Intermittent P0503 code 1999 Ford F-550

Question

Need Help , I have a 1999 Ford F-550 6.8 ltr Southwind 32v Motor Home 60k miles.

Problem, PCM code P0503 will appear and the transmission will start shifting erratic and hard. This code only happens after the motor is hot and let to sit idling for 15-20 minutes. I can clear the code by disconnect the battery but the code will re load and the problem comes back every time I let the vehical ideal for 15-20 minutes when hot.
While the vehical is moving and even short stops like stop lights the problem does not appear, only after longer periods of idling . We have tested the alternator for noise and even disconnect it during a drive an idle time to avoid potential noise from a bad diod but the problem still will happen with it out of the picture. Also have changed the VSS and pigtails by the transmission Shope .
I did have some electrical problems with the coach battery before all this
happened. Aux batt Leades got reversed by accident and I believe there is a charging path to the main batt , could I have damage to PCM maybe.
Wondering if heat is causing the PCM to read this code should I try replacing thePCM

Answer 1

Hi there

This code goes from the speed sensor, to the speedometer, then to the computer. So if this code is happening, but the speedo is working fine, it could be the speedo. If its not sending the signal from the speedo to the computer it's going to throw this code. You need a scan tool that can see the live data from the speed sensor, drive the truck and watch the speed. If it's not showing up on the scan tool screen, then it's the speedometer.

P0503 FORD - Vehicle Speed Sensor 'A' Intermittent

Read more: https://www.autocodes.com/p0503_ford.html