Tach input and noise

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Tach input and noise

Postby spiderman » Tue Dec 26, 2017 4:32 am

Hello all,

I have setup to my 91 190E v3.00 board with MS3. I have tested all the sensor inputs ignition and injection outputs without any problems.

However there are problems on reading crankshaft rotation. I have a Hall Effect sensor in order to read crankshaft rotation and I am using VR circuit assembled on MS v3.00 board. I am successfully reading the rotation of the crankshaft while cranking without any "NOT SYNC"s when ignitions and injectors are not working (*). However whenever ignitions and injections are active "RPM NOT SYNC"s are occurring. (*) I assume that these RPM NOT SYNCs are occurring because of the high noise that is generated by ignitors and injectors.

I have worked on the circuit that is built on v3.00 board and by changing R52 and R56 I have tried to eliminate the noise and generate a meaningful signal for microcontroller's interrupt pin however I cannot succeed.

I have also tried VR sensor without any success.

After searching on internet I found that, VR circuit that is built on v3.00 board is not so successful on handling the noise and some management systems are using ICs from TI LM1815 or MAXIM's MAX992x. Additionally there are some boards that use these ICs and can be used with v3.00 board (http://jbperf.com/dual_VR/v2_1.html , http://brickems.com/brickrpm/)

At this point I want to try one of these ICs. So here are my questions:
1) Should I use one of these ICs or insist on default built-in circuit?
2) Which one is best TI or Maxim?
3) Do these ICs successful in eliminating the noise?

Sincerely.
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Re: Tach input and noise

Postby SwedCharger-67 » Tue Dec 26, 2017 6:07 am

My guess is that you should start with looking at the grounding of your system, the 12V feed to coils and injectors, and check the grounding of the shielded cable to the crank sensor. Follow the MS schematics with relays, fuses and ground points.

Myself I have the shielded crank sensor cable going in parallel with the power and signal cables on top of my V8. I had problems with some sync errors at high loads, but after shifting to a crank Hall sensor type DIYAutotune I haven't had any sync problems.
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Re: Tach input and noise

Postby DaveEFI » Tue Dec 26, 2017 7:41 am

A hall sensor produces a constant amplitude pulse - unlike a VR sensor - so should be far less susceptible to noise. And given the V3 board VR input is working well with many thousands of VR sensors, you can be pretty sure it isn't a design fault.

Firstly, is your MS grounded to the engine block directly? Use four appropriate sized cables to four separate ground pins on the DB37. To provide a really good low-Z ground connection. Your sensor grounds (including the hall one) should be wired to another ground pin on the DB37 - not anywhere else.

Use a decent twin screened cable to the hall sensor - and keep it clear of any HT (sparkplug) wiring. Only ground the screen at the MS end.

It is also good practice to run the +12v feed to MS direct from the battery terminal (via a relay controlled by the ignition switch) rather than from the existing car wiring loom.

What you are aiming for is to remove and sources of interference rather than trying to cancel them out afterwards.

A 'scope can be very useful for seeing any actual noise, and checking which steps taken reduce or eliminate it.
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Re: Tach input and noise

Postby spiderman » Fri Dec 29, 2017 3:12 am

First of all, thank you for your responses.

As far as I understood it is very possible that we have a noise problem in our harness. So it will be better to work on harness first.

MS is connected to to vehicle's battery via pins on DB37: 15-16-17-18-19. Five ground cables are connected to battery's ground terminal.

We are using 4 tower wasted spark logic coil in order to drive igniters. Ground terminal of this logic coil was connected to engine block directly. I don't know whether this causes a problem or not.

I am assuming that engine block is directed to battery's ground terminal. This is why spark igniters can work, since they are grounded via the engine block. So I don't know whether connecting logic coil directly to battery or connecting to engine block solves the noise problem. Could someone please inform me on this from a theoretical point of view?

Sincerely.
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Re: Tach input and noise

Postby DaveEFI » Fri Dec 29, 2017 3:41 am

The engine block has to be connected to battery ground via a very high current connection - for the starter motor. The positive from the battery for the starter motor usually goes direct to it - for the same reason. But the main loom on the car doesn't use as high current cable as to the starter.

My original factory EFI installation had the injection positive fed direct from the battery terminal via a relay, but the ground for the injection to the engine block. Since MS says to do much the same, I stuck with that. My ignition power ground also goes to the same engine block ground. For ignition power, since it takes more current than the original factory system, I added an extra circuit direct from the battery, again via a relay switched by the ignition switch. But you might be OK using the original factory ignition circuit - just check for voltage drop with your new one.
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