pcmhacking.net

Hi All,

My LS1 build is running short of vacuum for the brakes and I've purchased a VF UP28 electric vacuum pump and also got a hold of the Bosch 20876799 vacuum switch to turn the pump on and off.
Unfortunately it seems instead of this running a 12v out line to trigger the pump, it looks to use the PCM to control when the pump turns on.
Seems like a stupid idea to me, but anyway, it uses a 5v in, ground reference and an output wire back the PCM.
My guess is on the E39a PCMs and similar ones the OS has a table of sorts that basically says when the voltage reaches x then send 12v (or ground) to a relay, when the voltages goes back to y, shut it off.

Is there a table in the 0411 PCMS (currently using HPT RTT 1 Bar SD OS) that could be utilised to monitor the voltage (Vacuum at the sensor) and send a signal to engage the pump via relay?
If there's a solution to this other than fork out $150 for a CVR or Aeroflow branded pressure switch it would be preferable.

I can't fathom how or why the PCM should be involved in controlling the vacuum pump. You can't just use a 12v relay and a vacuum switch?

Cincinnatus wrote:I can't fathom how or why the PCM should be involved in controlling the vacuum pump. You can't just use a 12v relay and a vacuum switch?

Something to do with variable timing and low vacuum build up at idle, I also thought it was ridiculous but apparently they use it on Volvo C30/70s, Vauxhaul Astras and some BMWs I think.

Also the vacuum switch for this is $100 which I'm not gonna pay that much for a simple switch.

Found a solution for this:
Arduino set up and relay.
Arduino board measures the 5v input and I program it to open and close the circuit at set voltages.

This also allows me to customise the vacuum pressures and can even add additional triggers or alarms when pressure is too low or motors running for too long and detect vacuum leaks.

You might be able to use the CAGS / skip-shift output for this.
It's a binary output, and the parameters that control it use both RPM and MAP.

NSFW wrote:You might be able to use the CAGS / skip-shift output for this.
It's a binary output, and the parameters that control it use both RPM and MAP.

Ah ok, thanks for that.
I ended up going the Arduino route though.

It was pretty easy, $29 UNO R3 board, which already has a 5v output so wired the 5v and ground onto the board, put the signal wire on one of the analog inputs (Pin A0).

If anyone else needs to do this, here's the code:
float input_volt = 0.0;
float temp=0.0;
float r1=10000.0; //r1 value
float r2=100000.0; //r2 value

void setup()
{
Serial.begin(9600); // opens serial port, sets data rate to 9600 bps
pinMode(0, OUTPUT); // sets the digital pin 0 as output
}

void loop()
{
int analogvalue = analogRead(A0);
temp = (analogvalue * 5.0) / 1024.0; // FORMULA USED TO CONVERT THE VOLTAGE
input_volt = temp / (r2/(r1+r2));
if (input_volt < 2.1)
{
digitalWrite(13, HIGH); // sets the digital pin 13 on
}
else if(input_volt > 2.0)
{
digitalWrite(13, LOW);
}
Serial.print("Voltage= "); // prints the voltage value in the serial monitor
Serial.print(input_volt);
Serial.println(" Volts");
delay(1000);

}

I haven't set it up in the car yet to acrually measure what voltage matches to 18" vacuum, but once I know when I want it to cut in and out I'll set the voltages and reupload the code.

Pin 13 goes to a $10 4channel relay board which runs on 5v and switches the high voltage relays, so the vacuum pump is hot on ignition/accessories and the ground is swtiched on/off by the arduino relay.

The board has another 4 analog inputs and a bunch of PWM/digital input/outputs so you could add all sorts of things to it.

I'm looking to see if I can make a module to emulate the ABS module so my dash doesn't keep flashing the obvious that the car its in doesn't have ABS.