Comment by addaon
2 years ago
I have a 2009 Mercedes SLK that had the same symptom. It was absolutely fine 99.9% of the time, but one time in a thousand it wouldn't even try to start. No clicks, no indication that it even knew I was in there turning the key. Just dead. And then seven or eight hours later... it would start fine, as if nothing ever happened. Couldn't correlate it to anything.
Had it towed to a service center a few times when this happened. Every time, by the time they got around to trying it (a few hours later)... it would start fine, with nothing to diagnose.
Then, it was 99% of the time it was fine. I was with a group of folks car-camping off road to fly a human powered airplane for a couple days, and... no start. Finally started -- with no sign of any problems -- around noon the next day, and I high-tailed it out of there a day before the rest of the group, because getting stuck there /after/ the rest of the group would have started pushing my comfort level. So at this point it's actually interfering with my life.
I've tried all the usual stochastic troubleshooting (swapping out fuses, light to moderate percussive maintenance, alternate keys) and nothing. Finally it fails to start in my driveway, and I get it towed to an independent mechanic. It's short tow, and it fails to start when it gets there! So now he's seen the problem, and is as puzzled as I am. Of course, when he tries again the next morning, it starts fine.
He proposes two possible fixes: replacing some ECU module, or replacing the fuse box itself (under the theory that it's the connector or connection into the bottom of the fuse box that is having some moisture ingress or intermittent connection). Of course whatever we choose, I won't know if it was right or not until the next time I'm stuck. The ECU is multiple thousands of dollars, and the fuse box is < $200 with labor, so I make the easy choice.
This was six or seven years ago, and that car is still my main and only car. Hasn't had a single mechanical issue since swapping out that fuse box. A good independent mechanic and a good guess!
I had a truck do this kind of behavior once.
Finally figured it out after months when I moved a wire and it started, then move the wire close to another wire and it would no longer start. These were wires that would typically be close, so my guess is one of the wires was now generating enough noise that it was bleeding over into some other system and causing it to fail. As the car bounced around the proximity of the wires changed and lead to the random behavior.
EMF can do some seemingly crazy things. I built a kiln controller and the initial version would sometimes randomly lockup, reboot in the middle of an operation, or do other seemingly "crazy" things. Sometimes even the hardware watchdog would stop functioning.
Turns out contactors pulling in and out a 5000w load generates some strong EMF and sometimes that EMF is enough to cause random glitches to the CPU or other hardware.
Switching to high power solid state relays completely solved the problem while keeping the system compact. The actual silicon transistor was so big you could have drawn the mask by hand and it was attached to a heat sink half the size of an adult fist. I was initially worried about reliability but (knock on wood) 8 years later the system is still working without issue.
Yeah. I wrote the code for a controller that managed the temperature of a lubricant. The heater was a propane-fired burner that was mounted on the same skid. Every so often we'd get a random reboot. Finally, when it was very quiet, I heard a soft 'tic' just before I noticed that the CPU had rebooted again. EMI from the spark gap that ignited the propane was coupling back into the I/O lines and would occasionally reset the CPU.
One of those things where if it had happened 100% of the time we'd have figured it out quickly. But it was so infrequent that no one thought of that as a cause.
EMF is fun.
We had one access point (unifi) in our datacenter which was consistently failing.
fail, RMA, fail, RMA, probably replaced that thing 5 times. It was also incredibly unreliable.
Meanwhile all the other access points (probably 10 of them in total) had 0 issues.
Eventually realized that the cable for that AP was running perpendicularly over the conduits which fed power into our suite, so, about 1MW of power. Relocated the cable so it was farther from the conduit and it never had an issue again. Makes you wonder about the effect of working in such environments.
Most likely this solid state relay has a builtin flyback diode that was not in the circuit of the contactor. The diode should be as close as possible to the load to reduce the EMF.
Interesting that your car is a Mercedes...
I know an older guy at church, whose kids all graduated college - except for one.
His "failed" son is the top mechanic at a Mercedes dealership. He does some supervision, training, etc. But the reason the dealership is paying him $200K/year is his skill at figuring out and fixing problems like that, for the dealership's most desirable and profitable customers.
