Injector Problems

What do the tip of the spark plugs look like? White, you have a fuel issue. Almost black, you have a spark issue.

You check spark with a spark checker? Nice blue almost white spark? Switch the plugs from 7 & 8 into 5 & 6. Black like that tells mw you have a weak ot no spark condition on those two cylinders.

I'm fixing to dump a lot of very technical stuff on your plate, so you may want to print it out and highlight the sections that relate to your current problem.

You didn't include the model year of Your Mustang. The wiring changed several times between 1979 and 1995. In case you have a 1991 or later model car, let me know and I will modify this post to include the engine wiring diagram for those model year cars.

Do the Fuel injector test path only after you have verified that you have good spark at the affected spark plugs.
Check the distributor cap for cracks and carbon streaks.
Check the spark plug wires for visible damage and proper resistance - 5000-15000 Ohms
Replace the affected spark plugs.


Fuel injectors inoperative, one or more injectors either on all the time or will not squirt.

Revised 1-Jun-2014 to add computer connector pinout graphic and revise injector wiring resistance readings

Tools needed: Noid light, Multimeter (volts & ohms), 10 MM socket &
extension, & ratchet.

Note: Do all of the steps and do them in order. The results of the subsequent tests are based on the prior tests being successfully passed.

1.) Each injector has a red power wire to provide power to the injector. Turn the ignition switch to Run and remove each injector electrical connector and use the multimeter to check for 12 volts on the red wire. Each injector should have 12 volts +/- .5 volt. More voltage is always better than less voltage. No 12 volts on a singe injector and the wiring for that injector is broken inside the engine fuel injector harness.

No 12 volts on all injectors:
A.) Check for a bad connection at the 10 pin connector. Check for 12 volts at red wire on the MAF or TAD/TAB solenoids mounted on the aft side of the passenger strut tower. Good 12 volts there and you have a wiring problem with the 10 pin connectors or associated wiring.

See the graphic for the location of the 10 pin connectors:


[ See the graphic for the 10 pin connector circuit layout.


The injector power pin is the VPWR pin in the black 10 pin connector. [/b]

B.) Bad ECC power relay. The relay is on top of the computer, it provides power to the fuel injector system. This is relevant ONLY if you do not have power to all injectors.
It is somewhat difficult to get to, since it requires you to remove the computer from its 2 bolt mounting.. If the relay or socket is bad, you will not have 12 volts on any of the red wires in the engine compartment or to pins 37/57 on the computer. The pins 37/57 are the main power feed to the computer.



C.) Blown fuse link – The blue fuse link for the computer is up near the starter solenoid. Check for no 12 volts on the ECC relay socket or computer black/orange wires. No 12 volts and the fuse link is blown open. If the fuse link blows, there is no power for any of the computer functions.

D.) Bad wiring. Broken or damaged red wire to the chassis side of the 10 pin connectors.



Some basics about the computer:
Remember that the computer does not supply power for any actuator or relay. It provides the ground necessary to complete the circuit. That means one side of the circuit will always be hot, and the other side will go to ground or below 1 volt as the computer switches on that circuit.

2.) Get a noid light form AutoZone or other auto parts store, or even better a set of them.

This set is from http://www.toolking.com/ProductInfo.aspx?productid=12492 and costs about $25.

3.) Use the noid light to determine that the injector pulses and isn’t stuck in the on position.
It you have a set of them install all of them and compare the pulse intensity. Install the noid lights, turn the ignition switch to Run and crank the engine. A light that doesn’t pulse and stays on has a short to ground in the computer side of the circuit. That can be either a wiring fault or a failed computer.
If this is the case, remove the passenger side lick panel and disconnect the computer connector.
There is one 10 MM bolt holding it in place. Pull the connector all the way out of the computer so that you can see the computer side pins.

Use the list from the graphic below to find the fuel injector pins for the injectors that didn’t turn the noid light off.





The diagram Below is for 88-91 model cars


4.) Set the multimeter to low scale Ohms and measure between the computer ground located below the computer and the suspect fuel injector pins. You should see greater than 100 K Ohms resistance. If you see less than 100K Ohms, the wiring between the injector and the computer has an internal short to ground and needs service.
Check the harness and look for damage, kinks or frayed spots.

5.) A single noid light that never turns on is either a wiring fault, or a failed computer. Either the injector has no DC power or the computer has failed and cannot switch the injector circuit to ground. Determine if the injector has power by using the multimeter to check for 12 volts on the red wire on the suspect injector connector. No 12 volts and you have a wiring fault.
Check the harness and look for damage, kinks or frayed spots. Check the 10 pin salt & pepper shaker connectors for bent pins, corrosion and damage.


If none of the noid lights flash and you have 12 volts at each injector, check to see that you have good spark. Before you even think about replacing the computer, see step 6.

Next check the fuel injector wiring end to end. Each fuel injector has a red wire (power) and an non-read wire (computer controlled ground). Set the multimeter to low ohms and measure each non red fuel injector wire from
the fuel injector connector to the matching pin on the computer connector. You should see less than 1 Ohm. More than that means a bad connection or bad wiring.


6.) Use an ohmmeter set on the low resistance scale and measure the resistance of each injector across the two contacts inside the electrical connector. You should see between 11-16 ohms. More or less than that is a bad injector. Next measure between either one of the contacts and the metal on the injector body. You should see greater than 100,000 ohms. Don’t hold the metal probe tips with your bare hands when you make this measurement. It will give incorrect results if you do.

Once you have determined that the suspect injectors have good power and good wiring, the computer is the likely suspect, since a ground is required to complete a circuit and make it function. The computer provides the ground: if doesn’t, then the noid light will not flash. If the noid light stays on, the computer has an internal failure.

7) If you have gotten this far, then the problem is likely ignition related. Remember the noid test using all of the noid lights? All of them were supposed to be equally bright. Since you have already tested all the electrical side of the fuel injector circuit, the one remaining common item is the pip sensor inside the distributor. A failing pip sensor, damaged shutter wheel or bent distributor shaft could all cause the pulse delivered to the injectors to be faulty. A bad pip sensor will cause all the injectors not to fire and you will have no spark. Dumping the codes will usually show a code 14.


8) Spark plugs indicate one or more cylinders not firing: use the multimeter to measure the resistance of the spark plug wires. The wires should measure 2000 ohms per foot of length. A 2 foot wire would be 4000 ohms and a 3 foot wire would be 6000 ohms. Some Taylor and Accel wires have metal cores and will measure much less: that’s OK.
Next examine the spark plug wires very carefully for burn spots, cracks and damaged insulation. One good thing to try is to start the engine while the car is a very dark area, open the hood and look for sparks or blue glow. They indicate the electricity is leaking out of the spark plug wires.

Thanks to Tmoss & Stang&2birds at www.veryuseful.com/mustang/tech/engine/ for some of the graphics

Normally this would be a stepped process. Check spark from coil to coil wire. Then coil wire to dizzy cap, dizzy cap to each wire, each wire to its corresponding plug. Each time you are eliminating the object before it. What does the inside of the cap look like? Condition of the rotor button? Wires old with worn/brittle boots? Porcelin (sp?) have any cracks in it?

make any progress on this??