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FYI: crank trigger solution for $30, no trigger wheel needed
Folks looking to install a standalone have often had problems with using the factory speed/reference sensors. Most people either get an ECU that supports the 130+1 signal out of the box, or they get an aftermarket trigger wheel to mount on the front of the crank. These options both work great, but here is a 3rd option I just tested with great results.
In short, the issue boils down the 130 teeth (or 132 for NA 8v cars) on the ring gear. That's a lot of teeth and the stock DME has a special circuit for decoding it. Aftermarket ECUs generally need some kind of bespoke circuity or at least code for this pattern, at the very least. The single pin on the flywheel is not the problem. I was thinking about what other options are available to pick up the speed signal, and reading the cam popped up as an option. Porsche was very courteous and put a small inspection window that's 90 deg to the cam pulley, which has 40 teeth....
I used an inexpensive brass sleeve from McMaster-Carr, P/N 7815K62. This is a press fit into the inspection hole. Some Loctite 680 is recommended to keep it snug. I trimmed the sleeve flush but I didn't need to. Next, I cut threads in the sleeve with a M12 x 1.00mm tap.
The sensor is a generic gear tooth sensor (hall effect). Cherry GS100502, can be found for under $30 shipped. You will also need a 2.4 kOhm resistor. The power wire of the sensor gets switched 12v, and the ground wire goes to your ECU's signal/sensor ground. The signal wire goes to your crank sensor input, in my case, this was VR- for Megasquirt (VR+ left floating). The resistor connects at some point along the 12v wire, and tees into your sensor's signal wire to act as a pull-up resistor.
The sensor simply threads in to the sleeve. When it bottomed out and contacted the cam pulley, I backed it off 1.5 turns (1.5mm due to thread pitch) and locked it with one of the supplied jam nuts.
To test this, I set it up on my 944 NA. It has a 36-1 trigger wheel that is used to run the engine normally. The 36-1 trigger wheel is in blue, which is running the engine. Green trace is the 40 tooth cam wheel. The "missing tooth" of the 36-1 wheel shows up after 35 pulses, and 20 teeth register on the green cam signal trace in this timeframe because the cam spins at 1/2 speed.
Note that this only takes care of the speed sensor. To use this approach, you would retain the stock reference sensor, since the ECU would need a TDC/sync reference. It becomes a 40 (cam) + 1 (crank) signal or effectively a 20+1 crank signal.
My only concern with this is the material that the cam sprocket is made off. Trigger wheels are generally made from low carbon steel because that meterial is not magnetised easily, however I suspect the cam wheel is of a sintered steel alloy construction which, over time, will be magnetised by the sensor and cause issues. I’m no expert, just something I read about a while ago on some other forum.
Interesting, didn't think about that. There is a very weak permanent magnet in the sensor. I'm hoping that with it being relatively small compared to the cam wheel, being 1.5mm away, and not being a powerful magnet will mitigate.
Originally Posted by Voith
Nice. I have one of those too.Where did you get it for $30?
I got this one from DIYAutoTune for $53 but you can get it much cheaper:
I’m looking into converting my 85 N/A 944 to Megasquirt and wanted to see how the cam sensor is working? Also what mega squirt model are you using with this setup? I’m also curious as to what the base timing is set at on the mega squirt?
08-09-2018, 12:09 AM
FYI: crank trigger solution for $30, no trigger wheel needed
