Contour Enthusiasts Group Archives Forums General General OBD II and the Contour

Hey all, I've got a big favor to ask about the OBD II system on the Contour/Mystique.

I'd like to know of a sensor or two that I could easily unplug to set off a check engine light and NOT kill my car. Car is a 96 Mystique Zetec ATX.

Background: For my senior project in school I've been working on an OBDII Scan Tool and I need to verify that my scan tool will retrieve DTCs correctly.

Thanks, James

Unplug the Throttle Position sensor, but do nothing but idle. The PCM should recognize the ~0 volts from the TPS as an error (~0.9 volts is its idle value), and air control at idle is only done though the IAC. The car would likely drive VERY poorly if you tried it then, but you shouldn't need to do that to throw a CEL. Mght need to idle for 5 minutes, shut off, then idle again though.

I'm not sure if the above will work, since a constant CEL is thrown only if the car has a problem with two out of three "drive cycles". Since a "drive cycle" is defined as driving your car, I don't know if idling will throw the CEL.

Anyway, another idea is to unhook the green vacuum hose from the EGR (exhaust gas recirculation) valve. I believe that this should throw a P0401 (insuffient EGR flow) after you have driven it around a few times and should not alter driveability that much.

Once you get the code, you can reattach the vacuum hose to put your car back the way it was.

Unhook one of the O2 sensors downstream of the cat. You should eventually get a cat efficiency code, but it won't hurt your car.
This sensor does nothing for setting fuel mixture or timing. Like I said, it
measures cat converter efficiency.

It may take a while, as mentioned above, drivecycles and all, so run out and do it now, drive it around for a while, then research it

You could probably leave your gas cap loose and get a
code from that as well. Don't think that does much harm.
Unless of course you go to the dealer and have them check
it for ~$100 to tell you to tighten the gas cap.

TB

Leaving the gas cap off will make the light come on within 20 miles of driving.

Putting the gas cap back on will make the light turn off....after a few days.

Brian

Disconnecting the EVR electrical connection would be the easiest way to consistently pop a code on a 96 without damaging anything (provided you don't run the engine so hard that EGR is needed to cool the engine). The gas cap should pop the code reliably, but some old PCM calibrations have a fairly poor completion frequency for the test that detects that fault (the PCM only looks for EVAP faults under very strict conditions due to all the variables that affect EVAP pressure changes).

If you are collecting the actual J1850 messages with a monitoring tool, I would be happy to review your messages. I am the PCM communications specialist for Ford assembly plant testing.

Thanks everybody!

Well, I'm more or less taking pieces out of the messages and displaying them. My scan tool is modeled around the AutoXRay which just displays relevant data not the entire message. Therefore, I pick off the header bytes (after making sure they are correct) and then the mode and pid. After that, the data is sent to an LCD for displaying.

Thanks again,

James

Which modes are you using? Just the OBD-II modes (0x01-0x09) or are you having fun with the enhanced diagnostics as well?

Well to limit the project, I'm just sticking with Mode 1 (PIDs 00,01, 04, 05 and 0A-11) and Mode 3.

I'd like to eventually work up to the other modes and Ford's enhanced modes, but that will come after I finish these. And change a few other things.

James

DiodeBoy,

Well, I've got some responses coming back from the car and they are looking good so far, but I've got a couple of questions for you.

1) What would you recommend for an acknowledge? I haven't implemented an "In frame response" yet and I was wondering if I had any options in this.

2) How many devices use the comm bus? I was using a logic analyzer to capture some messages and it seemed like only my scan tool and 0x10 were transmitting.

3) According to the SAE standards I have (SAE J1978 in particular) the scan tool is limited to requesting data only once per 100 ms. Is the standard different for the vehicle's computer? After a Mode 1 PID 0 request, my car would continuously send responses whenever there was an idle bus condition. Of course this has to do with my lack of acknowledging the data.

Oh yeah, this stuff is SOOOO COOL. After spending a week getting some other stuff working, I can say that seeing those responses from the car put a big ol' on my face.

Thanks, James