turbotaloon95

iTrader: (7)

Problem: Car doesn't start. Motor cranks, fuel pump works. No spark. No crank signal or RPMs. Reading 12v from crank sensor harness (to ecu) on power, but also reading 7v on ground and 4v on signal?!
Backstory: About 2 years ago, I started running into issues where the ecu fuse (#15) would pop under the hood while driving, randomly. I would replace the fuse, start it up and it would be fine for a while. Then I tried to start diagnosing why the fuse would pop, found some exposed wire at the crank sensor (on the ecu harness side). Tried to fix it, and then got to a point where the car wouldn't start, but the fuse was not yet blown. I was reading voltage on ground wires, and thought that was very bizarre. Someone convinced me that the wiring harness was foobar, so I relegated to replacing it and rebuilding the trans while I was at it because that needed to get done for a while. So fast forward to today, I bought a house and moved the car to the house. Took the motor out. Rebuilt the trans and transfer case, lots of R&R, replaced the wiring harness with a second hand one I picked up. Put everything back together and I'm finding the same issues I found before I started this whole adventure. I have absolutely no idea why I'm getting voltage on ground wires and on the signal wire, when the connector isn't even connected to the crank sensor! Is my ECU toast?

MinusPrevious

iTrader: (3)

Im no electrical expert but know enough circuitry to realize what a PITA this must be

Wonder what shops specialize in diagnosing issues like this? It may be something so simple, but yet not so obvious

Hopefully some of the guys might be able to offer up some constructive ideas

Hang in there / Cheers,

Biggiesacks

Maybe these will help. Is the ecu throwing codes? How do you know its not getting a crank signal, evoscan?

turbotaloon95

iTrader: (7)

Thanks for posting the diagrams. I have pulled them up myself, but I'm just really bad at understanding all this.

No codes from the ecu, but yes, I have evoscan and logged while cranking the motor over. RPMs were all 0, and crank sensor was 0 the entire time.

A friend had suggested a bad ground, so I took some resistance readings.

The battery was recently mounted in the trunk. Resistance from negative terminal to ground point was near 0. That's ok.

Up front, there are two chassis ground points on the driver's side. Resistance between the two are near 0. OK.

Checked resistance from the connector on the ecu harness that would plug into the sensor to the chassis ground (crank sensor NOT plugged in), I read "3" when the meter was set to "2000k". BAD
Checked resistance from the ground wire of the crank sensor (not plugged in to harness) to chassis ground: no circuit what so ever (expected).
Checked while sensor plugged in to the harness, resistance from ground wire to chassis ground, "3" at "2000k". BAD

In the cockpit, I checked some ground to the ECU. There are two pins (13 and 26) that get ground from the chassis. Unplugged this connector and read both pins, each read less than 3 ohms. OK
Next, I checked pin 40, which is the ground to the crank sensor (and cam and TPS sensor).
With the connector plugged in to the ECU, resistance between ground point and ground wire was "3" at "2000k" again. BAD
With the connector unplugged from the ECU, this wire read "483" at "2000k".

Now again, I'm really bad with all this, but I can't tell if the ground for pin 40 should be coming from the chassis ground on pins 13 and 26, or if it is grounded to the chassis at one of the points under the hood? Following the wiring from the crank sensor through the harness, it goes from passenger side to the driver's side by where the battery used to be. There are like 3 or 4 ground cables that come out from there and mount to chassis grounding point. The rest of the harness loops back along the firewall to the passenger side back in the cockpit to the ECU. I am assuming the wires that attach to the grounding point lead to pins 13 and 26 for "chassis ground". My assumption is that the ECU then connects that ground to the sensor ground. And if that's the case, my assumption leads me to believe the ECU is faulty here. Does that sound accurate?

Link to ECU pinout: https://www.roadraceengineering.com/...-ecuwiring.htm

Biggiesacks

I dont know what that pin 40 reading should be but 3 ohms does sound weird. Since you pretty much narrowed that down to the ecu I would pull it out and inspect the board. The board basically just pops out of the plastic chassis, you just need to pull back on the tabs. Then you can find pin 40 and follow the trace.

turbotaloon95

iTrader: (7)

Thanks! But when I said 3 ohms, if the meter is set to 2000k, then it's something like 3k ohms or whatever. Wayyyy too big.

Regardless. I decided to ground that wire for the sensors directly to a grounding point inside the cabin. After doing so, the ground wire at the crank sensor connection was less than 1 ohm (GOOD), power wire got 12v. Ground wire showed no volts as expected. Cranked the car over and sure enough, evoscan showed me some RPMs!!

So it's something in the ECU. I can try to pull it and inspect it, but I don't really know much about what I'm looking at or how to repair it. I have to decide if I want to try getting the ecu repaired, or go with another ecu option.

Biggiesacks

Your basically going to be looking for bad stuff, corrosion, charring etc. It would probably be a lot cheaper to just pick up a working used ecu off the forums here then have it repaired. Depending on the damage of course. Hopefully what ever killed it has already been repaired by the new harness and its just a matter Remove and replace.