I have been having a problem with my '96 F-250 with the 5.8. It keeps stalling going down the road, and sometimes not starting after it has been running. It's about as intermittent as can be, and either happens when I don't have my diagnostic tools with me, or as soon as I hook them up, it starts. Yesterday it didn't start for a while and I was able to run through some diagnostics.


I did replace the cap, rotor and coil yesterday, and still no spark at the plugs, or from the coil. I started going through the rest of the tests in the Haynes manual and there's voltage getting to it when the key is on. The next test called for putting test light on the tach terminal and looking for a bright, flashing light, but here's where I get stumped. I think maybe I was supposed to probe that with it off the coil, but I back probed it with it connected to the coil, and as soon as I touch the test light to the terminal the truck started. I grounded the connection, which I don't think is a good thing. The battery light on the dash came on and was one for a few trips, but the alternator was putting out voltage according to the gauge on the dash.

Correction: The engine started when I touched the positive side of the coil connector, not the TACH side. I think, but can't confirm now because the truck is starting, that the light wasn't flashing when I touched the tach terminal.


The light went out this morning. Later the truck wouldn't start again, but as soon as I back probed that lead with the test light she fired right up. The idiot light for the charging system didn't come on this time.


I'm hoping this is enough information that someone is going to say it's such and such for sure.



Steve

 

You have a bad plug on the coil.
This happens a lot. take the plug off and check the wires and the contacts to the coil on the plug.

 

Bump for my correction. Why does grounding out the positive lead make the truck start?

 

You are moving the wire or plug and it then makes contact.

 

I really don't think it's a bad contact. I'm pretty sure I have continuity through the coil when I back probe the connector. And when I do touch the back of the positive lead on the connector with the test light I don't have to touch it very hard at all.

Steve

 

The positive lead on the connector comes from the Ignition Switch and then the battery.
Touching the prob to this wire would NOT change anything electrical. You would have to be moving something at the connector of the coil.

 

OK, next time It happens I'll try without the test light grounded and see if moving it makes a difference. It's so hard to diagnose because it only happens occasionally.

Steve