When I start the truck the check engine light is off... while driving it will come on... when I restart again later it will be off... it repeats.

Part I am confused on is in every other vehicle I have owned once it is on you had to either 'clear the code' or 'disconnect the battery for a min' to get the light out.

Someone enlighten me thanks

 

Pull the codes and see what it says. It sounds to me like you may have an EGR that is slightly clogged. I had this happen months ago after a cleaning of the EGR and it was "on and off" for a while. Now it's on all the time and I have a code 71 EGR.

 

try checking the codes; the computer should store them and you should be able to retreive them, dispite the fact that the light may not be on at that moment

 

how exactly do you pull the codes? I know you jump something or other and count flashes... but searched and couldn't find all the details.

 

the little gray box next to the fuse box is the one you want then jump e1 to te1 turn the ignition on but don't start the engine the codes should flash for you.

Count them 1234 stop first number pause
flashing again 123 second number

 

Sweet ! thanks oldharleyman,

i had a brief check engine last week, about 30 seconds, and hasn't come on since, interested to see what's going on. I knew the terminals to jump but not how to count the numbers.

 

ok so I have code 42... speed sensor? what to do next?

 

change the speed sensor

 

I have a cable driven speedo ... not electric... where would I find a speed sensor? Also, could it have stopped because of low fluid in the tranny? read that in a diff thread... I did recently change the fluid.

 

I tried this testing procedure with my 91 pickup. First trying a paperclip and then with braided wire ( neither worked). How long do you have to wait before the light begins to flash. What might I be doing wrong?
Thanks

 

I have the same problem as sagorski

 

I just talked to a friend and he said it could be a broken wire to the computer or to the ground. "If it the ground its an easy fix but if its to the comp then you need to find out where the break is".

 

I am guessing that you did it backwards... when you look at the diagram on the gray plastic lid make sure you are jumping the correct 2 I read it backwards the first time and got nothing...

 

Just to update... i changed the speedo cable and the check engine light is gone... apparently there is a sensor in the cluster that can tell it is not working... I read that the sensors specific function is to match revs to the speed for safety... and since it senses revs but no speed it triggers the check engine... can also cause hesitation.. which I did experience.