blade7

I was out for a drive and all seemed good, stopped for gas after about an hour then only got 1/2 mile before the engine faded and stalled, it wouldn't restart so tried bridging the DME relay contacts, then pulled one injector connector but no go. I've got recovery/roadside assistance so called them, the first thing the mechanic did was check for spark by shorting the king lead to earth, there was power so he reconnected the lead and the car fired straight up and ran fine, a strange thing is my AFR's are now spot on where before idle and cruise has been a little rich for 2 or 3 months, so is it possible the DME/ injector drivers got a wake up by shorting the coil to earth ?

Paulyy

It's probably possible you had a poor ignition contact.

blade7

Maybe, though the distributor cap, rotor arm, plugs and HT leads aren't that old and I fitted them myself so fairly sure they were all secure. Checking when I got home the mech had barely pushed the coil lead back into the distributor cap yet it ran fine.

Paulyy

I don't know how the driver works but it could be a solid state ones (no moving parts) so i don't think you can "wake them up"

Just check the leads and see if they're clean. It's possible that they can be loose.

Droops83

Defnitely double check all of your wire connections, Pauly may well be right.

BUT, to answer your original question: I work on Porsches and BMWs for a living, old and new. Believe it or not, a relatively common "fix" for random unexplainable electrical issues on modern cars (say, late '90s and up) with onboard CAN networks is a procedure called a "battery reset." BMW actually prescribes it in certain cases.

This is a "hard" reset rather than a software reset, which is often stage one on such cars when the gremlins are unexplainable.

The battery reset operation involves removing both battery cables from the battery and shorting them together with a jumper wire for a period of time, preferably with a fixed resistance such as a 12V light bulb inline. 5-10 minutes is usually enough. This gradually drains multitude of capacitors in the various onboard electronic control modules, same as doing a "hard" reboot on your PC. The same process often works for many electronics when they "freeze", such as BluRay players, etc (unplug the device, hold down the power button for a few seconds and restart).

I am not an electrical engineer so I cannot explain exactly what occurs, but the capacitor-draining affect sometimes works.

I have never tried this on anything as old as a 944, so I have no idea if it would have the same affect, but it is worth a try if your engine management acts up again, and it should not hurt anything as long as you have a working light bulb or similar resistor inline when shorting the battery cables together.

Perhaps any resident electrical engineers can chime in. There is an electrical engineering firm right next door to my shop; I will ask the owner exactly what this achieves when I get a chance.

blade7

Thanks for the suggestions Paulyy and Chris. Around 15 years ago I had a 93 Audi 80 V6, coincidentally this was the last time I had to call the recovery service when the car wouldn't start. The mechanic tried several things and as a last try disconnected the battery leads and touched them together, the car started straight up and I was on the road. The Audi did the same thing a couple of times over the next year and each time I disconnected and touched the battery leads together to cure it. I mentioned this to the mechanic that came out to my 944 and he said they do this to reset ECU'S now. Good tip about using the 12v bulb as a connector.

mtnman82

What you're doing by touching the leads together is indeed draining any remaining 'juice' in any capacitors in the electrical system. This ensures everything is powered down completely, thereby achieving a hard reset. Capacitors act like little batteries and store charge. If you just disconnect the battery without touching the leads, some of the larger capacitors in the electrical system may retain enough power for long enough that some of the electronics are still powered even though you disconnect the battery. Hope that helps.

Edit: I should add touching the battery leads together is like touching the leads of the all the pertinent capacitors in the system together, shorting them out and draining any current. This isn't the best way to do things, as it not a graceful way of draining the charge - if you've ever touched the leads of a car battery together you know what I mean. Wiring a light bulb across the leads is a graceful and controllableway of draining the charge out of the system.

blade7

It's something else to try if I have the problem again. I'll be carrying a few extra tools in the future.

quinnfiske

A couple of years ago, I was watching an ALMS race on TV and one of the Ferrari F430's stopped out on the circuit. For some reason, the TV crew covered this and we heard the chatter between the driver and the pits. They instructed him to reboot the ECU. There must have been a reset switch or something within reach of the driver, because it worked and he drove off. I don't know if this car had a Bosch system but it was the first I ever heard of stuff like this.

On BMW's, wouldn't you have to re-sync the key fob and the ECU?

Tom M'Guinn

Interesting discussion, but I'd check all the likely suspects first, starting with the speed and ref sensor connectors, DME solder joints, etc.

blade7

Valid points but while the car is running fine how would you know if they're faulty, plus would shorting the coil lead have an effect on the ref/speed sensors ? The car is never usually slow to start.

Tom M'Guinn

Intermittent issues are the hardest to diagnose, no doubt. While anything is possible, the most likely explanation is that you have a connector going bad at the fuel injectors or speed/ref sensors, or a bad solder joint in the DME, or you have an ignition or fuel delivery problem -- i.e., one of the many things that routinely go wrong and create intermittent no-starts on these cars. Crazy Eddie had an infuriating intermittent issue, for example, where the car would randomly idle wildly and none of the usual suspects tested bad -- we finally found that the DME was randomly going into wide-open throttle mode from time to time because there was a cracked solder joint on the mother board. I guess another way to say it is that if grounding the ignition lead consistently causes the car to run (or run better) then something is not right somewhere....

Crazy Eddie

As Tom mentioned
https://rennlist.com/forums/944-turb...dle-issue.html
My issue was in the KLR but it could be either box
Hope it helps
Regards
Ed

blade7

Thanks, the pic of the failed joints in your link gave me an idea of something else to check if it happens again

blade7

Drove the car a couple of days ago and it ran fine, went out today a while it seemed generally ok a couple of times it stumbled and the AFR briefly went to 9-1.