JTEC Info - Turbo 2000 Jeep Wrangler Project

[sickgrin]

Hey guys,

Gearing up to add a turbo to my 2000 Jeep TJ. Currently learning/collecting as much information as I can about this ECU before actually installing the turbo. Just wanted to share some info I've put together so far.

Although I'm not exactly new to tuning, this is the first time I'm playing around with a JTEC. It is a far cry from Megasquirt or any of the aftermarket systems I've used before. So bear with me, I'm still figuring things out.

One of the things that I kept searching around for, and couldn't find much information on was the MAP scale multiplier and additive numbers and how those affect what the ECU ultimately sees. So I went ahead and attempted to reverse engineer some of the math. Now realize, this is all based on assumptions and / or guesses, but the numbers seem to be lining up for the most part.

((Sensor Voltage * 51) * MAP multiplier + MAP additive) / 623.5 = Kpa

The 51 converts the 0-5 volts from the MAP sensor to a byte range. I'm not sure if they're using more than one byte for the map sensor resolution. At one byte you'd get ~ 2/100th of a volt of resolution and stepping though the scanner logs I never saw less than 2/100th of movement in the MAP voltage. To know for sure I'd have to test the sensor plugged into the vehicle and use a vacuum pump to observe the ECU reported output. However, for the time being, the math appears to line up using 0-255. If anyone has more info on this, let me know.

After getting that formula sorted, I wanted to be able to scale the existing pulse widths to match the new MAP sensor and then interpolate all of the boosted cells. I also wanted to be able to scale the pulse widths based on the difference in injector sizes. So I put together a basic calculator to make the process a bit easier. Again, not sure if all the math is correct yet, so use with caution on your own vehicle.

http://jtec.sickgrin.com/

One thing I saw the SCT guys referring to was a max closed loop MAP value for the JTEC. Does anyone know where (or if) this is in the HP Tuners interface?

As I get further along, I'll be sure to post my progress here.

Thanks!

[LilSick]

well if the math is correct that is sick as fuck

thx

[sickgrin]

Thanks. Yeah, I started doing this on a spreadsheet but it quickly got a bit cumbersome.

One thing I discovered is that the MAP sensors don't exactly match up with their documented values and/or what the computer expects. For example the factory Jeep 4.0L sensor at 25.0 kpa actually outputs a voltage the JTEC sees as 23.9 kpa. This was totally throwing me off in the beginning when attempting figure out the math.

[LilSick]

Is it possible that HPT has it wrong and JTEC uses TORR or mmhg?

[sickgrin]

I certainly couldn't tell you for sure how it's being referenced in the JTEC ecu. However, I'd be willing to bet it's handling MAP values based on how many bits its ADC inputs are. So basically it's looking at the voltage in and converting it to 0-255 (8 bit) or 0 to 4095 (12 bit) or 0-65535 (16 bit), etc. The unit (kpa, torr. mmHg, inHg) is just a conversion factor for our sake. The computer is only referencing the bit conversion of the voltage. That would be my guess based on other micro controllers I've worked with.

[jbliss15]

anyone know how / where to get info about a map sensor to input? i'm using a honda 1bar