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Old 01-09-2019, 08:10 AM   #1
citizen_insane
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Stock cluster VSS signal parameters

I tried searching here and on msextra to figure out what signal is sent from the stock E30 cluster to the ECU, but I'm not finding much and it's kind of hard to test since it requires the vehicle to be moving.

As I understand it, the sensor in the diff is a reed switch which sends a signal to the cluster, which is then conditions into a square wave pulse with 9 pulses/wheel rev.

My issue is understanding what this "pulse" from the cluster actually is. Is it a switch to ground? Is it a 5 or 12V signal? This matters because it determines which input on the Megasquirt I wire to the signal. The MS3X "Launch In" supports ground switching inputs and the "Nitrous In" supports +12V switching, both are selectable as options in the VSS menu.

Last edited by citizen_insane; 01-09-2019 at 11:16 AM.
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Old 01-09-2019, 08:21 AM   #2
nando
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I don't recall what the voltage is, but the 'nitrous in' pin will work fine with the VSS output from the cluster.
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Old 01-09-2019, 08:29 AM   #3
Jaxx_
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Yes, it's a ground switching circuit. For my '84 I used the attached diagram. It might be different for other model years.

Connect C2 Pin 12 (Brown/Red) to Megasquirt ground switch input. I used Launch In myself.

Connect C2 Pin 8 (Brown/Violet) to Ground.

Here are my other settings:
VSS1 Input Type: Digital
VSS Position: Wheel
Speed Sensor Teeth: 9

VSS Dot smoothing:
Sample Interval(ms): 200
Lag factor: 50
VSS Sampling Interval(ms): 50


You can also setup gear detection as VSS/RPM, as I'm sure you know. I use my "6th" ratio as reverse.
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File Type: png boost.PNG (40.8 KB, 12 views)
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Old 01-09-2019, 08:33 AM   #4
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Don't attach the reed switch directly to Megasquirt. Let the cluster process it first. Just tap into the cruise control VSS output, which is right in that picture (C6). it's a black/white stripe wire. Actually, the stock engine harness likely already has a VSS wire, which you can also use.
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Old 01-09-2019, 08:35 AM   #5
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Quote:
Originally Posted by nando View Post
Don't attach the reed switch directly to Megasquirt. Let the cluster process it first. Just tap into the cruise control VSS output, which is right in that picture (C6). it's a black/white stripe wire. Actually, the stock engine harness likely already has a VSS wire, which you can also use.
I don't think it's necessary, but ok. I don't have a cluster anymore

edit: this is to say that I believe it is already a square wave from the diff--but I don't have a scope
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Old 01-09-2019, 09:19 AM   #6
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It's not though. Otherwise there would be no point in the cluster pre-processing it. It might be good enough to use for MS, but if you still have a cluster, the built in VSS wire makes a lot more sense.
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Old 01-09-2019, 09:32 AM   #7
bmwman91
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The reed switch in the diff grounds the line, which is pulled up to Vbatt normally.

The cluster simply acts as a buffer as far as I can tell. It is a ~11.5V square wave pulled to ground as a vane passes the red switch.
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Old 01-09-2019, 09:58 AM   #8
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It goes through a processor that's specifically designed for VSS signals. I have the datasheet somewhere, if I could remember what it was called.. lol.
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Old 01-09-2019, 11:16 AM   #9
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Quote:
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I don't recall what the voltage is, but the 'nitrous in' pin will work fine with the VSS output from the cluster.
Awesome, thanks for the confirmation!

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The reed switch in the diff grounds the line, which is pulled up to Vbatt normally.

The cluster simply acts as a buffer as far as I can tell. It is a ~11.5V square wave pulled to ground as a vane passes the red switch.
100% I plan to use the cluster to process the signal since I'm still running it (my secondary tablet dash will be mounted to the dashboard center console).

Based on this statement, I think the "Nitrous In" input is the correct one to use since it'll be providing voltage to the pins on the MS3X.

Jaxx, you're right on the money with the VSS/RPM gear calculations. My plan is to run a blend table for boost by gear to maximize traction.

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Old 01-09-2019, 02:47 PM   #10
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It goes through a processor that's specifically designed for VSS signals. I have the datasheet somewhere, if I could remember what it was called.. lol.

I assume it is to de-bounce the signal from the switch and isolate any possible EMI / ESD issues? If you find a PN or datasheet I'd be interested to see it.
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Old 01-09-2019, 02:51 PM   #11
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It also adjusts the output rate (k factor). If I find it, I'll post it.
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Old 01-09-2019, 08:41 PM   #12
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Quote:
Originally Posted by citizen_insane View Post
Awesome, thanks for the confirmation!


100% I plan to use the cluster to process the signal since I'm still running it (my secondary tablet dash will be mounted to the dashboard center console).

Based on this statement, I think the "Nitrous In" input is the correct one to use since it'll be providing voltage to the pins on the MS3X.

Jaxx, you're right on the money with the VSS/RPM gear calculations. My plan is to run a blend table for boost by gear to maximize traction.
That's right. You can use nitrous-in directly for vvs input from cluster. It's 0-12v square wave. And the cluster between the reed switch sensor and megasquirt is mandatory for debounce purposes.

But all this has catch. The speed how fast the megasquirt inputs can accept signal is quite limited. Even with that 9 pukses per rotation, the nitrous in input will saturate at about 200km/h speeds and the speed will go to zero. Making the boost lower. Like happened me on this 1/4 mile pull:


Only fast enough input is pt4, but it's 0-5v logic input. Which would need converter between. I have now second vss from front wheel abs sensor using max9926 vr-conditioner which signal goes to pt4 input. That works great on higher speeds also. And I csn use the tracktion control on ms3x too. But still because of the slow nitrous-in I have to switch of TC before 200km/h.
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Old 01-10-2019, 06:45 AM   #13
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That's good info. I can't say I ever went 200km/h with my MS2 though. I think MS3 has a faster CPU and more I/O, so this probably wouldn't be an issue.
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Old 01-10-2019, 10:19 AM   #14
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Pazi, thanks for the info. That sounds like an issue, the car will definitely be driven on track above 200km/h. I'll see what the rest of the MS3X community think and report back, but I'm thinking the best option would be to use the PT4 input with a voltage divider to reduce the signal to 5V max.
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Old 01-10-2019, 10:28 AM   #15
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if you have MS3x you should have plenty of inputs for VSS. I'm sure I have done over 120mph while logging (on a track) and I never saw a dropout, but I don't recall which input I used.
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