Three6GMC

iTrader: (11)

Alright well after tinkering with it for a hour think I have it figured out. You can plug it in straight into the intake manifold and won't cause any issues or service brake soon lights. But thats not what I'm wanting to do I'm trying to delete it completely without any issues. Basicly the sensor is just a map sensor has 5v going in and has to see resistance on the other 2 wires. The wires are labels 1-2-3, 3 being the 5v in. Now you would think just connect all 3 wires and done you get your 5v out to those wires but no. From what I can see you need 5v to them yes but they also need to see 3.8 ohms of resistance. What I'm about to go do is run to radio shack and get some resistors and run some tests. What I'm thinking is running resistors from wire (3) to wires 1 & 2. That should give me my correct resistance and voltage, hopefully. Also this sensor just reads one amount of resistance and voltage it doesn't range with vac or speed or anything like that. Its a pretty simple Bosch sensor just needs the right little bypass setup... Ill keep you all informed.



+++EDIT+++

Alright so with the sensor not plugged in it reads 4.89 k ohms across pin 3 & 2 and 3.7k ohms across pin 3 and 1... When powered up key forward the pins read over limit. Now with the sensor just plugged in but not connected to any vacuum source just in the free air it won't throw and service light, unplugged light comes on. I got the right resistors but its still throwing the brake light. Think i just need more resistors? Can anyone get a wire schematic for this sensor? Im sure I can fig it out but won't make it back to radio shack till tomorrow after work.

Wyttrash96

iTrader: (16)

This sounds like its way beyond me but I wish you good luck cause that SERVICE BRAKES SOON message is pissing me off!

AUTOT3K

iTrader: (3)

when you came up with the 4.89k was that checking on the sensor or the connector?
and the reason you got out of field powered on is cause you cant check the resistance of a live circuit

Three6GMC

iTrader: (11)

I was checking the sensor not the connector when I read the ohms but yeah makes sence about the live circuit reading. Thanks I didn't think about that. So far I can drive around with the sensor plugged into just the intake and the open side that plugged into the booster capped off and no service light but I'm going to continue working on a way to remove it. Slowly but surely lol. The sensor itself has a built in check valve so as long as the cap on the other side of the sensor hold vac should be good on a boosted application.

AUTOT3K

iTrader: (3)

my next step would be, put the sensor under a vacuum then check the resistance.
I'm guessing you probly checked it with the engine off?
maybe when you added your resistor...it was a value for engine off.

and just to make sure we're on the same page haha, your isolating the sensor then checking it? not just back probing?

11silverado

I spoke with a service writer I know and he researched with some techs that if the sensor reads between .13 and 3.3 volts it will not throw a code.
I have to figure out how to accomplish that.
Can you discuss the details on the three wires and if only one needs to read the volts or if they all do? Also is they way this works is the wire is at 5 volts and when the sensor read vacuum it would amount to a certain resistance and it wound bring voltage reading down to between .13-3.3 volts? If so do we just ignore sensor and just worn directly on wires / connector. Please excuse the basic questions but I build up to answers slow...

11silverado

ok looking at this further, a few observations and questions:

1) with my hydroboost conversion, I have hose to intake still on brake booster sensor, and the other end that went to old vacuum booster capped off and i still get service brakes soon in dic every start up which i can clear with pushing dic button

2) the wire that pushes the 5v is what color / position (center?)? I do not see any number lables such as 1-2-3?

3) based on info from the dealer service guy i spoke to , he said to get .13 to 3.3 volts reading and will be no code. doesnt this simply mean we need to get the 5 volt wire and have it go to each other wire (3 to 1 and 3 to 2) with a 6 ohm resistor between ? 6ohm will take 5v down to around 2v which is in spec.

4) i am going to try this in next few days with just getting 2 6ohm resistors and connecting between 3 and 1 / 3 and 2 right on the connector eliminating the sensor. i just need help identifying which cable is the 5 v. I can prob just pull a reading from a meter but if anyone can chime in to confirm which cable will help.
if works will figure out secure way to keep resistors on the connector..etc.

If any of this has already been tested let me know

11silverado

dang i just realized my 6 ohm resistor is based on 500ma (amps) and i have no idea currently what amps are being pushed on the 5v cable. once i figure that out it will allow me to figure out correct ohms for resistanc to put other two wires around 2v. will update when i test further

wardlws

Did you get rid of the "Service Brakes Soon" message? I also took some electrical readings before I did my conversion today. I measured about 2.5 Volts on the sensor/trigger/signal wire with the engine idling and the sensor in the vacuum booster. I was hoping that just capping the sensor would be ok, but it does not seem to be (at least not after two short drives). My next plan is the same as yours. Create a voltage divider by routing half of the current from the 5V wire to the signal wire and the other half of the current to the ground wire. Two resistors of matching resistance. Any resistance should be fine, and since this is low voltage, low wattage resistors should be just fine. I probably won't have time to work this issue for a few days. I have to deal with a leaking steering rack first.

Thanks!
Scott

wardlws

EDIT: Ignore this. I stepped on the brake when starting the engine and invalidated these tests.

Just reporting back with what I have tried (that didn't work).

1) I put a vacuum cap on the sensor and left it plugged into the harness and vacuum from the intake. This lasts the longest before throwing the message "Service Brakes Soon".
2) I unplugged the harness and built a voltage divider using two resistors that were the same (4.7k ohms). This gave a constant reading of 2V on the signal wire, but threw the message immediately.
3) I replaced one of the 4.7k ohm resistors with a 2.2 k ohm. This gave a constant reading of 2.8V on the signal wire, but also threw the message immediately.

My guess is the EBCM needs a before engine start reference signal of ~5V, then it looks for that voltage to drop as vacuum builds. Here is a snippet from a GM tech bulletin:
"Normal Brake Pressure Sensor voltage readings during service brake release and apply should range from .5 volts - 3.0 volts."

And here is a chart showing what the sensor is supposed to produce (engine idle is about 2.5V and about 50-70 kPa):


I'm getting a good OBD-II reader for Christmas that I hope will tell me more. My next thing to try will be to keep the vacuum sensor plugged in and either add a small constant voltage to the signal (in addition to the voltage fluctuating from the sensor) or remove some voltage. Both of these can easily be done with a resistor.

EDIT: Ignore this. I stepped on the brake when starting the engine and invalidated these tests.