operating on a daily basis. I hope this mechanic information has helped you out and maybe you will have it as easy as I did with repairing this failure.

Mercedes MBE Diesel Engine Air Compressor Cycles Too Often

We recently had a problem with a C2 School Bus that had a air system problem. The compressor was unloading every minute, discharging the air dryer. The compressor and air dryer were both doing their job. The air system build time was OK and the air dryer [fyi: filters the air and gets rid of moisture /oil] was doing it's part.

So we focused on an air leak in the unloader circuit. Getting back to basics the air compressor is controlled by the governor which cycles the compressor between 90 psi and 125 psi. This extends compressor life allowing it to cool and take the load off until the air system pressure drops back down to 90 psi.

Also to remove the moisture and oil residue the air dryer unloads at 125 psi as well spitting out the contaminants.
So eventually we found a leak at the unloading air line to the compressor head. The engine is a Mercedes MBE 900 and the compressor is made by Mercedes. The main problem was the fitting which is a push and lock style that leaked down fast enough to allow the unloading air pressure to drain off to atmosphere and cause the compressor to kick in and go through it's cycle once again. The advantage of having an air dryer is the fact that the abnormal discharge of air every minute is very obvious.
So once the part [Thomas # 001F/23-13301-412] was replaced everybody was happy again. So if you have a constant discharge of the air dryer way before it's time it's most likely an air leak in the unloader circuit for the compressor. I hope this mechanic information has helped you...