P0502 Chevrolet -

Question

i have an ac signal to ecm on wires coming from speed sensor to ecm. 4 volts ac when in gear and wheels spinning. voltage increases with speed up to 7 volts. vehicle is not shifting on its own. suspect bad ecm. ecm was changed out by another shop with a used one. not sure if correct ecm is in truck. is there a way manually supply a signal on the output side of the ecm to engage speedometer and tcm to determine if the problem lies in the computor or if something else in the system is missing (ground, short.. etc) vehicle is coding p0502. alot of people have worked on this truck before it came to me. list of other things that were replaced by 3rd parties are as follows (fuel pump, engine mounts, valve cover gaskets, throttle position sensor, ecm, fuel injector spider, intake gaskets, idle control valve). also checked ignition voltage to pcm from ignition switch. power signal is ok. not the ignition switch as i have seen that one before.

Answer 1

P0502 Vehicle speed sensor circuit low input

When you get this code the ECM/TCM default in just 2nd gear.The best thing for you to do is measure for its reference voltage its 5 volts.the purple white wire key on engine off you should measure 5 volts,