Friday, May 06, 2016

Park Brake Not Releasing C2 School Bus

Park Brake Not Releasing C2 School Bus from John Whelan on Vimeo.
The park brake on a Thomas C2 school bus has an interlock that requires the service brake to be applied before the park brake releases. Most of the issues we had was the stop lamp module and park brake module located at the air management unit depending on the year of the bus the bank of modules are at the very back in front of the differential housing or behind the transmission.

There was a recall years ago regarding the stop lamp switch located on the right side of the AMU looking towards the back end. The problem was the internal switch wasn't contacting properly so Thomas revised the design and there have been very few problems since then. If you go into  accessfreightliner you can bring up the AMU layout for the VIN you're working on.

The park brake light on the dash is a warning to the driver not to drive away with the park brake on, When a driver pulls out the park brake valve the modules sense this and through the J1939 messaging system air pressure changes trigger a signal that is sent back to the dash illuminating the park brake light. When you come to a stop and shut down the engine if the park brake valve is not activated this triggers an alarm until the park brakes are applied.

Unless there's an air leak most of the trouble with AMU modules is the switch contacts. The first check is to see if the brake lights are working if not the stop lamp switch in the module can be tested by jumping the signal to the bulk head module bypassing the internal switch in the module. If the brake lights work there is a problem internally with the stop lamp module.

The same test can be done with the park brake module. In the video the two terminals that sends signals to the BHM are jumped and if the park brake indicator light comes on then there is an internal problem in the module itself. There are only two wires to this module so it's a straight forward test. Most of our troubles have been with these 2 modules.

Also you can not rule out opens and resistance in the wiring harnesses. This is another possibility that you might have to deal with. If you have a Freightliner dealer in your city you need to get a user and  password to the accessfreightliner website. Here you will find the schematics and troubleshooting required to fix an electrical problem. Without this information it's very difficult to blindly find an electrical fault especially with multiplexing that's here to stay for a long time.

10 comments :

Dani said...

So, I am looking to kickstart a project of buying a diesel school bus and converting it into a bio fueled engine. I know nothing about mechanics but I want to learn and find someone to help me do this. when I find them I don't want to be so ignorant to the process that I can't be involved. Do you recommend any beginner books that can teach me about the parts of a bus Diesel engine and how they work? Also, what area are you from? I'm southern Illinois/Missouri.

Dani said...

So, I am looking to kickstart a project of buying a diesel school bus and converting it into a bio fueled engine. I know nothing about mechanics but I want to learn and find someone to help me do this. when I find them I don't want to be so ignorant to the process that I can't be involved. Do you recommend any beginner books that can teach me about the parts of a bus Diesel engine and how they work? Also, what area are you from? I'm southern Illinois/Missouri.

Dani said...

So, I am looking to kickstart a project of buying a diesel school bus and converting it into a bio fueled engine. I know nothing about mechanics but I want to learn and find someone to help me do this. when I find them I don't want to be so ignorant to the process that I can't be involved. Do you recommend any beginner books that can teach me about the parts of a bus Diesel engine and how they work? Also, what area are you from? I'm southern Illinois/Missouri.

Armando Santos said...

Hi John, i am antonio from chile, been watchig your videos and i have a emergency w a cummins isc fuel pump, we broke in the midle of the desert so is kind of a complicate situation. please if you can contact me to my email antonio@briceno.cl or by whatsapp +56998627240 i will really apreciated your help we been stock for a week now. thanks

Anonymous said...

Hi John, i am antonio from chile, been watchig your videos and i have a emergency w a cummins isc fuel pump, we broke in the midle of the desert so is kind of a complicate situation. please if you can contact me to my email antonio@briceno.cl or by whatsapp +56998627240 i will really apreciated your help we been stock for a week now. thanks

Armando Santos said...

Hi John, i am antonio from chile, been watchig your videos and i have a emergency w a cummins isc fuel pump, we broke in the midle of the desert so is kind of a complicate situation. please if you can contact me to my email antonio@briceno.cl or by whatsapp +56998627240 i will really apreciated your help we been stock for a week now. thanks

Armando Santos said...

i John, i am antonio from chile, been watchig your videos and i have a emergency w a cummins isc fuel pump, we broke in the midle of the desert so is kind of a complicate situation. please if you can contact me to my email antonio@briceno.cl or by whatsapp +56998627240 i will really apreciated your help we been stock for a week now. thanks

randy south said...

I jumped wires on park bake ,and stop light, stop light has 12 volts,but no voltage on the park brake, no voltage coming out of the module,replacEd module still no voltage ,aNY idea's I

John Whelan said...

Randy looks like wiring to the park brake valve is faulty. If you disconnect 2 wire connector at the PB valve and jump them does the brake light indicator come on at the dash? If not check the wrg.

John Whelan said...

Armando what exactly is happening? You have a no run?