Check Engine Light

[drl911]

 

1993 GMC Z71 (~165,000 miles)

 

Recently the check engine light came on for the first time in my 4+ years of ownership. This truck is only driven a few times per month, but I believe at about the same time as the check engine light coming on the transmission started to not shift automatically. In fact it seems to always start out in top gear. The transmission will change gears "manually" via the gear select lever on the steeering column, but not automatically while driving. The ECU will not give any codes at the local auto parts stores. I have tried 3 different places all with the ability to read OBD1. So far I have changed the throttle position sensor with no improvement. I bought a speed sensor, but they sold me the wrong one so that has not been tried yet. Someone suggested I need to replace the ECU. Does this sound right because it will not give any codes and the electronic transmission control is not working either???

 

Thanks in advance for your suggestions,

Dan

[Shav0]

I had a similiar problem in a 02 elantra. What happened was the ECU lost imput from the transmission input speed sensor and resulted to "safe mode." What safe mode is, is your transmission will automatically go into 3rd gear and stay put. This is to basically allow you to still get yourself to a garage to get it fixed, but it isn't reccomended that you drive like this for a long period of time. This problem will also illuminate the check engine light.

[drl911]

Thanks for sharing your experience. This truck does drive/behave exactly as you describe the "safe mode". I am going to take back the wrong speed sensor I was sold this past weekend and hopefully get the correct one to solve the problem.

 

Thanks again,

Dan

[Shav0]

Im not saying thats exactly the problem, but It sounds to me like your in some sort of "safe mode." If transmission is unsure of the speed it's actually going it will cause this same thing to happen.

[thecraft]

Just so you know, you can retrieve OBD-I codes yourself using a paper clip. You don't have to waste gas/money taking it to the local parts shop.

 

Instructions: http://tvoffer.tripod.com/gmcodes.htm

Codes: http://www.troublecodes.net/GM/

[drl911]

Just so you know, you can retrieve OBD-I codes yourself using a paper clip. You don't have to waste gas/money taking it to the local parts shop.

 

Instructions: http://tvoffer.tripod.com/gmcodes.htm

Codes: http://www.troublecodes.net/GM/

 

<{POST_SNAPBACK}>

 

 

 

So what does it mean if the ECU will not give a code under any circumstances (I have tried all of the above several times)? Nor will the "check engine soon" warning clear by disconnecting the battery.

 

Thanks for all of the input,

Dan

[GM-Tech]

So what does it mean if the ECU will not give a code under any circumstances (I have tried all of the above several times)?  Nor will the "check engine soon" warning clear by disconnecting the battery.

 

Thanks for all of the input,

Dan

 

<{POST_SNAPBACK}>

 

 

 

 

You should at least get a code 12. Make sure you're jumping the pins AFTER the key is on. Don't jump tthen turn the key on. However, if you can't clear the check eng light, it could be a bad ECM, or you have something wrong that's setting a code the instant you turn the key on.