Blew 3a MS3 fuse on racetrack...

[ohecht]

I'm hoping for some ideas from the experts since I'm unlikely to be able to recreate these conditions again anytime soon.

I was running in an HPDE event at Summit Point in my '82 Porsche 911, and started to get what felt like slight missing at very high rpms...7k in 2nd and 3rd gear. This seemed to start and get worse as the clouds cleared up a little and the temperatures climbed, although the engine was never what I would describe as extremely hot. Cruising oil temp is 180* F and it was maybe at 220* F at the peak...a level it often reaches just sitting in traffic on hot days and I've never had the same symptom. Eventually, the 3-amp fuse to the MS3 ECU itself blew and I coasted into the pits. Replaced the fuse and the car was fine for the 150-mile trip home. I didn't put it back on the track after that and I took it easy on the drive home. I did test hitting 6500-7000 rpm a few times once I got closer to home and did not have the same miss or fuse issue...the engine ran smoothly all the way to redline as usual.

I'm running the latest firmware of MS3, 6 Injector Dynamics ID725 injectors running sequentially for injection and twin EDIS ignition coils. The injectors are massive for my 3.2 liter, 235 hp engine, but the car idles fine based on the small pulse width test data from Injector Dynamics. I don't think injector duty cycle could be a factor as I've never seen it go above about 30% even at redline. Unfortunately I don't have any logs of the events on the track. I've also meticulously followed the recommendations for grounding the ECU and sensors and have each of those bundled separately and each grounded separately to the chassis...consensus has been to not use the engine "block" on the 911 as it's connection to the chassis and battery is not as direct as more traditional setups. My sensor readings have always been rock-solid since wiring everything that way.

My original theory was that the combination of slightly higher temps and very frequent revving to redline under load as I went around the track caused the MS3 to have trouble managing the current firing the injectors and EDIS coils so frequently and that first caused the missing before the main fuse eventually blew. Looking at the wiring diagrams again, though, there are separate fused circuits for the injectors and ignition circuits and neither of those fuses blew, so I wonder if it could be something else. I read some posts about external voltage spikes causing transient issues and/or the main ECU fuse to blow so now I'm thinking maybe my voltage regulator was what was suffering during the extreme use on the track. I wish I had logs to see if there were voltage spikes associated with the missing and eventual blown fuse. The alternator is about a year old with an internal regulator.

I'm just looking for thoughts about these theories or other ideas to investigate...I'd like to make the car reliable under track conditions but doubt I'll be able to easily create those conditions off the track or use a track for testing fixes easily or anytime soon.

Is there a way to better isolate the MS ECU from any voltage spikes if that is the likely cause?

Thanks in advance,

Olivier

[racingmini_mtl]

The MS3/MS3X cannot directly drive EDIS coils so how are you driving them? And how is everything connected?

Jean

[ohecht]

Sorry, left that part out. I'm running twin EDIS modules that drive the coils and I'm controlling those with Pin 36 on the main MS3 board. The injectors are running sequentially off the MSX board.