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post #1 of 12 Old 06-11-2009, 07:32 AM Thread Starter
 
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Need technical help...

I'm a mechanic working on a 2002 V6 Dakota that "suddenly died on the road".

I've narrowed the problem to the 12V coil only getting 6.5-7V while cranking the engine.

Tests include:
voltage from +pin to -pin at the coil connector = 6.5-7V while cranking.
voltage from +coil to -battery = 12V (battery voltage) while cranking.
resistance from coil pin to ECU is low resistance and not shorted out.

The coil itself tests fine and is putting out a weak yellow spark consistant with low voltage input. The rest of the ignition system wires, cap and rotor and plugs look good.

My understanding is that the ASD relay kills power to both fuel and spark. I do have fuel getting injected to the motor so I ruled it out.

That basically leaves the PCM grounding the coil.

Am I looking at replacing the PCM or have i missed something obvious or unique to this vehicle?

Thanks for any help you can provide!
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post #2 of 12 Old 06-11-2009, 07:36 AM
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Did you test the ASD circuit?


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post #3 of 12 Old 06-11-2009, 07:46 AM Thread Starter
 
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Now my posts are doing weird stuff...

I tested from coil +pin to the battery -pin i got 12V (actual battery voltage). Unless I'm misunderstanding the wire diagrams, this would be pulling 12V across the ASD relay to a good ground while cranking which shows the ASD is OK. I also have fuel.

So the answer to your question is 'no, I didn't test the relay itself'. I tested the circuit... )

Where is that relay located on a 2002 truck?

Last edited by Supra; 06-11-2009 at 07:51 AM.
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post #4 of 12 Old 06-11-2009, 07:49 AM
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In the realy/fuse box under the hood, it's is beside the raditor fan relay.

Wiring


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post #5 of 12 Old 06-11-2009, 07:50 AM
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My wiring diagram shows the dark green/orange wire going to the coil.


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post #6 of 12 Old 06-11-2009, 07:53 AM Thread Starter
 
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I'll need to track the owner down for a key. Might be a few minutes, might be a few hours if they are both working.

Thank you for your help and I'll post back with either my findings or to report it fixed...
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post #7 of 12 Old 06-11-2009, 09:30 AM Thread Starter
 
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OK here's the update:

Still no spark, definitely has fuel...

I found the relay and the relay itself checks out OK. I swapped it for another of the same part number and no change.

In the fuse panel, I found the DG/OR (more of an odd pink colored trace than orange but I digress) that goes to the coil. It has zero resistance pin to pin and no shorts. I get 7V from DG/OR to the other side of the coil connector (black/gray?) while cranking with the coil disconnected. If I move my meter's lead from the black/gray to the battery negative, I get "battery actual voltage" at DG/OR while cranking instead of 7V. My logic says the grounding side of this circuit is FUBAR which made me wonder if the PCM ground was bad...

I tested resistance across battery negative to both PCM grounds (pulled connector @ PCM). Surprise, surprise, I had resistance... I traced this to a corroded connection where a large body ground cable attaches to a post and a second harness connector, driver's side fender. I cleaned this connection and then had zero resistance from bat ground to PCM grounds.

Unfortunately, this didn't change anything when I repeated the tests above but I suppose two bad PCM grounds could have killed the PCM (if that proves to be the case).

Any other ideas or suggestions?

Last edited by Supra; 06-11-2009 at 09:43 AM.
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post #8 of 12 Old 06-12-2009, 09:37 PM Thread Starter
 
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update: ordered PCM, will arrive next Tuesday.
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post #9 of 12 Old 06-12-2009, 11:23 PM
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so how did you test that the PCM is dead? would hate to see you spend money for a PCM and that not be the case. I'm sure you know what your doing though


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post #10 of 12 Old 06-15-2009, 02:47 PM Thread Starter
 
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Wiring diagram says the circuit goes from Battery+ through the ASD relay to the coil, to the PCM to one of the two PCM grounds.

Following the circuit with 12V at the battery:

I get 7V at the coil from +pin to -pin with IGN on.
I get 12V at the coil +pin to -battery with IGN on (no voltage when off).
At the above suggestion, I tested the relay and also swapped relays with no change...
[This tells me the ASD is functioning and the relay contact is good]
[[Overall, this tells me the + side of the circuit is good.]]

ORIGINALLY:
I got decent resistance from PCM -pins to -battery. (Traced to a corroded ground point near the DS fender. FIXED)
[No change to problem]

CURRENTLY:
I get no resistance from PCM -pins to -battery.
[PCM is grounded OK]
The -coil wire to PCM pin tested with no resistance and no shorts to ground.
[electrically, the - side circuit is OK, excluding the PCM]

All of that basically tells me that the PCM isn't grounding out the coil properly.

Any arguements or suggestions are very welcomed.
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