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Old 03-07-2009, 09:13 PM   #1
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Default 99 Ranger turns over - no spark

Hi all - I have a 1999 Ranger XLT with a 3.0L V6 Flex Fuel engine. Two weeks ago, for the very first time, the engine would not run. Turns over fine, normal startup sequence, but no spark at the spark plug. I have checked the coil pack for resistance on the primaries of each coil and it still meets specifications (about 2.5 - 5 ohms). I've also confirmed that the PCM is not controlling the coil pack when engine is in this fault mode. Basically, this problem is intermittent. Some days it will start right up and run perfectly. Other days it will turn over but not run. In addition to checking the coil pack, I've also checked and confirmed proper operation of the crankshaft position sensor. Outputs a sine wave like frequency proportional to the engine RPM, everytime. Seems to me that this could be a connector problem in the engine compartment or a flaky ground but so far I haven't found any corrosion problems on the engine wiring harness connectors or the PCM bulkhead connector. I've also checked and tested all the grounds in and around the engine. All looks good. Maybe the PCM is going screwing up? Hard to say at this point. Any help would be greatly appreciated. Thanks...
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Old 03-07-2009, 09:20 PM   #2
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Check and see if it has 12V at the coilpack when it won't start. It is the red/lt green wire in the 4-wire primary connector. If it intermittent, it could be things like the PCM relay.
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Old 03-07-2009, 09:37 PM   #3
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Hi Bob, Thanks for the reply. When the engine will not run, I did check to see if there was +12 at the coil pack red wire and it is there, every time. So I'm getting power to the coil pack but the PCM is not switching the other side of the primaries to ground to fire the coil. I built a simple LED and resistor fixture to plug into the 4 pin coil pack connector so that I could see if the PCM was switching the coil pack. When the engine is running fine I'll see the LED's blinking away, as they should because the PCM is grounding out each coil in sequence. When the engine would not run, the LED's are all dark when turning over the engine. Also, I forgot to mention that the PCM is not generating any fault code or check engine light. Nothing...

Thanks.

Last edited by rustybolt; 03-12-2009 at 06:32 PM.
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Old 03-08-2009, 07:27 AM   #4
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Next thing would be the PCM power relay.
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Old 03-08-2009, 12:32 PM   #5
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I'll check the PCM power relay and see it's working properly. Thanks...
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Old 03-08-2009, 07:42 PM   #6
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Well, the PCM power relay is confirmed working and the PCM is powering up. If only I had a schematic for the PCM, how wonderful life would be. In any event, the truck seems to be in permanent failure mode which is a good thing in a way. I'm checking junk yards to see if I can find a deal on another PCM. Bob, any other ideas? I'm open.... TNX Chris.
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Old 03-08-2009, 08:05 PM   #7
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Chris, when you scoped out the signal from the CKP sensor, were you at the PCM socket? The signal from the CKP sensor is differential, and is connected to pins 21 (DARK BLUE WIRE) & 22 (GREY WIRE) of the PCM connector.

Also check to see if "unswitched" 12V is there.....pin 55, yellow wire.
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Old 03-08-2009, 08:18 PM   #8
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The PCM "switched" 12V comes in on pin 71 (red wire)


...addition......pin 97
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Last edited by Bob Ayers; 03-08-2009 at 08:23 PM.
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Old 03-08-2009, 08:22 PM   #9
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PCM ground pins are pins 24, 51, 76, 77, 103,
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Old 03-09-2009, 09:45 PM   #10
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Hi Bob. Thanks for the PCM header info. Greatly appreciated. When I checked the CKP with a scope, I was looking right at the output of the sensor. But I agree, it's better to check right where the signal goes into the PCM. I'll look there and confirm all the ground and power connections too. If all that stuff checks out, I'm half tempted to see if the PCM can be opened up and poke around there to trace back the ignition coil lines. (if possible). I'll let you know what I find. Thanks again. Chris...
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