J1850 PWM specs

**interceptor** · 10-19-2008, 03:18 PM

I am trying to build my own code reader using a PIC interfaced to a laptop for J1850 PWM to use with a Ford but having a hard time finding information about the protocol. Are there any resources on the web that explain it? I have various links bookmarked that explain the general workings of it but nothing really detailed.

Yes, I realize that I have to pay for the SAE documents but I am a poor college student doing this for a school project so I can't afford it. Any help is appreciated.

**interceptor** · 10-21-2008, 04:55 PM

I think I figured out most of the protocol but have a couple of questions still. Is the CRC byte always 0x0a?

I am going by examples here, Ford J1850 PWM

**p2psmurf** · 11-15-2008, 05:14 AM

This may help:
http://etd.lib.ttu.edu/theses/availa...5016605445.pdf

**sinclairvital** · 11-16-2008, 04:05 AM

Some timing information here:
http://www.kbmsystems.net/SAEJ1850.htm

**interceptor** · 11-16-2008, 02:50 PM

Thanks for the links. I was able to get a hold of the actual SAE documentation which I found rather confusing.

Never the less I am now able to receive data from the ECM. My question now is about IFR. I think I am missing something.

Here is what's happening right now. I am sending

SOF 0x61 0x6A 0xF1 0x01 0x00 0x0A EOF

The reply I get back is

SOF 0x41 0x6B 0x10 and either nothing else or a few 0x00 bytes

What am I missing? Am I suppose to send something back after I get 0x10 byte?

**interceptor** · 11-17-2008, 11:53 AM

Looks like I am doing something incorrectly. The response that I receive back is on the line constantly, regardless if I send anything to the ECU or not.

I guess my question would be, at what point should I be requesting data?