My "service engine soon" light flickers erratically when ignition is turned on, and flickers or fades in and out when the engine is running. I took the vehicle for a drive around the neighborhood, and it seemed to be doing fairly well until it suddenly died - no popping, spitting, struggling, or anything. I could not get it to even fire after that. I connected a fuel injector tester to the harness (one of those neat Actron devices) and the "Output" light would barely illuminate when I cranked the engine. I got it to start once more and run just fine, and it suddenly died about 30 seconds later. Ignition seems to be fine, fuel pressure is excellent, and timing is right on 6 BTDC. With the flickering SES light and the apparent failure to close the injector circuit, and considering that I had the SES light and injectors working consistently with the 1227747 ECM (though it did not know how to run the engine properly since it is a TBI computer), can anyone offer any opinion on the ECM's being bad? Thanks.

 

i can tell u when mine went bad the ecm no longer threw codes. so i would advise u checking to see if u even get a code 12 to make sure it is cool.

 

F'truck,

Most ECM problems are actually connection problems, either internally or externally. There are some instances where a failed external component will cause damage to a driver in the ECM, but usually not before the fuse protecting the ECM or device opens. Unfortunately, even if the problem is just a conenction in the ECM, there is little chance of repairing it, so you are left with checking the external connections and grounds.

Since your problem is intermittent, I would almost put money on connection failure. Again, all you can check are the externals. The sole exception to that is the PROM and Cal-Pak (older ECMs). Almost every ECM I've opened has a PROM that has thermally "crept" out of its socket at least a little. You can reseat the PROM/PROM carrier on the socket, and check all the card edge or pin/socket connectors to the ECM from the wiring harness.

Don't overlook power and ground connections to the ECM and engine sensors/devices. If you understand that the ECM actually controls most external loads by switching the ground side of the circuit instead of the positive voltage (as many people presume), you'll understand the importance of good grounding. The same is true for the positive power nodes to the ECM and chassis wiring. An intermittent connection at the fuel pump fuse holder will screw up everything for the ECM, since its main power is supplied from there.

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I would feel a lot better if the thing would at least give a code 12, but it doesn't even come close. The SEL doesn't even stay illuminated with the ignition on and the engine not running, so I don't suspect that it can be made to cooperate.

I did realize that most circuits on GM electronically controlled vehicles are actuated on the ground side, supposedely because there is less electrical noise on that end. I ensured that all grounds on the harness were tightly connected. I built the system using a butchered Firebird MAF harness and the interchangeable parts of my factory harness for the TBI 4.3L V-6, as there is actually a great deal that the two systems have in common. I traced each connection using either the harness or a wiring diagram, taking hours to do what could have been accomplished in far less time using the schematics on the DIY-EFI site. I suppose we sometimes learn a little more by taking the LONG way around. I am quite confident in the connections, and checked each one for continuity as it was assembled.

One condition which causes me to suspect the ECM more than anything is the fact that ECM 1227747 was functioning normally in the vehicle. It was obviously not the correct ECM, but it is set up to use many of the same components and was able to communicate trouble codes through the SEL. I checked the schematics of 1227747, which I was wrongly given with the system I purchased, compared to those of 1227165, and found that what the two ECMs did not have in common was not provided for in the less capable TBI computer (such as the absence of MAF inputs).

So, I simply cannot figure out why 1227747 would behave correctly aside from running the engine improperly where 1227165 flounders, unless the latter is malfunctioning. This logic may be far from perfect, but it seems to be the most that I have to hold on to so far.

Would it be likely that the PROM in the new ECM is corrupted? What I have read seems to indicate not. Isn't the diagnostic routine and SEL controlled directly by the ECM, based only on parameters read from the PROM? Could the Memcal be affected? Thanks for the help. I hope that I can whip this thing soon.

 

Well, it was the ECM. Now, as always, there is a new problem. The SEL works fine, and the injectors are pulsing, but it seems that the VATS is somehow engaging. The PROM I have is from a 1986 Corvette, and, as I understand, the VATS was not used until 1988, and then only on some vehicles. I am almost certain, however, that there is a VATS problem, because the engine seems to run great for about 2 seconds, then it just quits. It sounds like the fuel supply is being switched off, because the engine seems to attempt to fire for a few revolutions after it begins to shut down. The ECM flashes code 46, and the code was apparently stored the first time I turned the ignition on. This seems to never end. Any help would be greatly appreciated.

 

They started using the VATS system on all vettes in either 85 or 86 if I remember correctly. Camaros started getting it standard on all cars in 89. When I had problems with my VATS, the engine wouldn't even crank let alone run. According to what I read about the code you have, it sounds like the VATS portion of the ecm is not communicating properly with the VATS decoder module. It says if the engine cranks but won't stay running, there's a faulty communication between the ecm and the decoder module. I know from personal experience that when it's a key or resistance reading problem, you get absolutely nothing at all. No cranking. I'm wondering if you have the decoder module in your wiring since it sounds like you didn't have the VATS system originally. If you do have the decoder module, maybe it's not compatible with the vette prom? I guess you could wire in a decoder module if you don't have one. Don't even ask me where it's located at. I looked for mine and never found it. The other solution is an aftermarket chip that has the VATS system eliminated. I've heard of people getting custom chips that have it removed. GM Tech added the VATS system to his car, so he would probably be the one who could tell you for sure. You ought to post another topic with ATTN: GM Tech, and see what he says.

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