Well, I've had a minor setback on the resto-mod project, I've fried my ECM while doing a test crank of the engine... Any feedback on what could have happened would certainly be appreciated...

Here's a little background on the project..2004 LQ9 6.0L factory stock from a Cadillac Escalade + 4l60E transmission going into a '69 LeMans. Using all factory wiring harness, PCM, Fuse/relay box, accessory setup, etc..

I have the factory service manuals for the vehicle that the engine/ECM came from, so I used the factory schematics to wire in a temporary starting circuit using a selector switch (run circuit) and pushbutton (start circuit). All wiring was complete to the ECM other than the O2 sensors and unused sensors like the coolant level switch, oil level switch, etc...

The engine is in the vehicle, and all engine grounds are connected and read 0 ohms to the negative battery; the engine was grounded to the vehicle body; however the frame is not grounded at this point.

Prior to receiving the ECM from my PCM programmer (eliminating VATS, emissions stuff, etc etc), I jumpered a +12V source directly to the starter solenoid to turn the engine over to build oil pressure, it turned over normally and built 25 PSI of oil pressure which was good..

When I received the ECM, I powered it up, and checked for basic functionality (when "run" power applied, ignition relay pulls in, fuel pump relay cycles, etc.). I checked the DBW system and it functioned properly. I have an OBD-II program on my laptop and I was able to establish comms with the PCM and all of the sensors read normal and there were no fault codes present in the PCM.

I went to start the engine (fuel pump circuit not wired in at this point) (using proper +12V input to the PCM) and it turned over VERY sluggishly, and then died. I tried to start again, and the PCM was not communicating, and did not respond when run voltage was applied (did not pull in fuel pump relay, etc). I removed the 12V run voltage, and with the PCM still connected; I had 8 volts on the run power bus. I disconnected the PCM 1 fuse and the 8 volts disappeared. I re-applied the fuse and the 8 volts did not re-appear. I re-applied the run voltage, and the same thing happened, PCM was dead and after I removed the run voltage there was 8 volts on the run power bus.

I checked the PCM grounds and found 1.6K ohms on each one of the PCM grounds. I re-checked the same grounds two hours later (I did nothing to the wiring) and they were all at 0 ohms.

What I obersved when I started the engine; something drew a CRAPLOAD of current AWAY from the battery to make the engine start sluggishly, and I'm guessing the current went through the PCM and fried it. But looking at the schematics, I don't understand how this could be if everything is fused properly (I'm using all factory components from the vehicle it was removed from (salvage vehicle); nothing has changed...

I'm pretty confident the PCM is DOA; it's totally unresponsive; but my concern is I don't want to repeat the same thing with a new PCM....

Any thoughts that you have would certainly be appreciated!!

 

did you run starter current thru the pcm ?

 

I would definitely get the frame grounded too. You could have possibly backfed through the pcm and done damage that way, but it doesn't really sound likely. You said you had 8v huh. Is it possible that you just have a weak battery? You can't check grounds with an ohm meter if the circuit is live and has any power going through it. The meter will lie to you. That's why the readings changed. To really check the integrity of grounds you need to test the voltage drop over them while cranking to see if they can handle the current load. If a ground is loose and barely touching it can still read 0 ohms and not be able to carry much current.

 

Why did you do this and not run the 12 volt ignition wire from the car to the ignition relay and fuse in the fuse box from the caddy for the ECM?

 

sounds like a surge went through the pcm when you tried to start it.

 

I was attempting to test the motor before I finalized all of the wiring for the vehicle. The relay in the fuse box requires a ground to be completed to pull it in. It was a bit easier to bypass that and wire +12V to one side of the pushbutton and the other side of the PB to the starter solenoid...

"did you run starter current thru the pcm ? "

No, other than not using the vehicle keyswitch the rest of the circuit was factory stock..

"I would definitely get the frame grounded too. [Yes, definitely--In progress] You could have possibly backfed through the pcm and done damage that way, but it doesn't really sound likely. You said you had 8v huh. Is it possible that you just have a weak battery? [At that time the battery read +12V] You can't check grounds with an ohm meter if the circuit is live and has any power going through it. The meter will lie to you. That's why the readings changed. [Positive battery terminal was disconnected] To really check the integrity of grounds you need to test the voltage drop over them while cranking to see if they can handle the current load. If a ground is loose and barely touching it can still read 0 ohms and not be able to carry much current. " [This may be a lead; it seems like a high current load found a ground path that it should not have]

I've got one more question: I accidentally dropped my alternator while I had all the accessories off for engine painting. Could there be something going on with it? I checked from the +terminal to the case and it was in the MOhm range, and also from the case to the control terminals and they were also in the MOhm range...Is there anything that could be going on with it?

 

So you have no fuse protecting the ignition? So basically your are hotwiring the car..?