does anybody know which wire is the memmory wire on a 1991 ford f150 I have the 2 piece harness and does that model have a ground wire or is it grounded by the mounting bracket on the back.

 

Are you talking about the PCM?
If so pin #1 is memory (yellow wire) and Pins 40 & 60 (black wire with white stripe) are the signal ground.
Case ground is pin #20 (black wire).

 

Should be Pin #1, Yellow wire, routed to the HOT side of the starter realy. The ground wire is on Pin #60, comes off the computer harness and should be bolted to the upper fender near the hood hinge spring.

If you're looking to clear the stored codes, there are 2 ways from what I've read.

1-Disconnect the battery ground for at least 5 minutes. You will have to drive the vehicle for at least 10 minutes(or miles) for it to relearn the vehicle.
2-Perform the diagnostics test, when you turn the key on and the test begins, remove the jumper wire, then turn the key off.

I've never tried #2 but #1 always works for me.