Ok, I searched and search on finding the right formula to allow datalogging using the gauge 5v output (WHITE WIRE). I could not find the right answer to save my life. I found several incorrect answers, plugged them in, only to take the car for a ride and see that it was wrong & usually way wrong.
Yes livelink has popular equations in a drop down box in Analog Equations & yes AEM is one of them BUT that is for a wideband gauge and not a boost gauge. (v*2)+10 is the correct formula for the wb.
Yes you can splice a wire into your maf sensor as well, but why bother with that when you are in the car and not under the hood & the gauge is right in front of you? The gauge by the way, Ts into the vacuum line coming into the cabin (behind glove box) and actually reads the vacuum/boost. This is not electrical. It then converts that reading to a 0.0-5.0v signal. That signal is sent through the white wire that you use to hook into your sct unit.
My particular gauge is AEM Digital Display 30-4406 Model Number. It reads -30 to +35 vacuum boost. This is important. If you have one of there other boost gauges that reads a different amount, I know some read up to +50psi or so, you will need to figure out your own calculation, but relax, I will tell you how to figure it out, using mine as an example.
I got to thinking through my irritation with the wrong formulas found online.......what do the numbers in these equations actually represent? If I figure out how the WB works since it has a correct formula, then I can apply the theory to the boost gauge. Even though I run e85, I read gasoline AFR on my display because everyone is much more familiar with a stoich value of 14.7 (14.68) rather than the 9.02 of ethanol. There is a calibration chart in the manual with the wb gauge that tells me 2.34v from the white wire is stoich 14.68. AH-HA. That rang a bell.
I know my boost gage reads from -30 to +35psi. Common sense then told me that -30 must be 0v & +35 must be 5.0v. So I wrote that down on an integer line. Splitting the 5v down the middle is 2.5v, so I put that in the middle of the integer line. I then split the difference of 5v & 2.5v coming up with 3.75 in the middle of those 2. I also split the 0v & 2.5v, putting 1.25 in the middle of those, so it looked like:
0--------------------------1.25----------------------2.5------------------------3.75..........................5
--------------------------------------------------------------------------------------------------------
(-)30--------------------- --------------------- ------------------------ -----------------+35
All I had to do now was apply the same theory to the voltages, so +35-(-)30=65 then divide by 2=2.5
0--------------------------1.25----------------------2.5------------------------3.75..........................5
--------------------------------------------------------------------------------------------------------
(-)30--------------------- ---------------------2.5 ------------------------ -----------------+35
35-2.5=32.5 divided by 2=16.25 So 35-16.25=18.75
0--------------------------1.25----------------------2.5------------------------3.75..........................5
--------------------------------------------------------------------------------------------------------
(-)30---------------------- ---------------------2.5 ------------------------18.75---------------+35
Finally 2.5-(-)30=32.5 divided by 2=16.25 (duh) so 2.5-16.25= -13.75
0--------------------------1.25----------------------2.5------------------------3.75..........................5
--------------------------------------------------------------------------------------------------------
(-)30------------------- -(-)13.75 -------------------2.5 -----------------------18.75-----------------+35
All that needed to be done now was figure out what formula would make all of those true 0v = -30psi, 1.25v = -13.75psi, 2.5v = 2.5psi, 3.75v = 18.75psi & 5v = 35psi. The instructions gave: PSIA=12.5*(volts)-6.25* scale: .5v-4.5v = 0-50psia
Any how I took the difference of each voltage which is 1.25 & place that under the difference of the psi which is 16.25. 16.25/1.25 = 13. I knew this should be the multiplier then. After that is was just trial & error.
So 13x1.25v=16.25 I know at 1.25v the display needs to ready -13.75, so 16.25-??.??=-13.75 (16.25+13.75=30) Bingo! 16.25-30=-13.75!
Now apply to the others and see if it works: 13x0=0, 0-30= -30!
13x2.5=32.5, 32.5-30=2.5 !
13x3.75=48.75, 48.75-30= 18.75
Finally 13x5=65, & 65-30 does = 35! So your equation for the SCT Livelink is: (v*13)-30.0
One other little trick. You may or may not know on the Wideband gage, you can calibrate the gage itself with a voltmeter to read the correct voltage. You simply unplug the sensor & the gage should read 2.35v. If it does not, then there is a gain adjustment to correct it to that voltage. Unfortunately there is no gain adjustment on this boost gage. Remember, the digital display on the boost gage is actually measuring vacuum & psi so that reading is correct, unless you have a bad/broken gage. It may not match the number that you see on the livelink display. SIMPLE solution, adjust +30 in your equation to make up the difference. My car the gage at idle showed -24.0 while livelink show -21.0. I simply changed the -30 to -33 & they are in sync.
