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Jeremy, Certified Truck/Bus Technician
Category: Medium and Heavy Trucks
Satisfied Customers: 3375
Experience:  ASE Med/HD Truck Technician, NATEF Diesel Engines and Electrical/Electronics Technician, EPA 608/609 HVAC Certified, Steering/Suspension and ABS/Brakes Technician
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GMC 6500: Have a 2006 GMC 6500 chassis cab truck with Isuzu

Customer Question

Have a 2006 GMC 6500 chassis cab truck with Isuzu 7.8 liter - VIN is 1GDJ6C138F412732
The brake malfunction light comes on and then the alarm sounds almost anytime the vehicle is running. Last year it was very intermittent, but this year is almost always. Any idea what to look for or where to start looking?
Submitted: 2 years ago.
Category: Medium and Heavy Trucks
Expert:  Jeremy replied 2 years ago.
My name is ***** ***** I will assist you with your concerns.
Can you connect a scanner to it and see what code it has for the brake system? This will rule out the ABS on this vehicle.
Do the brakes feel spongy or hard at all?
Does the power steering feel like it is acting up?
If the truck key is off and you press the brakes, does the pump come on?
If the key is on, truck engine off, does the pump come on?
Link is an explanation of how the brake system works and what you can check in the mean time.
This link is a Word document. If you can't open it, let me know and I will convert it to pdf for you.
Expert:  Jeremy replied 2 years ago.
I see you viewed my info and the document I provided in the link.
Were you able to get to those switches to test for correct voltages during the on/off of the switch and to see if any of the switches are bad on the system, yet? Sometimes it can be easier to just replace those switches at first if you are not comfortable with the testing if the motor is doing its job with the key off pedal pressed, key on engine off and constant run of motor, engine on and motor off. That is the most important is to make sure fluids are topped, the motor works properly for the rest is electrical which is normally an issue with the switches if there is no power steering loss on the vehicle.
Customer: replied 2 years ago.
Jeremy, doing this from my phone.... Bear with me.
First of all, now the malfunction does not show up. As I said, it's been intermittent.
As for your I initial questions:
I have no scanner - is there a way to get codes without one?
Brakes feel fine
Power steering is fine
Truck key off and press brakes I hear no pump
Truck key on and engine off I hear no pump
I should clarify: my abs light does not come on and my primary brake light does not come on. It is a light located below the radio which shows a wrench and a brake lining pic with the word "brakes". I'm not sure if this is referred to as the aux brakes?
I have checked all fuses indicating they are for brakes and all are good. The brake fluid level is good, as is the p/s fluid level. I've unplugged and replugged the two switches on the mstr cyl and booster.
Is the pump you're referring to, the one directly connected to the booster?
Are these two switches the only ones? How is that pump supposed to function - only if there is a brake fluid or booster sensor failure?
Expert:  Jeremy replied 2 years ago.
Ok, the pump should come on with the key off and pressing the brake.
Since you don;t have the light on the dash, I'm going to say the motor is bad. Yes, it would be the motor on the booster.
Try testing the motor by putting power direct to it. If it doesn't come on, the motor needs to be replaced. If it does, then there is a issue in the brake control module. You will need a scanner to pull the codes. There is no way to do it without. I wish there was.
You are getting that light because the motor isn't coming on, which is the emergency backup braking system aka auxiliary brakes. Now that it is narrowed down to the system, the question is why is that motor not coming on. It is either the motor or the module mounted on the frame not turning the motor on. I hope it is just the motor for it is inexpensive to fix. If the module, it is very expensive and you have to program it.
Customer: replied 2 years ago.
Jeremy, I've just checked the booster motor out.
I have no voltage going to it w the key off and pressing the brake pedal.
When I unplug the motor and test for voltage, I have none with key off and pressing the pedal as stated above.
However I have 9 volts w the key on and not pressing the pedal. It doesn't change even when the pedal is pressed - 9 volts. That seems odd?!
I directly connected 12 volts to the booster motor and it immediately activates.
I followed the wire harness down along the frame and see another motor above the fuel tank but looks as though it has a compressor connected? Also has an inline fuel filter (but not fuel) and a relay among other things. I took a pic of it if I can attach. Is this the module?
Now, back near the booster motor, I see the connection of course and also a relay in the black box (again I have a pic to attach). Is this relay also in the system? Should I be cking these? Trying to be sure there is no other possibility other than the module if it's that expensive.
Now this morning when I turned the key on motor off, the alarm activated and aux brake light came on. Yesterday it didn't. Intermittent.
Expert:  Jeremy replied 2 years ago.
There is an issue going on with either the relay for the brake system or the module itself.If you can, borrow a scan tool for Chevrolet cars and see if you can pull the brake code on that. Sometimes the basic scanners will read the brake codes. I don;t want you buying the module and it not be the cause. With it being intermittent, that makes the search harder...