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Old 08-27-2008, 02:15 PM   #1
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Default 0.1uF cap mod on MSPNP

Since upgrading to high res, I am experiencing the random "hiccup" on my MSPNP9495 (tach drops out randomly for a split second under load).

I am about to do the JS8 to Ground, 0.1uF capacitor mod which should fix this issue. I've seen diagrams/photos for the 3.0 PCB but not the 3.57M which MSPNP uses.

Can you guys take a look at the photo below and tell me if this is correct? Thanks.

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Old 08-27-2008, 02:26 PM   #2
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looks correct to me.
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Old 08-28-2008, 10:49 AM   #3
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Thanks. I installed the cap yesterday. Took the car for a quick spin. Haven't tested thoroughly but so far so good.
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Old 08-28-2008, 01:02 PM   #4
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What is JS8 anyway?
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Old 08-28-2008, 01:19 PM   #5
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input for tach from the CAS, IIRC.
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Old 08-28-2008, 01:40 PM   #6
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Yup. JS8 is just one of many "spare" I/O pins of the CPU brought out to a via on the board. When one is using the second trigger input, the MS is hard-coded to look at JS8 for that input signal. For those following DIY's How To... writeup, JS8 winds up getting wired out to the IAC1A pin of the DB37, with a pullup resistor.
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Old 08-28-2008, 01:56 PM   #7
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So you're putting this CAP between the tach input and ground?

Reason I'm asking is because some AEM ECU's do the same thing.
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Old 08-28-2008, 02:42 PM   #8
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Schematically, this is what it works out to:



CMP is the open-collector input from the sensor, JS8 is the CPU pin. The original design (and the one still currently posted at the DIY howto) omits the capacitor.
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Old 08-29-2008, 12:40 PM   #9
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We added it to the text of the how to article a while ago, but need to update the pictures.
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Old 09-27-2008, 12:49 PM   #10
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Go a picture of mine here if it's any use to you?

Also pics for the TPS mod...


http://www.mx5nutz.com/forum/index.p...12887&hl=*pnp*
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Old 11-17-2008, 03:54 PM   #11
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Just to make sure I'm doing this right.

On a MS2, I would need to do this to JS10, not JS8, right?
Or are the CMP/CKP pullups not needed for the '99 sensors?

Last edited by WestfieldMX5; 11-17-2008 at 04:41 PM.
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Old 11-18-2008, 10:20 AM   #12
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Don't confuse this with a pullup. A pullup is a resistor that biases the pin in question towards +5, expecting an external pull to ground. So yes, the NB sensors need pullups too.

This mod is just a filter to try and combat noise on the input in question. I've not heard of people having this issue with MS2, but if you were to need it, then yes, it would be on JS10.
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Old 11-18-2008, 02:32 PM   #13
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Thx for explaining that Joe. I'll be building similar input circuits as Abe's, so I assume these completely replace the CMP circuit posted above (and the corresponding CKP circuit), right?
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Old 11-18-2008, 07:56 PM   #14
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Yup. They go from the CAS directly to the CPU, bypassing all the on-board circuits. Make sure that you have no jumpers installed on the MS PCA in either TSel or TachSelect.
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