(That I've heard, none of the mechanic's "successful" siblings are making that kind of money.)
Troubleshooting can be a very valuable skill.
I know a story of a certain large engineering firm, dating back to the second World War. They had a senior engineer who habitually came to work drunk and slept through meetings. Every once in a while, they'd wake him up, and he'd save them a couple of million dollars. He had a gift for finding clever solutions.
He probably would have had a better life if he'd gotten his act together. But knowing how to fix subtle issues, or how to design good processes, can be a ridiculously valuable skill.
That definitely sounds like a loose connection, not an ECU problem. Usually it's a bad clamp on the battery terminal. Often it's the ground clamp in particular.
The easiest diagnosis is to rotate the cables on the terminal several times to rub off oxide build up, then leave them in an orientation so that the natural tension of the cable forces the clamp into good contact.
A "dead" (low voltage) battery will still cause some indicators/lights to come on when you crank, while a bad connection usually acts like zero volts.
I was about to say the same thing. My car (nowhere near as nice as a Merc) had the same issue. There would be oxide deposit on the contact points and the minuscule amount of rust and crack on the clamp would render it useless on a bumpy road randomly (1 in 10 times approx). It was scary the first few times but when I figured out the symptoms, it took me less than a minute to get it to start.
Today's car mechanics are a different breed. The ability to diagnose and triangulate problems without advanced tools made car mechanics quite intuitive and resourceful in the past. I take my car to one such mechanic. I always leave with a bill lower than any other estimate I get and he always finds and fixes the problem.
Wow, it's crazy to see this comment because I drive the same type of car and recently experienced a similar problem.
The car was dead for long enough that I could get it towed to the mechanic and they educed it was a problem with the EIS (electronic ignition system).
The EIS computer was sent off to mercedes for diagnosis, they reported that the computer itself was fine but it was an issue with power. The mechanics traced it back to a bad connection of a wire somewhere.
wonder if removing and replacing the fuse box (reseating all the fuses, reconnecting all wires, etc) would have done the same thing? sigh.
It would have required removing and replacing the box itself... the fuse box has a connector going into the back side, and then a separate harness inside the box that splits out the individual fuse sockets; that connector seems to have been the problem. At various points I swapped/reseated every fuse in there trying to fix things, never occurred to me to remove the box itself.
I have seen similar behavior on vehicles that have a dying alternator. The issue is that one vane of the rotor (or maybe stator?) has shorted and no longer functions. If when the car is shut off and the rotor stops in a particular position the alternator won’t work. Give it some time and with heating/cooling moving things around just a bit plus the act of repeatedly trying to turn it on potentially making it move just a bit, and it works fine.
Alternators aren't needed for starting. Perhaps a similar issue with the starter motor? But even then you'd hear the thunk of the starter solenoid. Any audible clue definitely makes root-causing simpler.
I have a crank sensor that is going out, and though it still works most of the time if the crankshaft stops in just the right position it becomes an absolute bear to start, and runs in limp mode when it does.
Any of the other positions it works just fine.
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Seen things like this before. Often a bad connection somewhere, possibly involving material that expands/contracts significantly in response to a change in temperature (turning the module into an inadvertent temperature controlled switch).
What happened with the human-powered airplane?? Is that a thing normies can do?
https://en.wikipedia.org/wiki/DaSH_PA
I don't know if Alec Proudfoot counts as a normy, but DaSH PA flew a bunch of times, with many different pilots. That particular event, which was an attempt to try some records, was a bust; there had been rain recently and the dry lake beds weren't. Landing gear was a recurring problem (light weight and robust don't go together), and even some last-minute attempts to build a runway out of 4x8 plywood sheets was unsuccessful.
I have some very, very fond memories of flight days at Moffett, though. I had gotten to work on the Moffett runway for work previously (so cool out at the bay end of the runway at night, totally silent), and getting back there to help run ground ops (including assembling DaSH PA before sunrise so first flights would hit the calmest possible air) was just lovely.
That's really cool! I was only aware of the Gossamer Condor/Albatross and MIT's Daedalus in that field.
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