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My 95 Mistsubishi Montero runs rough periodically. It has 3…

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My 95 Mistsubishi Montero runs...
My 95 Mistsubishi Montero runs rough periodically. It has 3 coils and I see joice coming to all coils. When the engin starts and works rough, I found that the two spark plugs connected to coil 3 won't spark. I exchanged the coils 2 and 3 to see if coil is bad but it didn't make any difference (coil is OK) and the same spark plus are not sparking.
Submitted: 8 years ago.Category: Car
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Answered in 5 hours by:
2/4/2010
Auto Mechanic: Randall C, ASE Certified Technician replied 8 years ago
Randall C
Randall C, ASE Certified Technician
Category: Car
Satisfied Customers: 13,367
Experience: 32 years Foreign / Domestic Cars All aspects of Diag and Repair
Verified
Hello, each coil is controlled for firing by 2 other items. The ECM ( main computer) sends the signal to the Power transisitor ( ign control module) then to each individual coil. The fact that one is not firing leaves the ignition module or ECM as the problem. below is location of the power transistor and tests for it. If it tests ok, suspect ECM. The power transisitor ( ign module) is a more common failure of the 2.

NOTE: An analog-type circuit tester should be used.

Ignition Power Transistor

Ignition Power Transistor Continuity Table


No. 1 - No. 4 Coil Side

  1. Connect the negative (-) terminal of the 1.5 V power supply to terminal (4) of the ignition power transistor; then check whether there is continuity between terminal (13) and terminal (4) when terminal (3) and the positive (+) terminal are connected and disconnected. NOTE: Connect the (-) probe of the circuit tester to terminal (13).
  2. Replace the ignition power transistor if there is a malfunction.
Ignition Power Transistor

Ignition Power Transistor Continuity Table


No. 2 - No. 5 Coil Side

  1. Connect the negative (-) terminal of the 1.5 V power supply to terminal (4) of the ignition power transistor; then check whether there is continuity between terminal (12) and terminal (4) when terminal (2) and the positive (+) terminal are connected and disconnected. NOTE: Connect the (-) probe of the circuit tester to terminal (12).
  2. Replace the ignition power transistor if there is a malfunction.
Ignition Power Transistor

Ignition Power Transistor Continuity Table


No. 3 - No. 6 Coil Side

  1. Connect the negative (-) terminal of the 1.5 V power supply to terminal (4) of the ignition power transistor; then check whether there is continuity between terminal (11) and terminal (4) when terminal (1) and the positive (+) terminal are connected and disconnected. NOTE: Connect the (-) probe of the circuit tester to terminal (11).
  2. Replace the ignition power transistor if there is a malfunction.

graphic
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Customer reply replied 7 years ago
Sorry was in Tibet past few month and came back recently. I have bought and installed IGN module but the problem still exists. I know next guess could be changing computer and have seen many people changed the computer and didn't help. Is there any test to narrow down the issue?
Auto Mechanic: Randall C, ASE Certified Technician replied 7 years ago
No specific computer test for this. The computer controls 3 coil circuits within module then to each coil. Since you did coils, module and still same, yes, you may very well have bad computer. this is true only if all wires to and from computer/module etc are ok. You have replaced everything asiide from computer so I am leaning towrds that as the problem. I wish there was a 100% way to test. You may want to wiggle all wring to and from module /coils etc to see if bad connection.
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Customer reply replied 7 years ago
I have asked similar question and waiting for response. however this answer may help me in diagnosing the problem but I can't see the pictures.
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Randall C, ASE Certified Technician
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