Sensor noise. Need Help. - Miata Turbo Forum - Boost cars, acquire cats.

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Old 04-25-2009, 11:53 AM   #1
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Default Sensor noise. Need Help.

I have an AEM installed in my 94 Miata with an AEM 3.5 bar MAP sensor. For some reason, the MAP Volts signal gets erratic above 5000 rpm when under boost (boost is set to between 8 and 9 psi). With MAP Filter set to 4 as it was in the initial startup cal, Engine Load drops to around 4 psi. The boost gauge and the dyno's map sensor confirm that boost does not drop when this happens. Obviously it is bad for the MAP sensor to be not reporting pressure correctly at positive pressure and high RPM's. I have swapped MAP sensors, vacuum source, 5 volt, and sensor signal wires to the sensor trying to figure this out. I have also verified, using the boost gauge and the dyno's map sensor that the vacuum line Im using has the right vacuum. Nothing has helped. The car drives great under 5000 rpm.

The tuner I took the car to (Tony Szirka at UMS Tuning in Meza, Az) got the AEM tech rep on the phone and they decided it must be the AEM box that was doing something strange, so I sent it back for troubleshooting and repair. They could not find anything wrong with it, but replaced the guts based on Tony's recommendation. I have it back in the car and nothing has changed. I'm desperate at this point. Does anybody have any experience with this kind of problem and the AEM EMS?

What I've done recently:

1) ran a 4 gauge grounding wire from the battery to the engine block, alternator, and other 16 gauge wires to all the stock grounding locations from the 4 gauge wire at the block.

2) Replaced my stock coils with toyota COP's. Stock units split open and died. I read on here how people had seen reduced noise with them.

3) Installed a filter to the stock 12v permanent power wire to the ECU.

4) Verified that the sensor ground is not shorted to chassis. It is not.


See this post for additional information:

Map signal going crazy
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Old 04-25-2009, 01:30 PM   #2
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John, I recall reading your original thread in the AEM forum.

I would suggest trying another MAP sensor if you have someone close that can lend you one.

Wait for J_man and Jason to see if they have other suggestions.

In the meantime; why don't you post your latest calibration so they can take a look at it?
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Old 04-26-2009, 01:55 PM   #3
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Originally Posted by Rafa View Post
John, I recall reading your original thread in the AEM forum.

I would suggest trying another MAP sensor if you have someone close that can lend you one.

Wait for J_man and Jason to see if they have other suggestions.

In the meantime; why don't you post your latest calibration so they can take a look at it?
We swapped on 2 different MAP sensors at the Dyno. And its all the sensors on the sensor ground/5v ref lines. I'm convinced this is a hardware problem, but I'll post the cal files when I can get to my tuning computer.
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Old 04-26-2009, 02:56 PM   #4
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Originally Posted by stinkonamonkey View Post
We swapped on 2 different MAP sensors at the Dyno. And its all the sensors on the sensor ground/5v ref lines. I'm convinced this is a hardware problem, but I'll post the cal files when I can get to my tuning computer.
You don't need to post the cal. I think it can be found in your AEM forum thread. I don't have my issues solved but at least I don't have problems with my MAP sensor. I did a quick "compare" between your cali and mine and at least our MAP sensors settings are different. Granted, I have a 2bar one from a Supra.

Hope you can fix your problems. I know how it feels to be in your shoes.
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Old 04-27-2009, 12:45 PM   #5
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Hey John. Did you find your problem?

I was comparing your calibration to mine and we have different MAP sensor settings and also different voltage. I know yours is a 3.5 bar AEM one but the voltage doesn't sound right.

Shoot me a pm.
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Old 04-27-2009, 02:13 PM   #6
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Did you connect an oscilloscope to the 5V and signal wires of the map sensor? That's the first thing you should've done IMHO. Log those two, post the scope graph and if it is ok, then start looking for AEM hardware or software problems.

P.S. If the excuse still is that you don't have an oscilloscope, a decent USB two-channel one costs less that $200 brand new and is a great tool to have
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Old 04-27-2009, 04:15 PM   #7
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Originally Posted by j_man View Post
Did you connect an oscilloscope to the 5V and signal wires of the map sensor? That's the first thing you should've done IMHO. Log those two, post the scope graph and if it is ok, then start looking for AEM hardware or software problems.

P.S. If the excuse still is that you don't have an oscilloscope, a decent USB two-channel one costs less that $200 brand new and is a great tool to have
I still do not have an oscilloscope. Can you give a link for one you recommend?
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Old 04-28-2009, 02:35 AM   #8
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I still do not have an oscilloscope. Can you give a link for one you recommend?
I use this one:

PC USB Oscilloscope / Logic Analyzer for $169.50 (UART, SPI, I2C, 1-Wire)

It's been working great. The only issue I had with it was one of the BNC connectors wasn't making good contact when took it out of the box brand new, but that was just a 1 min fix.

You can probably find similar ones in the $100-$200 price range if you search ebay, google/froogle, etc.
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Old 04-28-2009, 11:32 AM   #9
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Originally Posted by j_man View Post
I use this one:

PC USB Oscilloscope / Logic Analyzer for $169.50 (UART, SPI, I2C, 1-Wire)

It's been working great. The only issue I had with it was one of the BNC connectors wasn't making good contact when took it out of the box brand new, but that was just a 1 min fix.

You can probably find similar ones in the $100-$200 price range if you search ebay, google/froogle, etc.
Ok. I'll pick that one up. Whats your phone number? So you can tell my wife why I need to spend another $200 on my car.
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Old 04-30-2009, 01:24 PM   #10
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Originally Posted by stinkonamonkey View Post
Ok. I'll pick that one up. Whats your phone number? So you can tell my wife why I need to spend another $200 on my car.
Comeon, that's an universal tool - you can use it for other purposes too - i.e. to fix her computerized hairdryer or smth ...

Btw I just ran a quick google search and found even cheaper one:

Parallax USB PC Oscilloscope


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