Check Engine Light, How to CLEAR

I have a 1991 chevy, 3500, 4x4, 454 engine, auto trans. pickup.. My check engine light is on. Engine runs rough, backfires through throttle body. I have put a jumper between A and B terminals on plug, no response. I have disconnected the battery for days. The check engine light will not clear. What type of OBD1 scanner do I need to read and reset computer..I have tried a cheap plug in model put that doesn't work either. Please HELP.....

91 should flash codes from jumping A&B pins.
Verfiy A & B terminals.
Look on back of plug and see if any wire on A and B pins.
Sometimes the plug gets turn over or wire from it to pc gets broken.
http://www.troublecodes.net/GM/

Backfiring up through the TBI usually if not always indicates a problem with the timing. You should check the timing before doing anything else.

To check the timing there is a connector under passengers side of dash where the wiring harness comes through firewall and you will see a tan with a black striped wire with a connector connecting the wires together, this is the est bypass wire, you need to unplug this connector and set timing to 0 degrees then turn off engine then reconnect the connector.

If the timing was off then hopefully this will fix your problem. Post your findings.

I'm also moving this thread to the C/K forums.

Mt-2500 Thanks for the info. I thought maybe a wire was broken but I didn't check it because I have never pluged anything into it. I will check it and get back to you..Thanks again, Pjksr195591 should flash codes from jumping A&B pins.
Verfiy A & B terminals.
Look on back of plug and see if any wire on A and B pins.
Sometimes the plug gets turn over or wire from it to pc gets broken.
http://www.troublecodes.net/GM/

You are welcome.
Post back what you find.

Shortbus, I just want to make sure that I got the right wire. There is a connector that has three wires going into it and three wires coming out of it. The three wires coming in are from the main wiring harness. The three going out, go to a solid plug [ dead end ]. The three wire colors coming in to the connector are black with a white stripe, pink with a black stripe and gray. Coming out of the connector are black with a white stripe, brown with a white stripe and pink with a black stripe. I put a timing light on, started the engine, the timing was on zero. With the engine running I unpluged the connector and the timing didn't change. I just wanted to see if timing would change. I pluged it back in then started it back up, timing was still on zero , shut the engine off like you said and reconnected the plug. I am not sure if this is the right wire. The wire you are talking about, tan with a black stripe, is it a single wire? Should the timing have changed when I unpluged the connector. Thanks again, pjksr1955 Backfiring up through the TBI usually if not always indicates a problem with the timing. You should check the timing before doing anything else.

To check the timing there is a connector under passengers side of dash where the wiring harness comes through firewall and you will see a tan with a black striped wire with a connector connecting the wires together, this is the est bypass wire, you need to unplug this connector and set timing to 0 degrees then turn off engine then reconnect the connector.

If the timing was off then hopefully this will fix your problem. Post your findings.

I'm also moving this thread to the C/K forums.

MT-2500, I checked the wires coming into the back of the plug and they were good. I will have to trace the wires and see where they go and make sure they are not broke. Thanks, pjksr195591 should flash codes from jumping A&B pins.
Verfiy A & B terminals.
Look on back of plug and see if any wire on A and B pins.
Sometimes the plug gets turn over or wire from it to pc gets broken.
http://www.troublecodes.net/GM/

The EST Bypass is a single wire (Tan with Black Strip). With the wire disconnected, set timing to 0. With wire connected, timing usually will be 20 degrees before TDC.

Also note that with EST Bypass disconnected, the Check Engine Light will come on (code 43, I believe). If the timing does not change when the EST Bypass is reconnected, it might be a wiring issue at the connector.

//2000CAYukon

2000 CAYukon, This wire, is it under the dash or could it be on the firewall. Thanks for the info.....pjksr1955The EST Bypass is a single wire (Tan with Black Strip). With the wire disconnected, set timing to 0. With wire connected, timing usually will be 20 degrees before TDC.

Also note that with EST Bypass disconnected, the Check Engine Light will come on (code 43, I believe). If the timing does not change when the EST Bypass is reconnected, it might be a wiring issue at the connector.

