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Senior Charter Member
348 Posts
Discussion Starter · #1 ·
My engine and AAW wiring is complete. Now comes the circuit testing before I attempt to start up the beast. As stated in my previous posts, I am mentally challenged when it comes to electrical work.

With my Ohm meter, what is the procedure to test circuits? Any particular strategies?



Charter Member
4,001 Posts
It depends on what you want to find out.
If you want to know that a circuit is complete from one contact to another( checking for continuity)put your scale on the VOM to the ohm position.
You should get 0 resistance if your circuit is complete.
If there is no movement on the scale or readout you have a problem.
If you want to know that B+ 12V is going from a connector to a relay for example place the scale to volts no less than 12 and put the negative probe to a known good chassis ground and the positive to the relay trminal,you should read the approximate battery voltage minus a little drop.
This is real basic information,you probably should invest in a basic electrical book that can give you much more detailed information than what I written here.
Good luck,
Paul M.

Junior Charter Member
815 Posts
I am going to assume you are EFI.

From the 2007 FFRP catalog:

• Always disconnect the battery before doing any wiring!
• The single leading cause of most electrical problems is due to poorly grounded circuits.
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.
EFI computers measure the voltage returned from the engine sensors to directly influence the parameters for proper air/fuel ratio and spark timing for the engine.
An example of how a high ground or connection resistance can have a serious effect on an engine is as follows. This particular case applies to a 2005 Mustang GT, but can easily be
extended to any electronically controlled Ford vehicle:
Very small changes in the MAF return voltage can have a huge and potentially catastrophic effect on the engine. Consider the case where a PCM is reading a MAF 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…all 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 imperative 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.
• Doing a resistance test should be done with the key OFF. Having voltage going through the system can return a false reading of excessive resistance. Additionally, it is possible to have
a ground that may test acceptable while the engine is cold, but can test poorly when the engine is hot. Heat increases resistance, so as a rule of thumb, you should run these tests on
a warm engine when possible.
To test for an adequate ground circuit in the fuel injection system for a 1986 to 1995 Mustang, use a Volt/Ohm meter to check the resistance of the following circuits.
– To verify a proper ground to the computer, 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 computer harness, check the resistance from the Mass Air Flow (MAF) meter 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.
– While 0.2 ohms is desirable, the resistance can be considered ‘acceptable’ up to a reading as high as 0.5 ohms. If you have 0.51 ohms or more, this would be considered
completely unacceptable and is susceptible to drivability concerns.
• A weak ground connection can also cause the computer’s internal reference voltage regulator to function incorrectly. This can be checked at the Throttle Position Sensor (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 and 5.3 volts.
General Procedures and Settings
• The Powertrain Control Module (PCM), or ‘computer’, should be mounted inside the vehicle whenever possible, to protect the PCM and connections from water damage. The PCM
module should be mounted with the connector at the bottom to avoid trapping any water.
• When setting the voltage at the TPS, you should check the voltage between the Black and Green wires. This voltage should be somewhere between 0.96 and 0.98 volts. If the key
is on while the engine is off, set the voltage at 0.96 volts. If the engine is running, set it at 0.98 volts. 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. This will help avoid 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 or O2) sensors, NEVER lengthen the wires of the
O2 sensor itself. These wires are made up of a unique material and you will disrupt the signal coming from the O2 sensor even if they are soldered correctly! If you must alter the
length of the leads to the O2 sensor, always lengthen the wires on the wiring harness side of the O2 sensor. Many aftermarket companies offer O2 Sensor extensions that work quite
well and are an easy solution to this problem.
• When soldering two or more wires together, you should “tin” the bare ends to be soldered. This will prevent cold solder joints and make the process easier. “Crimp” style or “solderless”
connectors are not recommended. Over time, these have a tendency to loosen and permit corrosion. Additionally, these connectors can commonly allow short circuits to develop
within the connection. Many of these problems within the harness can be difficult to locate. Use weather-tight heat shrink over all soldered joints.
• If the factory coolant tubes are not used, the Engine Coolant Temp (ECT) sensor should be installed directly into the threaded boss in the intake manifold near the thermostat. This is a
coolant passage.
• The ACT sensor should not be moved from the factory location. Some after-market companies offer ACT relocation kits while making false claims of increased horsepower by reading
cooler air. While it is true that a cooler engine can make more power, this “trick” is not cooling the incoming air charge. It is merely reading the air temp from a different location. Since
the computer is no longer measuring air from the position that it was originally calibrated for, it will offer a false reading of the incoming air temperature to the computer, and can have a
negative effect on overall engine performance. On a forced induction engine the ACT sensor needs to be located after the power adder as additional heat is generated by the power adder.
• Protect the air filter element from turbulence created by the cooling fan. This is commonly referred to as “Fan Wash”. If you are using an open element air filter on the end of the mass
air meter, it is strongly advised that you use a shield to eliminate this problem.
• It’s best if the air filter gets cold air from in front of the radiator. If the filter is located in the engine compartment, as in many street rod applications, the inlet air temperature can be
up to 60 degrees hotter which can result in a 5% torque loss from the air density change. The PCM will also retard ignition timing for the hotter air which will result in an additional
5-10% torque loss.
• An improperly functioning charging system can cause engine running problems. Under-drive pulleys spin the accessories slower meaning that they consume less power from the
engine. This results in a greater net horsepower available at the flywheel. Normally this is not a problem, but some systems may not perform properly if you under-drive the alternator
excessively. If the alternator does not generate enough voltage to keep the system adequately charged, it can have an adverse effect on the fuel injection system PCM.

Senior Charter Member
348 Posts
Discussion Starter · #4 ·
Thanks Paul and Lurker for the good info.

Premium Member
9,698 Posts
I used a Radio Shack 12V lantern battery to check my circuits. Not enough amps to hurt anything, but it will still allow you to check for voltages and shorts. It would power up everything, except the fuel pump and the horn.

FFCobra Fanatic
12,975 Posts
Also used a small battery for checks. had a motocylce battery with an inline 10 amp fuse on the battery jumper wire used. Being carbed the main checks were of the dash and lighting curcuits. Starter solenoid with starter disconnected. Cooling fan, power to the dist.

Senior Charter Member
348 Posts
Discussion Starter · #7 ·
I have a jetski battery that I can use with your suggested 10amp fuse.

Great idea.
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