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So, I paid my 20 bucks to get 3 days of GM Service Information from acdelcotds.com. It looks like this is our problem:
"No vacuum is detected in the power brake booster for approximately 10 seconds after start-up.
Low vacuum is detected in the power brake booster for approximately 60 seconds or longer after start-up, or is detected while driving."
So, my next plan is to figure out how to determine when the engine starts and trip a relay from that signal. Then I just need to build this circuit that should simulate the vacuum sensor with 5V before engine start and 2.5V when running:
It won't fluctuate with engine load and won't change when the brake is pressed, but it sounds like from reading through the DTC documents and Service Information that those are not needed.
EDIT: I learned latter that the signal voltage must drop on brake press and rise on brake release... see message below and new post coming soon.
Couldn't you just install a small vacuum pump and actually put a few inches of vacuum on it?
I currently have the vacuum brake booster sensor connected to the intake manifold; as it came from the factory. I don't have any significant engine mods that would effect the amount of vacuum the engine would pull, so this should be ideal. However, the difference is that my sensor is stuffed into a small vacuum cap, instead of a large chambered vacuum booster. I believe the difference in chamber size is the reason for the "Service Brakes Soon" message. Also, I would like to remove this sensor from relying on engine vacuum, for all of those that have a turbo or high-lift cam, or whatever. Tomorrow, I should be able to do some more experimentation on simulating the voltage that it sends to the EBCM.
Almost solved! So, my 20 bucks to GM Service Information was a bit of a waste. The diagnostic and repair info for the DTC is missing key information. DTC C0299 states that as long as the voltage reads between 0.5V and 3.0V all is good. However, that is not the whole story.
Today, I finally had time to do some experimentation. I removed the vacuum brake booster sensor and replaced it with a variable potentiometer (pot). The pot is just like two resistors that divide the voltage, but allows me to vary the output voltage. I played around with starting the engine and varying the throttle and brake pedals. I found that the DTC is correct for starting and running the engine... the voltage on the signal wire can be anything from 0.5V to 3.0V. It does not matter if you hit the gas pedal, nor if the engine is under load... any voltage in that range is good.
However, the voltage must drop and rise again when the brake pedal is pushed. This is why capping the sensor fails after a short drive. My testing indicates that it does not matter how much the voltage drops and rises, as long as it is still in the 0.5V to 3.0V range. I was hoping to use the signal from the brake position sensor (that rotary stop light switch inside), but the signal from it increases in voltage when the brake is pushed. Fortunately, I used the GMT800 adjustable brake pedal, which has a longer stud to connect the booster rod. I've ordered a stop light switch and pigtail that should be here later this week. This stop light switch is just an on/off switch, so I'm going to use it to trip a relay that will reduce the voltage when the brake is applied and return it to 2V when the brake is not applied:
I'm off to build my circuit. I'll reply again with results.
Update 01/02/2022: Still waiting on the connector for the stop light switch. Updated the circuit to produce the correct voltages. The previous circuit was giving higher voltages due to multi-direction current flows... I'm pretty sure I got an A in my Basic Circuits class, but that was like 25 years ago.
Update 01/12/2022: I've just installed the wiring circuit and so far, so good. I haven't driven enough yet to call it a success. I've got it all written up and will follow up here and a new post with the write-up.
Success! My "Service Brakes Soon" message is gone!
However, my brakes are no longer linear. This may be a 2012 and up issue, since they have the brake pedal rotary position sensor; instead of the simple stop light switch. It seems that the EBCM has an ability to provide brake boost from the ABS pump: “The Brake Booster Vacuum Sensor is an input to the EBCM, and operates the ABS pump motor to precharge the brake system.“
I'm going to try a couple of more changes to increase or decrease the voltage when the brake pedal is pushed. My goal is to have a circuit that keeps the EBCM happy (no DTC) and keeps the brakes linear. With the circuit that I have now, the brake pedal is linear until I have a harder braking event. When the pedal is pushed past a certain point, the EBCM triggers the ABS pump to "help". Good thing no one was behind me the first time it decided to "help"... When it "helped", the brake pedal was easier to push and the brakes engaged very strong... not sure I need to upgrade to four piston calipers anymore.
Lots of below freezing whether this week / weekend. Hope to have time to get to this soon.
12-23-2021, 02:37 PM
