SOLVED BELOW!! I have a 1990 gmc tbi 350, I have the motor in a jeep. I have no spark advance. Bypass plug is connected, I traced all wires with multimeter from ECM plugs to distributor and Esc module and everything checks out. 12v on ESC and grounds good on water neck. I thought it was the module in the dist so I just installed a new distributor. Engine starts great and runs, just sets at base timing 0 degrees even when revved a-little. I'm stumped, I switched out the Esc module with another I had but same prob. Would the ECM run the engine great but lose timing control and be fried? I have reset and unplugged battery several times thinking the ECM needed it. But nothin. Any help would be great.

The ECM I guess needs a vss input but I don't think that's the issue.

 

What does your scan tool show for spark advance? Knock-sensor retard?

 

I don't have Aldl tools but with a timing light it just sets at base timing. 0 degrees. Yet the bypass is hooked up. I'll see if I can figure out this aldl connector to get more info.

 

You won't get much from the ALDL on a '90 TBI vehicle. I doubt that the spark advance is even in the data stream.

After you set the base timing, did you turn off the engine (key-off) for at least ten seconds before restarting with the EST/BYPASS connector connected?

Opening the EST/BYPASS connector sets code 42 which locks in. Until the ECM is reset (key-off for at least 10 seconds), the ECM won't control the spark advance.

RBob.

 

Ya I've even unhooked the battery. My mind is blown cuz it runs so good at 0 degrees but I haven't revved pass 2000 rpm cuz it holds timing exact at base. Idk what to do. Does the Esc have to be mounted? when I switched out them I just plugged them in and laid it on the intake manifold. Is there a guaranteed reset of the ecu, I've touched disconnected battery terminals together to try hard reset.nothin...

 

Well I switched out coil, checked grounds at the ecu plug. Everything good but no advance. Can just the ignition control in ecu be fried? The old distr had a lot of corrosion on the module inside the dist. I replaced with new but could it have killed the ecu?

 

Another test to try, with the EST/BYPASS open check the base timing, should be at 0* BTDC. While watching the timing slowly increase the RPM until about 2K RPM. Before that point, usually in the 1600 - 1800 RPM range, the spark advance (SA) should jump up to 10 - 12* BTDC.

That is limp mode advance that is built into the ICM. If the SA just stays at 0* then the ICM is bad.

Is there a SES or CEL light installed? If so does it do the blink-off at key-on, engine-off? And does it go off and stay off with the engine running?

The ESC doesn't need to be mounted, and it isn't likely involved. All it does is report detonation (perceived) to the ECM.

RBob.

 

Is the ignition control module in distributer? I replaced dist. I'll try your tests and see. Thank you.

 

Tested icm, no advance threw 2500 ish rmp. Switched it out with a new one. Still no advance. Tried resetting computer with bypass connected back and still nothing. Can this be the Esc or ecu?

 

With the new icm no advance thru 2500 rpm too.

 

HOW did you test them?


Perhaps. Seems unlikely.

So the ECM has no vehicle speed data?

 

No not yet, I have a one w buffer. Wanted to get it running then kindof lost motivation when timing was not advancing.

Hooked up ses light and was blinking like a strobe. Codes are 21 and 51, looks like prom is faulty. Thanks for the help, idk how just a prom fries but my luck sucks so doesn't surprise me. Thanks again!!

 

SOLVED!! Took the ecu apart and removed the larger chip, put it back in, installed the ecu and bam! Timing and no ses light! For anyone out there with ecu prom problems, remove it and put it back in. Mine wasn't corroded or anything, just 20+ years caused bad connection. At least I have tons of spare parts now! Thanks everyone!!

 

What do you mean "hooked up SES light"???

If the SES light isn't functional, the ECM won't work properly.

First Guess: All you needed to do was connect the SES light, and then disconnect the battery for a few seconds to re-set the ECM.

 

Well man, you just saved my bacon. I have a ‘92 Chevy 2500 that never ran perfectly or got good mpg. I noticed with a light that the timing wasn’t advancing. I replaced everything that I could think of... Came across your “Solved” thread, unplugged the main chip and snapped it back in, and BAM 💥. I don’t know how you thought to do it, but thank you! I would have NEVER thought to do that.

 

Omg. Thank you so much for this post. Didnt drive my 87 chevy tbi 350 for like 3 weeks and then it started having problems. Looked at the timing as the issue was only in drive or under load. Popping, stalling, burning rich. I could not figure it out. Until i saw the timing was not advancing under load. Thought it was the dist or wiring but recently changed cap rotor ign module. Read this post then decided just to try and unplug the computer to see what happened. Did it once then immediately starting advancing under load. It had a few hiccups so i did it again and again just to clear any built up corrosion it might have had. Then took it for a drive and it finally drives without stalling and wanting to die. Life saver! I was stumped and was going to replace the computer next! Thanks!