If the SECU blocking starter from turning is a anti theft feature, then it shouldn't be possible to push start the engine in this condition. As push starting works, it don't see any issue with having the starter circuit bypassing the SECU unit.
Strange thing: My P12 did the same thing on (nearly?) the same date as Covjek!
The story of my starter: All winter starter has been slow turning, but always starting. It's cold and battery is 8 years old, so I blamed the battery without actually checking anything. One evening i january I parked outside a shop and vent in for 5 minutes. When I got back starter was dead, no sound of it trying to turn over at all. I figured I would try to push start the car. Well, it was snowing heavily and even with help we could not even push the car out of the parking. So I put it in 6th gear and jiggled the car a bit forward and backward. Then tried the starter, and it started.
On February 26th the same thing happened again, 2000km from home, barely any tools in the trunk, in a country where noone speaks english. But this time pushing in 6th gear did not help. With the help of random people we push started the car. I typed "battery" in google maps and found a place that only sell car batteries. One guy there happened to speak English. He put a voltmeter and a 0,3ohm resistor over the battery. Pretty simple way to test battery starting capacity. He concluded that the battery was not a problem. Then he called a friend in a company named "Starter". Well, you guessed it - that company is highly specialized they only replace starter motors! They had a refurbished starter in stock that would fit, so the guys at the batteryshop pushed me out of their parking lot and I drove to "Starter".
At Starter the guy came out to look at the problem, but here the car started every time we tried. I was told via google translate to come back the next morning.
That evening I was smart enough to park on the top of the only hill for miles, cause the next morning the car didn't start. And trust me, pushstarting a cold diesel engine in winter takes a lot more than a hot engine. I pushran the car, jumped in, failed to start, jumped out, running, pushing, jumping in, jumping out.... finally, 5m before the bottom of hill, it started and I could drive to Starter.
They took the starter out and disassembled it. The brushes were almost not existing. The contact plate in the magnetic switch on the starter was nearly broken in half. Surely the problem was located, I though. New (refurbished) starter in, and car started perfectly. It was no longer slow turning over either.
10 days later, starter did not turn over. Not even the sound of the magnetic switch. I'm sure I had the sound of the magnetic switch when testing at the batteryshop.
The car was push started, and has be working ever since. But I try to park front down anywhere I see a hill.
I've been thinking in the same path as Covjek - either bypass the SECU, or bypass the starter switch. But what I'll end up doing (when/if I get home) is to put a switch under the hood that connects the starter terminal directly to the battery. This will bypass everything. So if the problem occurs again, I have a workaround that will work even if the source of the problem is not truly identified.
If/when the problem becomes more frequent, I'll be able to do a better debug than now where it can be weeks between its occurrences.
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