This red beast is still an electrical nightmare. But I got out there and did some testing today. The short version is that if you have a suby, with after market ECU I would suggest a 1.6kohm to 2.2kohm 1 watt resistor be installed at the ECU or just inside the ECU. This loading resistor will help get a better electrical signal to the ECU. See below for a slightly more detailed analysis below.
In this case, the slow cranking pulses are above 85mS, so the MAX9926 zero-crossing threshold will self adapt to about 4mV to 30mV, typically 15mV. The chip typically self adapts to 33%, so when in these low cranking RPM situations, you want your VR signal to be at least 30mV * 3 = 90mV, which is 180mV peak to peak. By adding a resistor to the circuit at the ECU, you can decrease the effects of noise. However if you put on a loading resistor with small ohms, you can also prevent slow RPM tooth detection. In my case, I have a very noisy signal, and I'm looking to decrease the noise by adding a loading resistor.
I was hoping that I could minimize the noise and get valid readings from my suby with questionable wiring. I set out to see what ohms of impedance I could put on the VR to minimize the noise, and decrease false tooth detection.
Here is what my suby signal commonly looks like while cranking. Please note, this is probably not the typical. the prior owner did some really messed up wiring, it's a 98 with a 2002 engine harness. The battery has been relocated to the trunk, and I have many brown scotch connectors. This wiring is very sketchy and bad. I don't drive this thing, as it's just waiting to catch fire. I need to gut it completely and wire it properly. However I have not gotten around to doing that yet.
620ohm_5ms_0.3Vpp_noisy.png
The above is with a 620ohm resistor across the VR.
Here is what my noise looks like with the key on and not cranking.
620ohm_no-crank_1ms.png
You can see that as soon as I start cranking, I get massive noise issues. As well you can see that even with a 620 ohm resistor, I'm still getting about 480mVp-p which is more than the required 180Vp-p. You can see the noise floor is very low, so this noise is not from ambient stuff, it's from the car. Also the hood was open, and the scope was placed outside the car near the front passenger fender. Such that the wires were as far away from potential noise generating wire as possible. The shield was connected to the frame ground, so the shield should be mostly functional. Also I have disconnected any devices I can get my hands on. However there are many things buried in the engine.
Here's a picture of where I captured the signals. I'm showing the scope probes on 10X and that I'm on that side of the wiring connector. When I did the testing, I ran the wires 180 degrees from how shown, such that they went over the fender and left the engine bay right there.
Setup.jpg
Any how, despite my attempts to keep the noise down, it's still there for me. I don't see enough of a change from 10k to 2.2k to prevent false tooth detection's. To me it does look like the 10k is more noisy than the 2.2k. As well it looks like 620ohms is less noisy, and I expect that lower ohms will decrease the effects of the noise.
So I wonder, how low in ohms can I go? I really don't know. It depends on a bunch of details which I do not have. I don't know the gaps, or inductance of the VR, etc. I don't know how many mJ of energy the VR will produce at full RPM. I know a 1/8 watt 620 ohm resistor, could handle about 9Vrms, so about 12Vp-p. I expect VR's to typically get up to 50vp-p ish. So I would think that 40Vrms ^2 / 0.125W = 13k ohms is all you can get out of an 1/8 watt resistor. I would expect a 1W could handle 1.6k ohm. However I don't know the voltage that would be produced by this sensor at full RPM, so that's just guess work. It would be handy if I could get this sensor spinning at 6kRPM, then I could measure it's voltage and make good suggestion for what size resistor it could handle.
Any how, the conclusion I would make from this is that if you use a 1.6kohm or a 2.2kohm resistor, you want to install it across the leads at the ECU or if you can inside the ECU. This will help decrease the electrical noise and will help prevent noise from generating false triggers. I also know you can electrically go down to 620ohms and you would still have sufficient signal for the MAX9926 to detect, however you are likely hitting thermal limits when you go below 1.6kohms. If do install a loading resistor, I suggest using a 1 Watt resistor, and that you provide it with thermal cooling. Granted you aren't going to be full RPM all the time, so you would probably be OK with a smaller resistor. However keep in mind you may end up doing engine braking as you are heading down a mountain or long hill, and it would suck if it overheated. Also there really isn't a difference between 1/2 watt and 1/8 watt, so you might a well spend the extra $0.50 and get the 1W resistor.
Also for archival purposes, and to leave a bread crumb trail for kb1gtt here's my notes of this testing.
Subaru_Crank_Sniffing.7z
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Here's the basic specs.
