Crank no start...I may have fried my computer

A 1998 Chevy truck, 5.7 gas engine. I disconnected the battery to make a repair and I needed to unlock the steering wheel so I turned the key to "run" position. After making the repair I attempted (many times) to reconnect my battery only to strip the internal threads of the battery. After I made that repair I connected the battery and went inside the cab to find the key still in the "run" position. I turned the key too "off" waiting a few minutes then tried to start the truck...crank only, no start. No codes, but my reader does indicate "pending" codes, but it will not display what they are. I checked both ignition fuses and they are fine but I have no spark. I checked the Crank sensor (a three wire circuit) and I have 12v, ground and 5v while cranking on the appropriate pins. The cam sensor has it's required 12v, ground but the 5v signal is at 14 volts. At my knowledge level I don't know if that means the cam sensor was damaged or the PCM. I will research this further but I think I fried my computer because in the repair manual is a "caution" not to connect battery power with the key on or it can damage the PCM. Dang it.

Good quick check if you're not sure would be to check for 5v on another sensor like the TPS or MAP.
I've seen some real stupid things done when connecting batteries, but I've never seen a PCM killed over hooking up the battery with the key on.

I have seen on GM cars cranking no starts and lack of 5v ref due to crappy battery terminals and the use of bolts, nuts and washers instead of the appropriate terminal ends.
I'm not suggesting you hacked the repair, just sharing a prior experience.

Thanks so much! I will recheck my repair but I am getting 12v to the two sensors mentioned. But I hear ya!

Don't feel too bad, sir, mistakes happen!

Just so I'm clear, it's the brown/white wire at the CMP sensor that has 14V on it? Genuinely curious where the extra voltage is coming from...

This may sound unusual, but I think I'd try a soft reset at this point. Disconnect both battery cable ends and connect them together with a jumper wire, have a snack, come back and try to start it again. I've seen low/intermittent battery voltage do STRANGE things to computers, and not just GM's. Nearly ended up replacing a transmission module in a Prius over a weak 12V battery. Sometimes, all it needs is a good 'reset'.

Like Noah said, not saying you did a bad battery repair. Some PCM's will tolerate all kinds of voltage spikes, some seem to be made of glass

I appreciate your feedback. I'll try your reset idea...can't hurt anything. I'm away from the truck right now and I don't remember the color of the wire but I do have a pin diagram that gave me the direction. I have a Vantage tool and the display on the signal wire (back probed) was showing a max of 14 volts (engine cranking). Yeah, a strange one. I'm going out now to recheck that indication.

Let us know what you find! I've got my fingers crossed for the soft reset, but we can go down other diagnostic paths if needed.

Soft reset didn't work. Since I know it's a lack of spark, I'm checking into that side of things. More later...

Sorry sir, thanks for trying anyway Let us know what you find with spark. I'd be very curious to follow the CMP signal back to the computer, if possible.

I also doubt you hurt the computer.
The cam sensor designs GM has used forever are 12v pull-up circuits. So I am not concerned about having higher voltage on the cam than you do on the crank but the signal cannot be higher than the feed. It's just not possible given the circuit design. The cam sensor produces the signal and sends it to the PCM.
Double check me on that. With the key on, unplug the cam sensor and give me the readings on all three wires.

Thanks for the input Paul. I appreciate your time here. I will recheck the cam sensor signal wire tomorrow and post my findings. I used the Vantage in "graph" view so maybe I'll view it in "digital" to make sure I'm seeing it correctly. In the mean time the book I'm using says I should have between 1 and 4 volts on the wire that comes from the crank sensor to the ignition module and I'm seeing no voltage at all. Again, it's worth rechecking to be sure. I'll post my findings on both. It seems strange to me that it ran one second and after reconnecting the battery (multiple times with the key to the run position) I now have no spark. But the learning continues...good thing I have another car.

wyatt wrote: In the mean time the book I'm using says I should have between 1 and 4 volts on the wire that comes from the crank sensor to the ignition module and I'm seeing no voltage at all.

That's number is probably an average reading using a DMM.
I thought you said you had a steady 5 on the crank and 14 on the cam signals? And you were using the graphing meter during cranking right?

That's correct. The Vantage in "graph view" indicated 5 volts on the signal wire from the crank sensor and 14 volts on the signal wire from the cam sensor (back probed, engine cranking). I did have the battery charger on while cranking to check the cam sensor so maybe that explains the extra voltage? I understand this design is a feedback to the computer and does not directly effect drivability. Both sensors have 12 volts on the pins that require 12 volts and I have good grounds (but only verified by an ohms check). I also found that the crank signal goes to the ignition module and there is suppose to be 1 to 4 volts at the Ign. module, the Vantage read 0 volts but I will verify my connections today. The only thing between this truck running and not running was disconnecting the battery. When I tried to reconnect the battery I stripped out the threads on the negative side which required a longer bolt to repair the connection. That process caused the power to be on and off many times while the key was on. I'm going to produce a video on my channel about this process because this may be a good case study for others to benefit from. Thank you again for your interest Paul.

wyatt wrote: That's correct. The Vantage in "graph view" indicated 5 volts on the signal wire from the crank sensor and 14 volts on the signal wire from the cam sensor (back probed, engine cranking). I did have the battery charger on while cranking to check the cam sensor so maybe that explains the extra voltage?

Ahhhhh OK, I can go for that, as long as the 12V sensor feed was at a similar voltage level. Man, I was having a tough time figuring out how the PCM was stepping up sensor voltage! :lol:

Just looked at some known good waveforms, the CMP does use a 12V signal.

No signal at the white wire at the IC module? If you're positive about that, then we may be on the right track with cam/crank signals, then powers/grounds for the PCM. I'd love to see a video on this one if you have the opportunity!

I verified no signal at the white wire again this morning (video has been shot, editing it now). I also rechecked that output of 14 volts and it is in fact closer to 12 volts. Everything checks except that white wire (no signal). That white wire comes from the ignition module. The input to the module comes from the PCM. The specs asked for between 1 and 4 volts. I had 3.52. Now I'm suspecting the ignition control module. Since it's Sunday my local NAPA dealer is closed. I have to wait until tomorrow to finalize this repair. I'll let you know how it goes. And there will be a short part two on the video if this fixes it! Thanks brother!

Video will be posted tonight. I'm downloading it now.

wyatt wrote: I verified no signal at the white wire again this morning (video has been shot, editing it now). I also rechecked that output of 14 volts and it is in fact closer to 12 volts. Everything checks except that white wire (no signal). That white wire comes from the ignition module. The input to the module comes from the PCM. The specs asked for between 1 and 4 volts. I had 3.52. Now I'm suspecting the ignition control module. Since it's Sunday my local NAPA dealer is closed. I have to wait until tomorrow to finalize this repair. I'll let you know how it goes. And there will be a short part two on the video if this fixes it! Thanks brother!

3.52V at the white wire during cranking? Yeah, I'd say the ICM is getting the signal to fire from the PCM. Unless you have already, then I'd be looking for ignition coil control at the white/black wire at the coil itself. Test light to B+ would work.

If you've covered all this in your video, my bad!

I'm not sure I know the term "control". I tested the other wires at the coil and they all tested good. The video has been posted if you'd like to see the steps I took. Go to 6:22 for the ending to save time if you like. Thanks for the input!

Can you post a link to you're video ? I would like to help thanks

Could you link me, sir?

Hope that works.