//2000CAYukon

2000 CAYukon, This wire, is it under the dash or could it be on the firewall. Thanks for the info.....pjksr1955
I am not 2000 CAYukon.
But.
On that model I find most of them on firewall behind the black plaster wiring cover where fuel pump relay and fuse is.
Usually taped to the side of wiring harness that runs up and down.
And a few on the wiring from left side to dist

But some pickups they put it under glove box and under heater cores housing/at or below carpet.
Kinda they hid it and you have to look for it.:lol2:
Good Luck

On my 90 K1500 it is under the black plastic cover on the firewall.

Let me know if you need to see a picture of it.

//2000CAYukon

MT-2500 I found the wire. It was taped to the wiring harness on the fire wall like you said. I disconnected the wire, started the engine. The timing was on zero. I shut the engine off, connected the wire and started the engine. The timing was still on zero. It didn't change. I did this when the engine was cold and then again when the engine reached operating temperature. I have noticed the past couple times when I start the engine [cold ] the engine is running really rough. When it reaches operating temp. it does smooth out some. It will still backfire through the TB [ hot or cold ] when you push on the throttle real fast back and forth. I still have the problem not being able to read the codes on the computer. I will have to trace the wires this weekend to see if one is broke if I get a chance. This motor has around 200,000 miles on it. It ran really good. I used the truck one day, the next day went to use it and it was running like shit..Could it be a timing chain...or gears... Thanks, pjksr1955 I am not 2000 CAYukon.
But.
On that model I find most of them on firewall behind the black plaster wiring cover where fuel pump relay and fuse is.
Usually taped to the side of wiring harness that runs up and down.
And a few on the wiring from left side to dist

But some pickups they put it under glove box and under heater cores housing/at or below carpet.
Kinda they hid it and you have to look for it.:lol2:
Good Luck

MT-2500 I found the wire. It was taped to the wiring harness on the fire wall like you said. I disconnected the wire, started the engine. The timing was on zero. I shut the engine off, connected the wire and started the engine. The timing was still on zero. It didn't change. I did this when the engine was cold and then again when the engine reached operating temperature. I have noticed the past couple times when I start the engine [cold ] the engine is running really rough. When it reaches operating temp. it does smooth out some. It will still backfire through the TB [ hot or cold ] when you push on the throttle real fast back and forth. I still have the problem not being able to read the codes on the computer. I will have to trace the wires this weekend to see if one is broke if I get a chance. This motor has around 200,000 miles on it. It ran really good. I used the truck one day, the next day went to use it and it was running like shit..Could it be a timing chain...or gears... Thanks, pjksr1955


Engine warmed up and wire pluged up you should see some spark advance.

Try a battery disconnect after you set base timing and after wire est wire is pluged back up.

Wire upluged usually sets a check engine light and code.

Recheck timing specs.
I show 4 degrees for yours.

Bump up timing and see if that helps.

You may need to get it on a good engine capable scanner and get the codes and watch time advance in computer.

MT-2500 I was looking at Autozones website. My truck is suppose to have a Electronic Spark Control which advances or retards the timing. Do you know where it is located on my truck and do you think this could be the problem with my truck backfiring.....thanks, pjksr1955Recheck timing specs.
I show 4 degrees for yours.

Bump up timing and see if that helps.

You may need to get it on a good engine capable scanner and get the codes and watch time advance in computer.

MT-2500 I was looking at Autozones website. My truck is suppose to have a Electronic Spark Control which advances or retards the timing. Do you know where it is located on my truck and do you think this could be the problem with my truck backfiring.....thanks, pjksr1955


If you are not getting spark advance yes.
Spark control module is on firewall.
Sorry about firewall make that right side of engine above valve cover sometimes on coil bracket.
But a lot of things can cause it not to work.
Knock senor/sensors or wiring or plugin power feeds.

If advance is not advancing get some good repair info with a code 43 repair flow chart and go threw the repair flow chart.

A factory manual or a all dat online sub will led you threw the check and repair on it.
http://www.alldata.com/products/diy/index.html

Let us know what you find and how it goes.
Good Luck

Here is a link that might explain/help.
http://www.troublecodes.net/articles/gmmsundrstd42.shtml

MT-2500 Thanks for all the info. pjksr1955Here is a link that might explain/help.
http://www.troublecodes.net/articles/gmmsundrstd42.shtml

MT-2500 Thanks for all the info. pjksr1955

You are welcome And let us know how it goes.