No spark
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No spark
I have a 1988 F-250 7.5L with a history of intermittent stall. I have replaced cap rotor, plugs and wires, pip sensor inside distributor, ICM on out side of distributor. Been fine for a few months then started again. Now it stalled and will not start. Checked with test light at tach- coil and found no light pulse. Checked coil supply 12V and coil. When plug pulled from ICM I find only 4 volts at pin 2 where it should be 12V !!!! Traced wire back to ECM and checked for shorts and continuity, checked ok. Only getting 4 volts out of ECM ??? Should I replace computer or look for other problems ??? Any help would be very appreciated. Thank you, The Ghosttowner
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Joined: Jun 2006
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From: Ottawa, Ontario
What brand ICM did you use? Do you still have the old one?
I'd suspect the PIP module inside the distributor was your original problem and these things don't often fail so the TFI(ICM) is the current suspect, and note you will still get spark even with the PCM(computer) removed as long as the TFI and PIP are functioning properly. The reason I ask if you still have the original TFI is because the aftermarket modules have a terrible track record, sometimes they last a year, sometimes only a month, and some are DOA right out of the box. So if you could swap on another TFI just for a spark test it would be a quick way to narrow this down.
I'd suspect the PIP module inside the distributor was your original problem and these things don't often fail so the TFI(ICM) is the current suspect, and note you will still get spark even with the PCM(computer) removed as long as the TFI and PIP are functioning properly. The reason I ask if you still have the original TFI is because the aftermarket modules have a terrible track record, sometimes they last a year, sometimes only a month, and some are DOA right out of the box. So if you could swap on another TFI just for a spark test it would be a quick way to narrow this down.
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Joined: Mar 2014
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From: southern maryland
does your MIL go out while cranking if so this usually indicates you have an rpm signal. one note if you have a bad coil many times it will kill the pulsing of your testlight and make it appear you have no signal when you have a malfunctioning coil.
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I noticed the coil was warm, almost hot. I removed it to check all windings etc. All checked to spec. I had coil removed when I was checking for voltage at TFI/ ICM, that May have been why it was only 4 volts. I will re install coil and try to find my old TFI to test for spark. Thanks for the help.
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Joined: Apr 2002
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From: Waterloo, Iowa
Is that an old school type coil? They can get shorted windings in the secondary that are tough to measure, and the intensity will be OK but the duration is degraded and/or will become intermittent when hot, etc. Worth swapping a known good coil in.
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Coil
It is the OE coil. I was concerned when it was hot after about 10 min with the key on while doing diagnostic and voltage checks. That is why I removed it and checked resistance on primary and secondary circuits and to ground. They all where in spec. I was already looking at after market coil for a upgrade. This stall problem finally last long enough for me to maybe find it. Thanks everyone for your help. 
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From: Waterloo, Iowa
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From: southern maryland
resistance checks are next to useless when diagnosing things like this. you can override coil negative if you need to isolate the coil.
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Thanks everyone for your help. I found the OE Motorcraft TFI, and it started right up, although running rough. Ran less than one min. noticed coil too hot to touch. I am headed down to Sumitt Racing for a performance TFI and matching coil. Thanks again for all your help. The Ghosttowner
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Stalling again!!!!
Truck ran fine for one week then started intermittent stall again !!! Then died. ICM out again. Replaced with Motorcraft ICM at parts house, truck stalled on way home. I have replaced everything but the wire harness and computer. THIS TRUCK IS DRIVING ME CRAZY!!!! Some thing is knocking out the Dist mount computer. I am debating on running a hot lead from the battery to a fused switch on dash back to the ignition. Any Ideas would greatly be appreciated. Thanks the Ghosttowner
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I believe I am losing spark. Originally started stalling while driving, then would restart on its own before I could pull off road. Now when it stalls I have to get to a stop, turn key to off and retry to start several times before starting. I carry a spare ICM in glove box for when it won't restart. Taken ICM to parts store and they tell me it is bad. Last one a Mortorcraft test good several times. My original motorcraft ICM test good half of the time. The intermittent loss of voltage or short to the ignition system must be knocking out the ICM. The truck ran fine all summer, it was a problem that has gotten worse since last winter. I have done resistance checks on all ignition wires and to ground while moving etc. I think my next step is to hot wire the ignition with a dash mounted switch.