check engine light on but cant pull any codes

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Mar 16, 2011 | 04:18 PM

#16

jordan101

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ok ill do that

Mar 16, 2011 | 04:48 PM

#17

EPNCSU2006

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From: Concord, NC

Quote:

That tells me ST0 is getting grounded which in turn triggers the PCM to start the KOEO tests. I suspect the problem is the Data Link connections on ST1 are corroded or some other reason to have a bad connection.

A quick note - it's STI and STO not 1 and 0. STI stands for self test input. This is the wire that needs to be grounded or shorted to the signal return wire to initiate the self test. STO is the self test output, which is the same circuit that connects to the check engine light.

See the diagram here: EEC IV Self Test hookup

Mar 16, 2011 | 04:56 PM

#18

jordan101

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i have all of that figured out. Im now trying to figure the code 121 out.

Mar 16, 2011 | 05:11 PM

#19

EPNCSU2006

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From: Concord, NC

Right, I was just making sure if someone found this thread through a search that they wouldn't ground the STO expecting it to trigger the computer, or put 12V on the STI and fry the computer.

As for the 121, have you checked closed throttle voltage with a voltmeter? I'm not sure what color wires were used in '94, but from memory, the gray wire is the signal return or sensor ground circuit. If you back probe the TPS connector, you should be able to touch the contacts inside the connector to get a reading. Of the other two wires, one is a constant reference voltage of 5V and the other is a variable voltage signal wire. You want to measure voltage between the signal and the signal return wires.

The fuel pump relay code can come up if you switch tanks while the engine is running. It won't hurt anything to do that, but it can trigger the code. I wouldn't worry too much about that one.

Mar 16, 2011 | 06:03 PM

#20

rla2005

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From: Kentucky

Quote:

Originally Posted by EPNCSU2006

Wow, Was I a doofus or what....

Mar 16, 2011 | 06:16 PM

#21

jordan101

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lol i guess so. i knew what you meant bc i figured it out. Now freaking taking the tb off the person i bought truck from used a homemade gasket brittle lines everywhere just little stuff really didnt notice when buying it. Heater hoses cracked.

Mar 16, 2011 | 06:22 PM

#22

rla2005

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From: Kentucky

Sorry for the nomenclature snafu on my part, it was not my intention to lead anyone astray. I am glad you made it this far.

Those vacuum lines are a devil in disguise, I do not trust any of them. I use a handheld vacuum pump/gauge to test each circuit. Beware, the EVR/EGR circuit is a controlled leak. Testing with the vacuum pump/gauge can lead one off the path if you go that route to test.

Mar 17, 2011 | 05:04 AM

#23

EPNCSU2006

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From: Concord, NC

Quote:

Sorry for the nomenclature snafu on my part, it was not my intention to lead anyone astray. I am glad you made it this far.

Don't sweat it - I'm not right all the time. I just hope if I post something wrong that someone else will correct my mistake too. That's the good thing about having so many people here.