944 doesn't start anymore after bucking
Hello,
I’m hoping to get some help here regarding my 1985.5 944. I’ve found some similar forum posts, but nothing that exactly describes my problem.
Just to give you some context: I started the engine, and at first I noticed a significant fluctuation in RPM, ranging from about 500 to 1000. This was the second time it had happened. Once I started driving, it was fine. On the way, I noticed that the engine started to jerk at 2500 RPM and above. In my opinion, it wasn’t gear-dependent. I only tested it in gears 3–5, but every time there was this bucking above 2500 RPM. The car drove completely normally, except when it reached that RPM. On a long straight stretch, I tried to figure out where the problem was coming from, so I also tried to drive past that bucking—which wasn’t my best idea. After severe bucking, the tachometer dropped to zero, but the engine was still running. Shortly after, the warning light on the speedometer came on, and the engine stalled. We were coasting through a traffic light and had to brake because of it. The car wouldn’t start anymore, but it cranks. I quickly pulled the car to the side of the road, but all attempts to start it were unsuccessful.
Just for your information: There is no spark.
I initially suspected the DME relay, so I replaced it. However, this did not resolve the issue. Today I focused on further diagnostics.
I then suspected the two speed and reference sensors. These are located next to the OT display window. I removed and cleaned them, as there were a few metal shavings on them. I disconnected the DME connector in the passenger footwell and then measured pins 8–27 with an oscilloscope. According to Clarks Garage, the signal is correct; a sawtooth wave is visible. However, according to Clarks Garage, this should have a voltage of >2V. I measured a voltage of approx. 0.2V. I rule out the possibility that I read it incorrectly. This voltage would definitely be too low.
I also measured pins 25-26; the signal is correct there as well. The voltage was also 0.2V.
I then measured the resistance at the sensor connectors to check them.
25-26 ~1k ohms
25-78 nothing
8-27 ~1k ohms
8-23 nothing
So the sensor itself is also OK.
I also rule out the possibility that the sensor distance has changed suddenly; the mounting plate looks correctly installed—it has worked fine up until now.
I would be very grateful if anyone has any tips that could point me in the right direction.