You will need to supply more information as to what kind of vehicle, make, year and engine size. There is no standard place for every manufacture.
In the Power Distribution Box ( which is " live " ) located in the engine compartment Full size relay ( # 304 ) is the PCM relay according to the Owner Guide
On a 2000 Lincoln LS : In the FRONT power distribution box ( which is " live " ) located in the engine compartment ( on the passenger side of the engine compartment ) ( relay # 14 is for the PCM power )
On a 2001 Ford Ranger : In the Power Distribution Box ( which is " live " ) located in the engine compartment , on the drivers side ( the # 54 relay is the relay for the Powertrain Control Module / PCM )
Power Control Module
1996 ford thunderbird pcm power relay location?
in the power distribution box under the hood
The power control module (PCM) is the brain of the vehicles engine functions. The relay is a module as part of its power supply circuit.
On a 2004 Ford Ranger ( for the 3.0 and 4.0 liter V6 engines ) In the power distribution box ( which is " live " ) in the engine compartment ( # 47 is the PCM relay ) ( # 48A is the fuel pump relay )
The 1991 crown Victoria has several relays group in a relay center loacated under the hood near the master brake cylinder. In this relay center its the air conditioning WOT cutout relay, fuel pump, PCM power relay, and PCM power diode. P.S. defective relays must be replace as a unit.
Is the device (relay) that provides power to the Power Control Module, if this fails the car will be unable to start
under the hood, left side, large box above wheel well, relays inside --------------------------------------------------------------------------------------------------- Just adding to the answer , it's a full size relay ( # 304 for the PCM ) Helpfull
In the Power Distribution Box ( which is " live " ) located in the engine compartment Relay location ( # 54 ) is the PCM relay You can view the 2002 Ford Ranger owners manual online at : www . motorcraft service . com ( no spaces ) Click on Owner Guides It includes diagrams for the Power Distribution Box and fuse panel