I have a 91 7.3 IDI, and every now and then, the tach just decides to stop working. When that happens, I have to manually shift through the gears. It’s got an E4OD in it, and I can’t figure out why it’s doing this. Has anyone else dealt with this? How did you fix it?
The tach sensor is on top of the timing cover. Usually, the wires coming off it get worn out and can rub together, which makes the tach stop working and messes up the transmission shifting. Did you check those wires?
@Reese
Yeah, I actually checked that already. I forgot to mention it in the post. The wires looked fine, but I swapped it out with another sensor I had just to be sure. Still having the same issue. When the tach stops, the overdrive light also comes on, and I can’t turn it off even if I press the button. Does that help narrow it down?
I picked up a cheap code reader for $30 at AutoZone. You don’t really need one, but it does make things easier.
Reese said:
I picked up a cheap code reader for $30 at AutoZone. You don’t really need one, but it does make things easier.
Good idea! I’ll probably grab one since I’ll be working on other older vehicles anyway. Thanks for the tip.
@Ash
Yeah, I got mine because I was too lazy to count the flashes manually 
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Have you tried connecting to the EEC connector to see if any codes pop up?
Reese said:
Have you tried connecting to the EEC connector to see if any codes pop up?
I haven’t done that yet. I’ve got scanners for anything newer than OBD2, but not for this. Looks like I need to pick one up.
@Ash
You can actually pull codes using a paperclip. Check this guide: https://www.ford-trucks.com/forums/1715941-how-to-retrieve-trouble-codes-from-fords-eec-iv-with-a-paper-clip.html