You are welcome.
Yes livelink has popular equations in a drop down box in Analog Equations & yes AEM is one of them BUT that is for a wideband gauge and not a boost gauge. (v*2)+10 is the correct formula for the wb.
Yes you can splice a wire into your maf sensor as well, but why bother with that when you are in the car and not under the hood & the gauge is right in front of you? The gauge by the way, Ts into the vacuum line coming into the cabin (behind glove box) and actually reads the vacuum/boost. This is not electrical. It then converts that reading to a 0.0-5.0v signal. That signal is sent through the white wire that you use to hook into your sct unit.
My particular gauge is AEM Digital Display 30-4406 Model Number. It reads -30 to +35 vacuum boost. This is important. If you have one of there other boost gauges that reads a different amount, I know some read up to +50psi or so, you will need to figure out your own calculation, but relax, I will tell you how to figure it out, using mine as an example.
I got to thinking through my irritation with the wrong formulas found online.......what do the numbers in these equations actually represent? If I figure out how the WB works since it has a correct formula, then I can apply the theory to the boost gauge. Even though I run e85, I read gasoline AFR on my display because everyone is much more familiar with a stoich value of 14.7 (14.68) rather than the 9.02 of ethanol. There is a calibration chart in the manual with the wb gauge that tells me 2.34v from the white wire is stoich 14.68. AH-HA. That rang a bell.
I know my boost gage reads from -30 to +35psi. Common sense then told me that -30 must be 0v & +35 must be 5.0v. So I wrote that down on an integer line. Splitting the 5v down the middle is 2.5v, so I put that in the middle of the integer line. I then split the difference of 5v & 2.5v coming up with 3.75 in the middle of those 2. I also split the 0v & 2.5v, putting 1.25 in the middle of those, so it looked like:
0--------------------------1.25----------------------2.5------------------------3.75..........................5
--------------------------------------------------------------------------------------------------------
(-)30--------------------- --------------------- ------------------------ -----------------+35
All I had to do now was apply the same theory to the voltages, so +35-(-)30=65 then divide by 2=2.5
0--------------------------1.25----------------------2.5------------------------3.75..........................5
--------------------------------------------------------------------------------------------------------
(-)30--------------------- ---------------------2.5 ------------------------ -----------------+35
35-2.5=32.5 divided by 2=16.25 So 35-16.25=18.75
0--------------------------1.25----------------------2.5------------------------3.75..........................5
--------------------------------------------------------------------------------------------------------
(-)30---------------------- ---------------------2.5 ------------------------18.75---------------+35
Finally 2.5-(-)30=32.5 divided by 2=16.25 (duh) so 2.5-16.25= -13.75
0--------------------------1.25----------------------2.5------------------------3.75..........................5
--------------------------------------------------------------------------------------------------------
(-)30------------------- -(-)13.75 -------------------2.5 -----------------------18.75-----------------+35
All that needed to be done now was figure out what formula would make all of those true 0v = -30psi, 1.25v = -13.75psi, 2.5v = 2.5psi, 3.75v = 18.75psi & 5v = 35psi. The instructions gave: PSIA=12.5*(volts)-6.25* scale: .5v-4.5v = 0-50psia
Any how I took the difference of each voltage which is 1.25 & place that under the difference of the psi which is 16.25. 16.25/1.25 = 13. I knew this should be the multiplier then. After that is was just trial & error.
So 13x1.25v=16.25 I know at 1.25v the display needs to ready -13.75, so 16.25-??.??=-13.75 (16.25+13.75=30) Bingo! 16.25-30=-13.75!
Now apply to the others and see if it works: 13x0=0, 0-30= -30!
13x2.5=32.5, 32.5-30=2.5 !
13x3.75=48.75, 48.75-30= 18.75
Finally 13x5=65, & 65-30 does = 35! So your equation for the SCT Livelink is: (v*13)-30.0
One other little trick. You may or may not know on the Wideband gage, you can calibrate the gage itself with a voltmeter to read the correct voltage. You simply unplug the sensor & the gage should read 2.35v. If it does not, then there is a gain adjustment to correct it to that voltage. Unfortunately there is no gain adjustment on this boost gage. Remember, the digital display on the boost gage is actually measuring vacuum & psi so that reading is correct, unless you have a bad/broken gage. It may not match the number that you see on the livelink display. SIMPLE solution, adjust +30 in your equation to make up the difference. My car the gage at idle showed -24.0 while livelink show -21.0. I simply changed the -30 to -33 & they are in sync.
You are welcome.
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