tach issues

[Andylad13]

my tach stopped working today. it was running just for the a/c because i was working on a steel building and needed to cool down. everything was good, but the tach suddenly died. when i drove home, it didnt go into OD, and never down shifted, i had to start in 1st, manually shift to 2nd and also to drive. if its in drive, it acts like a 5 speed trying to take off in 4th gear, super, super slow and really high rpms. anyway....

checked the fuses and all is good, so where is the sensor for the tach, and what is it reading??

 

[GenLightening]

Look on the drivers side of the IP housing and you'll find a large nut with 2 wires coming out of it. Check the wires for cracked insulation or a loose connector in the harness. Worse case is you need to get a new one from Ford for about $50.

The E4OD computer uses the tach signal for shifting and such and without that signal it goes into limp mode.

 

[Andylad13]

thank you

 

[Andylad13]

well, i got the part in, and though the magnet is a lot stronger, its still not working. whats going on with this? is there supposed to be power coming to the sensor at all??? something i can test for?? i cant drive the truck in limp mode to work, so this needs to go away in 2 days max.

 

[GenLightening]

There has been a few threads with tach issues. Here's one:

Both tach sensor wires go to the transmission computer, not the tach. The green/red wire goes in the left column 4 from the bottom, the tan/yellow 3 from the bottom. The computer sends out it's own signal to the tach, a white pink wire center row, 5 from the top.

The tach sensor should have a resistance reading of 2000-3000 ohm.

 

[Andylad13]

so your thinking that the trans itself is messed up?

 

[GenLightening]

It's just that they're linked and if one is bad the other has issues. The trans computer can't work without the tach input (as well as TPS and ABS) and the tach can't work if the trans computer isn't happy. Add to that, any connection between them can be loose and cause a problem. Try pulling the connections apart at the computer, trans, TPS and tach and check for any corrosion or loose pins.

 

[Andylad13]

thats what i did today and i have come up empty. i think the scanner had a dead battery so im going to get one and try to scan it. i pulled the ecm plug off and checked the pins and everythings good. i tested the wire coming from the trans to the tach and there is a signal, i dont know what to make of that because its not working but it's doing something.

this is a complete mystery to me. the truck was in park when this happened.