I cannot read the obd 1 error codes

[sino1981]

I have 93 GMC sierra k1500. Engine 5.7
I cannot read the obd 1 error codes, i noticed that when I turn the key to on position. (Engine not running) CHECK ENGINE LIGHT comes on and stays on. normally the check engine light come on and goes off with car not running.
the car runs very bad. It hesitates when I accelerate, idle poorly. ..
A friend of mine told me I may have a open circuit or a bad relay, I checked all the relays they are all fine.
Note: the first time I pulled the relays the engine light comes on and goes off like normal, and I managed to get codes, it was code 12 and indicated that Ecm is operating correctly I think. So I put the relays back, I took the car for a drive , everything was perfect.
The next day, I turned the car to on position and engine light come back and stays on again and I could not get any error codes too. So what can cause that problem, I will appreciate any advices. Thank you

[carriedi]

are you using a scanner to get codes or are you jumpering a and be manually?
[f] [e] [d] [c] [a]
[g] [ ] [ ] [ ] [ ] [m]

if you are manually getting codes 12 will flash three times before the first code. which will flash three times and so forth.

If your using a scanner that reads fine when the truck is running normal and no communication when the engine light is on and it's running bad, check for a bad ground. Check the ground wire from the battery to the fender. Engine ground. and the ecm ground.

the bad or poor ground won't usually happen once driving. It will usually show up randomly on start up.

let me know what you find out.

[sino1981]

i am using just a paper clip to get the codes, ok i will check the grounds and let you know. thank you for the advice. i am just confused why when i pulled the relays out and put them back. the car start running fine with no check engine light. but the next day the problem come back again.

[sino1981]

also i checked the fuses , they all seems ok, except when testing them with a test light with key on, some of the fuses have power, some are not, is that normal?

[carriedi]

yes, not all the fuses are powered up until the circuit is on. your problem should show up only when you cannot get codes and the light stays on. that's when you need to check the grounds and ignition power to the ecm

[sino1981]

yes you were right brother, today i had the day off, i checked all the grounds, and clean them. so turned the ignition key on, no check engine light, also i can read the obd1 the codes again, and the truck is running perfect again, thank you very much

are you using a scanner to get codes or are you jumpering a and be manually?
[f] [e] [d] [c] [a]
[g] [ ] [ ] [ ] [ ] [m]

if you are manually getting codes 12 will flash three times before the first code. which will flash three times and so forth.

If your using a scanner that reads fine when the truck is running normal and no communication when the engine light is on and it's running bad, check for a bad ground. Check the ground wire from the battery to the fender. Engine ground. and the ecm ground.

the bad or poor ground won't usually happen once driving. It will usually show up randomly on start up.

let me know what you find out.

[GUEST]

I think I'm having the same issue on my 1990 silverado 350. Sometimes it runs fine but most of the time I have these symptoms. When I try to get codes it gives me 12 three times then the check engine turns off. Sometimes not even that... Did you clean the grounds on the fuse panel? I clean the body and engine grounds and the grounds on firewall on the passenger side with no improvments. I have power and ground to the pcm.

[carriedi]

when you get code 12 three times and then nothing more, it means the computer thinks everything is fine. No problems. Does the check engine light come on when the engine is running bad? The computer systems during obd1 are nothing like the current obd2. You should see if someone would scan your vehicle a professional scanner and see if there are any stored or pending codes that may no be current codes.

there are many mechanical things that can make the truck run poorly that won't give codes. The obd1 system is for emissions related problems.

the best thing to do would be to have a scanner hooked up to the truck and be driving around until the truck runs bad and see the graphs show is happening. Having the engine running and acting up while on a scope would be even better. Scope and scan while the engine is running poorly would be the best thing but I know it tough to get that done when something is intermittently happening. Is there some way to make the driveability issue happen or is it just unpredictable?