Think I fried my daughter's PCM in a moment of stupidity yesterday. Can anyone tell me how you can have 5V on the crank sensor return with the sensor unplugged? I hit the A/C 5V sense wire with 12V by mistake, slip of the finger. The engine immediately shut off and of course no restart. 2001 Dakota 4.7, auto.

I have verified that I still have my 5V reference at CKP, CMP, TPS, and A/C. I still have my grounds at all sensors. I have proper 12 supplied and ground to PCM.

Using a scope the CKP is pinned at 5V until I start cranking then it goes high/low. Shouldn't this be at 0V and go high or is this possible?

I have no 12V on coils, nor engine relay engaging, nor fuel pump engagement. Thought I knew where I could borrow a PCM but that isn't looking very good right now either. Don't mind buying one since I screwed it up but also don't want to get it here and find something I missed either.

No good deed goes unpunished lately it seems. Thanks guys.

 

Crank sensor is essentially a tone generator. So, it will pulse as the crankshaft turns. (I think a square wave, but, not 100% sure of that.)

 

I do have a high and low but it sits on high. Also, when I pull the plug from the CKP sensor I still have 5 volts on 2 of the three wires.

 

That sounds correctish I think.

Of the 3 wires, one will be ground, the other +5, and third is the "input" to the PCM. IIRC it is held high when material is present, then biases to low at the window. Each window is 5 degrees of duration. The coil is signaled at the transition from window to material "rising edge". I might have high/low backwards, but i know the spark triggers on the back edge of the windows.

You should be able to hear the fuel pump prime at key-up regardless of CKP condition.

Depending on the voltage regulator design, zapping the 5V output rail to 12V could pop an internal p/n transition or the regulating zener, or it could have been sunk out through a safety buffer (I would like to think the latter, esp since you still have voltage).

If you zapped 12V to the "sense" wire that goes to the PCM, you're screwed, as I doubt they have much for protective diodes on that wire since it needs a very fast alternating rate.

 

Let me run out and check for fuel pump prime but I don't recall hearing it with all the noise yesterday.

Update: no prime with key on and engine relay also does not close. FP will prime if I manually hold relay so power is there.

The relay marked engine relay must also be known as the ASD? No ASD relay labeled anywhere I can find.

 

sounds like you nuked the PCM.

PCM is powered by Fuse 2 - 20A (Circuit A14), Fuse 11 - 30A (Circuit A142), and that's all that I see.

 

Looks like I'll order up a PCM today. Thank you guys for assistance in diagnosing. Will let you know how I made out for future reference of others.