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99 Ranger turns over - no spark


rustybolt

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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...
 


Bob Ayers

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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.
 

rustybolt

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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.
 
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Bob Ayers

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Next thing would be the PCM power relay.
 

rustybolt

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I'll check the PCM power relay and see it's working properly. Thanks...
 

rustybolt

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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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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.
 

Bob Ayers

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The PCM "switched" 12V comes in on pin 71 (red wire)


...addition......pin 97
 
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Bob Ayers

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PCM ground pins are pins 24, 51, 76, 77, 103,
 

rustybolt

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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...
 

Bob Ayers

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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...
There are some open collector drivers driving the coil pack. I doubt that they are the problem (all three). The chances of the problem being the PCM is slim. Years ago, however, a friend of mine had a Porsche 924, that would intermittently shut off. I traced the problem to the DME (Porsche for PCM),
and dug into it. I found that the 5V for the digital circuits was going to 0V.
The 5V regulator was made up of discrete components, which I replaced with
a better solution....a linear 5V regulator, which you can see in this photo.
This solved the problem, and saved my friend some big $$$$.



 

Ranger Rod

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So no spark you say... Back when I was an avid smoker the ole zippo failed to spark at times, I found the best way to restore combustion was to re: & re: the flint with the recomended brand name replacement. Sorry I can't be of more help but if your well insured you are welcome to borrow my zippo. TEE HEE
 

rustybolt

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I haven't done anything more on the Ranger yet. The work place has been real crazy this week, long hours to accomplish seemingly very little. Sign of the times I guess. Thanks for the PCM picture. Pulling the PCM apart will be the last step in this process but I agree with your comment about the likely hood of the PCM being the source of the problem. Engine controllers are usually built to survive that abusive environment. But things happen. Since the whole coil pack is inoperative I doubt the FET's or TO3's (whatever their using to switch the coils) are bad so it has to be something that is common like a bad regulator or cold solder joint that is disabling the circuit. I'm still amazed that PCM doesn't generate a fault code but I guess that it was designed to do only only so much. This Saturday, bright and early, I'll be in the garage working on the truck and I'll let you know the results Saturday night. Thanks again. Chris.
 
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fatboyf150

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this may be a long shot, but doesnt all the trucks this body style have the pats chip in the key that tell the pcm to let the truck fire. is it possible that the key is dead. have you made a new key that hasnt been flashed for the truck. just thought i'd throw that out there.
 

betomonroy

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:icon_thumby:
You can also check the CMP before You disassemble the PCM.
:icon_bounceblue:
 

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