Page 144 of 2008 Performance Parts by Ford Racing Performance Parts

electronic fuel injection efi system tips always remember to disconnect the battery before doing any wiring on your vehicle electrical grounds the single leading cause of most electrical problems is a poor ground the ground for the fuel injection system should connect directly to the battery at the negative post using the steel chassis or engine block as a ground will commonly create excessive resistance causing the computer to function improperly an example of how a high ground or connection resistance can have very serious effects is as follows this particular case applies to a 2005 mustang gt but can easily be extended to any electronically controlled ford vehicle consider the case where a powertrain control module pcm is reading a maf sensor signal of 4.1 v due to a high ground or connection resistance when it should really be reading 4.3 v this equates to a difference in measured air mass of 13 that is the maf will be telling the pcm that there is 13 less air entering the engine than there really is let s say this happens at wot where air/fuel ratio is critical not only to performance but also to engine durability the result is that the actual air/fuel ratio can go from a safe 12.5:1 to a potentially damaging 14.1:1 just from a 0.2 v change in the maf return signal all pcm sensors not just the maf are affected in a similar fashion so it is absolutely critical that all electrical connections are solid and that the grounds are reliable the potential penalty for a bad ground can range from strange drivability issues that are difficult to diagnose all the way to a damaged engine as in the above example all resistance tests should be done with the ignition key in the off position having voltage going through the system can return a false reading of excessive resistance additionally it is possible to have a ground that tests ok when the engine is cold but not when the engine is hot heat increases resistance so these tests should be performed on a warm engine when possible to test for an adequate ground circuit in the efi system for a 1986 to 1993 5.0l mustang use a volt/ohm meter to check the resistance of the following circuits · to verify a proper ground to the pcm check the resistance from pin 40 and pin 60 directly to the negative side of the battery resistance should be no greater than 0.2 ohms · to verify a proper ground to the main pcm harness check the resistance from the maf sensor at pin `b directly to the negative side of the battery resistance should be no greater than 0.2 ohms · to verify a proper ground to the engine harness check the resistance from the black wire at the throttle position sensor tps directly to the negative side of the battery resistance should be no greater than 0.3 ohms note that while 0.2 ohms or less is desirable a resistance as high as 0.5 ohms is considered acceptable greater than 0.5 ohms is excessive and could result in drivability concerns a weak ground connection can also cause the pcm s internal reference voltage regulator to function incorrectly this can be checked at the tps by checking voltage between the black ground wire and the orange reference voltage wire with the key on this voltage signal should be somewhere between 4.7 v and 5.3 v general tips · whenever possible the pcm should be mounted inside the vehicle to protect it from water damage the pcm should also be mounted with the electrical connectors at the bottom to avoid trapping water some pcms on newer model cars are mounted under the hood but they are sealed against moisture and designed to operate in such an environment when in doubt mount the pcm inside the vehicle · when setting the voltage at the tps you should check the voltage between the black and green wires 1986-1993 5.0l mustang this voltage should be somewhere between 0.96 v and 0.98 v if the key is on while the engine is off set the voltage at 0.96 v if the engine is running set it at 0.98 v the tps can be set by loosening the mounting screws and slightly rotating the sensor if you are unable to achieve the proper setting you may need to elongate the tps mounting holes · if you ever need to lengthen any of the harness leads for your specific application it is strongly advised that you lengthen only one wire at a time which will help to avoid making mistakes · if you are using long tube headers and need to lengthen the leads of the harness to reach the heated exhaust gas oxygen hego also known as o2 or oxygen sensors never lengthen the wires of the o2 sensor itself these wires are made up of a unique 142 techline 800 ford788 not legal for sale or use on pollution-controlled motor vehicles direct replacement part see pages 2-8 for important safety emissions and warranty information.

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