G4+ Thunder Intermittent 5V supply problem

[007]

There's a bit of background I need to establish here, so apologies if this is slightly long.

I was hooning around one night as usual and on the way back, ran into a couple of extremely lousy speed bumps in quick succession. If sounded like the downpipe and / or oil sump bore the brunt of the blows. After the second hit, I noticed my CEL and oil pressure warning light come on, and the engine died.

When I broke out my laptop and connected to the ECU, imagine my surprise when every single sensor was spazzing out. Even the ECU temp read -12C, which it pretty much impossible. The root cause was the 5V supply malfunctioning. The real time display of the 5V supply read under 1V. The MAP sensor readout was swinging from -ve all the way to thousands of kPa.

I naturally assumed that the hits must have shorted out a sensor and that was causing the 5V rail to keep cycling. However, I wasn't able to find anything obviously broken or damaged. Shaking the harness wherever it had some play didn't seem to change anything either, so that ruled out a loose contact.

After about 30 minutes of futile attempts, I was ready to leave the car where it was (pretty close to my destination), and come back with a tow strap in the morning. However, I decided to give it one last shot and cranked the car, and was rewarded with a slight cough from the engine. So I continued cranking with the pedal mashed to the floor and the engine sputtered to life, even though the MAP was still all over the place. With the pedal still mashed all the way, I kept riding the clutch to get the car to move. It died thrice in the next 100m and probably pissed off the neighbourhood, but the fourth time around, while I was still trying to ride the clutch, everything seemed to come back to life just as suddenly as it had died, and the car started to run like a champ once again as if absolutely nothing had happened 30 minutes ago.

The next day, I lifted the car and checked for underbody damage, and found absolutely nothing. All electrical components and connections were solid too. I started the car up and shook whatever I could, and the engine didn't skip a single beat.

I'm not sure how to make sense of any of this. How could a sharp impact have caused this issue that seemed to solve itself? Is this something I should be worried about? Obviously, besides being more aware of my surroundings, should I also be looking out for something in the ECU? From what I gather, everything in the 5V loop is automotive grade and there are no moving parts here that can short on impact only to be magically fixed with no input from my end.

Has anybody had a similar experience that can shed some light on this?

[Adamw]

The most common cause of a 5V rail short I have experienced are from pressure sensors.  In my last job looking after the electronics on a category of 20+ single seater race cars I saw this happen probably 6 or 7 times in a season.  For whatever reason often when they fail they go low resistance between 5V & gnd.  And as you say, they will often just come right on their own if left for a while, so you have got to get in quick and diagnose while the short is present.

So unfortunately to diagnose you really need to wait for it to happen again, then the process is to watch the 5V runtime in the software while you unplug sensors, it will instantly jump back to 5V when you unplug the faulty one.

 

[007]

I'm definitely now going to crunch the car on any speed breakers. LOL.

Thanks for the information though. *If* this happens the next time and I'm in a position to have the car towed straight back to the shop, I'll try to hunt for the short.

Just wanted to make sure that this is usual behaviour under the circumstances and the ECU is fine.

Thanks Adam!