Check Engine light not lighting up

[adaml23]

1989 crew cab dually

So I'm trying to get the ALDL to work so I can find out why my new truck is stumbling, but my Check Engine light isn't working. I tried putting a new bulb in and it still doesn't work. Any suggestions on what I need to check or test? I don't see any breaks in the printed circuit board but electrical stuff is not my strong suit...

 

[1987 GMC Jimmy]

1989 crew cab dually

So I'm trying to get the ALDL to work so I can find out why my new truck is stumbling, but my Check Engine light isn't working. I tried putting a new bulb in and it still doesn't work. Any suggestions on what I need to check or test? I don't see any breaks in the printed circuit board but electrical stuff is not my strong suit...

What I did back when I was having ECM problems was I unplugged the computer and turned the key to on. Your SES light should come on then. If it does, I think it's fair to say your ECM is bad, at least the diagnostic circuit and other stuff in it, too. If it doesn't light up after you do that, you're probably looking at an electrical problem. Your ECM is gonna be behind the glovebox. I know that's not a very technical test, and a troubleshooting diagram would be the more official way to go about it, but I did this, and it worked fine for me.

 

[adaml23]

Isn't it supposed to light up for a few seconds when you first turn the key on? Mine doesn't even do that.

 

[1987 GMC Jimmy]

Yes, it's supposed to self-test when you first turn the key. The fact that it doesn't, discounting potential electrical problems, says that the ECM needs to be replaced. Oh, but make sure you check your ECM A and B fuses. Can't forget that. Same exact thing happened to me. Twice. The first time, the ECM was burned, and I replaced it. I got it all back, and two weeks later a rat chewed some wires and caused some major electrical shorts. This somehow blew out only the diagnostic circuit, but the rest of the ECM worked fine. I replaced it under warranty, and I've been going good for many months now.

 

[adaml23]

Where are those fuses? On the ecm, I assume, or somewhere else? Gonna try to get a look at it tmrw over lunch.

 

[1987 GMC Jimmy]

Where are those fuses? On the ecm, I assume, or somewhere else? Gonna try to get a look at it tmrw over lunch.

No, those are in the fuse box.

 

[adaml23]

SES still didn't come in with the ECM unhooked. The wire going from the SES light bulb to the ECM looked fine . Still don't know what's going on with it...

 

[1987 GMC Jimmy]

SES still didn't come in with the ECM unhooked. The wire going from the SES light bulb to the ECM looked fine . Still don't know what's going on with it...

And the fuses were good?

 

[HotRodPC]

Yes, do check the ECM fuses in the fuse block under the dash on the firewall.

Man oh man, it's been a long time I dealt with this issue and it was only 1 time too. But it seems to me, we were able to use a tool or made a tool with a bulb socket and the wires to the bulb socket as a jumper and able to recover codes that way direct from the ALDL. It might have enen been a damn OBD 1 Reader for GM cars and trucks I don't recall now for sure buddy, but it's worth finding the ALDL pinout to see if it's possible.

 

[adaml23]

Fuses are all good. I forgot to say that I checked that before making this thread.

Still no luck with the CEL but found out I've got a buddy with a obd1 scanner so I'm gonna use it at some point. Pulled the distributor cap and the coil wire nipples were corroded some so I cleaned them up good and it runs a little better but still stumbling. Ordered wires and plugs so hopefully that helps me out.

Checked my fuel pressure and its on the low side (8-9 lbs without a load). I know one of my tank pumps is dead so I ordered a denso pump as well. If the plugs and wires don't take care of the stumble, then I'll change the pump a little sooner than later...

 

[1987 GMC Jimmy]

Yeah, it sounds like the pump is gonna have to be done fairly soon. If you want, you can probe the wire coming off the A5 pin on the ECM. It should have zero volts key on, engine off, and it should have fourteen volts engine on. If that wire isn't producing fourteen volts, it's the ECM's fault. Like I said, I've had the ECM work for the engine alright with the diagnostic circuit damaged. I've never had to go digging around in the ECM area of a square body, but I have in a GMT400. You should check to see if there's a ground strap in there for the ECM and how it's looking. I'll attach a pinout for your computer so you can check any of the voltages you want.

 

[adaml23]

Well I put in some new led dash bulbs and the SES light started working?! I don't know why, maybe just due to the fact that it had been sitting of a while... don't really care as long as nothings gonna mess up later...haha

Stuck a paper clip in it and its throwing a Code 44 error - 02 sensor reading lean condition...

 

[HotRodPC]

Good deal on the SES light. Now you've got something to work with. Yep, that Fuel Pressure is a tad low too. How about the fuel filter? Is it maybe needing replaced making the fuel pump work hard and also the reason for low fp?

 

[adaml23]

Replaced fuel filter last weekend. No change. Also ran some seafoam thru the intake. Gonna hopefully change the o2 sensor tmrw and will change the plugs and wires whenever they come in (love/hate relationship with rockauto... The prices make the wait worth it... Plus the magnets...)

Any suggestions on what may make it throw that code besides a bad o2 sensor or a vacuum leak? I've sprayed carb cleaner around the intranet with no changes.

Don't know if this thread would be more appropriate somewhere else now so feel free to move it if a Mod wants to...

 

[adaml23]

Just in case anyone wondering what we're working on... We all know we love seeing pics! Haha!