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Old 06-11-2011, 01:48 AM   #1
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Default MSII Wont Turn on Anymore, ST_SIG Fuse

Hey guys,

Ok, I have a buddy's ECU, and we've been trying to get it started. When we finally got everything all done ready to roll, I tried to start the car. I did the fuel pump mod on his megasquirt, and everything seemed to be working all right.

The car fired up, but ran extremely rough. Then it just died. Don't know why, or how. Then I asked him If he pulled the ST_SIG fuse, and lo and behold, it was still in. Pulled it out, and I found a thread about someone who left it in, saying you need to replace the Q5 transistor.

Move to today. I changed out the Q5 transistor on the heat sink, and the MS still doesnt power up. What else could have gone wrong? I also pulled out the fuel pump circuit and Im going to rebuild it, but the MS still doesnt start. Need help guys!

-Marc

Last edited by Marc D; 06-11-2011 at 04:24 AM.
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Old 06-11-2011, 02:10 AM   #2
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Been checking with a multimeter around for some points specific on the MS.

I have an old MegaStim, just using it to power it up on the bench.

I checked to make sure the MS is getting power on some points.

S12c where there are flyback diodes from the idle and boost mods are getting 12V.
Q1 and Q5, both transistors on the left side pin when looking down at the MS is getting 12V.
The banded side of D9, or the power regulator diode that is close to U5 only getting 0.03V
The S12C pin on the back side of the MS that is closest to the MSII chip is only showing 0.03V
The 5V pins on the board close to the ProtoArea is showing -0.04V.

Any ideas?
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Old 06-11-2011, 11:24 AM   #3
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leaving the st_sign fuse on a 90-93 MSPNP would not stop the operation of the unit...only the fuel pump.
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Old 06-11-2011, 01:35 PM   #4
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Then something else must have gone wrong.

This is not a MSPNP unit, it is a DIY MS unit with a MSII chip on it. Do you know what else could have gone wrong? is it best to send it in to DIY to see what else went wrong?

This went on a 90-93 car.
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Old 06-11-2011, 02:24 PM   #5
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So I looked at the points again and I tried jumping 12Vs to the power regulator on the board (U5) from one of the empty transistor holes getting 12Vs, and the MS powered right up. Could a lead have been destroyed somewhere? I dont exactly want to run the MS this way unless I know it will be ok.
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Old 06-12-2011, 05:52 PM   #6
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anyone? Could a blown transistor be drawing too much power somewhere Are there any other transistors or other components I can suspect on why my MS isn't turning on normally? (without me having to jump a wire to the power voltage regulator?)

or should i ask on the msextra/msefi forums?
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Old 06-18-2011, 05:51 AM   #7
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Just for sake of completion, I found out the problem and I'm posting it here.

I used the MS 3.0V schematic and traced it down to a cold solder joint on D10 or D12. I replaced both diodes on these and now the unit powers up completely.
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Old 09-06-2011, 02:52 PM   #8
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*edit*
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