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89 F150 4.9L no spark

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3 weeks 6 days ago #79875 by Grawg72
Hello, please forgive me if this has been posted. I'm working on my 89 F150, with the 4.9L inline 6, 4x4, manual transmission. Driving with my son, the truck just died. Did not starve for fuel, the tank is nearly full. I've done several tests to check for spark, and everything looks ok, except when checking ohms from the TACH wire on the coil to ground and the ohms should be over 10k. I read 35 ohms. I'm hoping one of you experts can help me rewire this specific TACH wire and bypass the short. When I checked the TACH wire to the ICM pin 2 read 4 ohms, right in the range it should be in. The distributor has been changed, with new ICM, cap, rotor, plugs etc. Even have a new coil to go in, but all tests show coil is fine, so i was planning on returning it. Thank you for your help.

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3 weeks 6 days ago - 3 weeks 6 days ago #79925 by ferris48
Replied by ferris48 on topic 89 F150 4.9L no spark
 

It appears you have a short to ground on the coil control wire which I have highlighted in green in the diagram above

If you have already located the short and want to bypass it, the wiring diagrams are available on charm.li/

If you're having trouble finding the short, might I suggest a test light connected from battery positive and touching this wire may serve as a visual indicator of a ground path on the wire and it would be a simple task of moving the harness around until the light flickers or turns off. This may give you a general direction where the short may be located.

When I checked the TACH wire to the ICM pin 2 read 4 ohms


ohm checking a wire simply tell you the wire is not open completely, and if the wiring diagram is correct, checking this wire at the icm will also show a 35 ish ohm short to ground.
Last edit: 3 weeks 6 days ago by ferris48.

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2 weeks 6 days ago #80675 by Grawg72
Replied by Grawg72 on topic 89 F150 4.9L no spark
before I went chasing down the short, I swapped out the coil just to see. Everything I had checked up to that point was a control issue, not the coil. But it turns out the coil was the culprit. Thanks for the info, I'll keep it in my back pocket for next time.
The following user(s) said Thank You: ferris48

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2 weeks 5 days ago #80757 by ferris48
Replied by ferris48 on topic 89 F150 4.9L no spark
Don't forget to disconnect the battery if doing ohm checks. You DVOM, voltmeter, multimeter is sending out voltage. You don't want any other voltage in the circuit BESIDES your meter.

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