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MetricMalibu
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Trying to Log RPM - Eratic Readings - Have MSD 8918 Pick-Up- LS Motor

I am trying to hook up my Logger on Channel 5 to read RPM and I am getting eratic RPM readings.

I first tried wiring the logger right to the ignition box out put that goes to the tach but the RPM would jump all over the place. I would hold it at 1200rpm and it would bounce between 0 and 14,000rpm.

I bought a MSD 8918 Tach signal GMR pick up and hooked it up to the coil 12 volts as per the instructions with the washers on top so there is no air gap. the RPM was steady until it got to over 1000rpm then it was all over the place and jumped up and down. I then tried putting the pick up on the tach output wire off the ignition box and it didn't provide any reading at all.

I put it back to the coil and tried moving the washers around still nothing.

Can you provide any advice to hook the MSD 8918 GMR pick up with my LS motor to the AQ-1 logger.

 

I also spoke to the tech line and the person I talked to said you can use the logger to power the sensor but the sensor output is 5v and the MSD 8918 GMR pickup requires 12volts to work.

I ended up wiring it right to switched power to get it to work.

Thanks in advance.

 

KP

AEM_NS
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Last seen: 1 year 4 months ago
Joined: 06/06/2014 - 09:31
Is this a carb'd LS or

Is this a carb'd LS or something? Is that why you're using an "ignition box" and not an ECU? What exactly is the ignition box? FWIW, MSD tach outputs have been notoriously noisy.  Also, you need to make sure that the tach output from the ignition box is a square wave and not a coil primary signal. A square wave signal will be capped in amplitude to 12-14 volts but if it's a coil primary signal, there will be voltage spikes into the many hundreds of volts that will damage the AQ-1 circuitry.

I've fixed noisy MSD tach signals by implementing a simple low pass filter.  All a low pass filter is is a series resistor and a parallel capacitor that ties the signal wire to ground.  The combination between the resistor and cap creates a cut off where high frequency noise is shunted to ground and the low frequency signal we actually want gets passed through.  There are a handful of low pass filter calculators online but I've gone ahead and run some numbers.

Figuring 8 pulses per cycle at 10,000 rpm, that's 667Hz.  A 2.4Kohm resistor with a 0.1uF capacitor is what you want.  It'd get laid out like this:

Vin is the signal from the ignition box and Vout is the condititioned signal that goes to the AQ-1.  What the cap connects to is ground.  Note that the signal goes through the resistor before it gets to the cap.  This is important.

MetricMalibu
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Joined: 07/09/2017 - 21:41
Sorry for not clarifying that

Sorry for not clarifying that I am using a Carb on my LS motor, and have both a MSD 6012 and Daytona Sensors Smart Spark LS Ignition Box, currently Smart Spark box installed.

I tried the tach output which is a 12v square wave signal out of the box, which I have confirmed with both ignition box companies.

As per the AEM instructions for the box I purchased the MSD 8918, but can't seem to get it to work.

I think I may have had it attached to the wrong coil wire, I think I have it on the 12v switched power but it needs to be on the trigger wire to get a tach signal.

I will try it the other way tonight and report back my findings. If it doesn't work I will install a low pass filter as you have described above.

Thanks,

KP

 

AEM_NS
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No, you probably have it

No, you probably have it wired correctly. The GMR pickups sense the current change in the ignition circuitry. The coil trigger is a 5v square wave that sinks no current so it won't trigger the pickup. Also, not that it helps any now, but the GMR pickup is really only needed when the only tach signal you have comes from the coil primary. Using the GMR pickup doesn't really guarantee that the signal it outputs will be any less noisy than the original signal. The more appropriate course of action is to filter the signal like I described.

MetricMalibu
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Last seen: 2 years 3 months ago
Joined: 07/09/2017 - 21:41
Looking at your Calculations

Looking at your Calculations for the filter, how did you calcuate 667hz from 8pulses per cycle and 10,000rpm?

Talking with the Daytona Sensor Ignition box tech department, they thought it comes out a 4 pulses per revolution.

I just want to know what that would look like to see what size Capcitor and Resistor I would need.

 

Also could I hook up the ground off the filter to the "sensor ground" out of the AQ-1 or should that go to chassis ground?

Thanks again for all your help, much appreciated!

AEM_NS
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Last seen: 1 year 4 months ago
Joined: 06/06/2014 - 09:31
10,000 rpm is 6ms per rev. 4

10,000 rpm is 6ms per rev. 4 pulses per rev would be 1.5ms per pulse. Frequency is the inverse of period so 1000ms/1.5ms is 667. That's how you get 667Hz at 10,000 rpm. 

I gave you the RC values in my first post.  2.4kOhm and 0.1uF.  If you decrease the capacitance by a factor of 10 (.01uF vs .1uF), you have to increase the resistance by the same factor (24kOhm vs 2.4kOhm).  Guess it depends on what you can get for parts but all of these are fairly common values that you should be able to get from an electronics supply place.

The AQ-1's regular chassis ground will work fine.

MetricMalibu
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Looking at this further, I

Looking at this further, I understand the reasoning behind your statement:

Using 8 pulses per cycle at 10,000 rpm, = 667Hz.  With a 2.4Kohm resistor with a 0.1uF capacitor for the filter.

Would I then after installing the filter as you have noted program the AQ-1 logger to "4 pulses per engine revolution"?

 

 

AEM_NS
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Yep, you got it.

Yep, you got it.

MetricMalibu
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Your filter worked perfect!

Your filter worked perfect! Thanks for the help, much appreciated!

AEM_NS
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Joined: 06/06/2014 - 09:31
Sweet!  

Sweet!  yes