Recently I went through hell trying to figure out why my Bronco wouldn't start. I had fuel pressure and I knew my fuel pump was working... but why no spark?
After utilizing Fullsize Bronco, other various forums, and pounding my head against a wall trying to follow my repair manual... I've compiled this 30-Minute Ignition Systems test.
Note: this write-up applies to a Bronco with the Remote Mounted TFI, that cranks but does not start, once fuel has been ruled out as the cause for the no start condition. In about 30 minutes (with or without a helper) this tactic will test / verify your distributor cap and rotor, spark plug wires, coil, TFI (ICM), various harness wires, and your PIP... indicating WHERE you have the problem, in order to get the damn thing fixed!
Makeshift LED Test Tool (see below)
Analog or Digital. Some guys insist that analog is the way to go, but I prefer a good Digital Multimeter over analog whenever possible (myself... I've yet had to use an analog meter).
A general lighted circuit tester, such as this one. Obviously, it must be 12v capable.
Should be capable of testing spark at the plug wires and at the ignition coil. Mine (pictured) allows you to adjust a vehicle specific gap so that you can see how good your spark is.
Makeshift LED Test Tool:
Radio Shack LED light in holder (Part #276-0270). I added some small gauge speaker wire, and solder-on alligator clips to mine. If I ever need it again I'll probably add a connector so that I can change leads between alligator clips and probes... that would have helped!
Again, this test applies to your Bronco that cranks but does not start. Fuel has already been proved out OK!!!
Step One: Testing for Spark at the Spark Plugs.
This portion of the test checks to see if you are getting spark from the Distributor Cap and Rotor. To complete this step you'll be using your Multimeter and the Spark Tester.
- Verify proper battery voltage using your multimeter (image 1).
- Attach to the Ground Battery Terminal, a length of wire long enough to move around the engine compartment for testing. You can attach the wire to another ground location if desired, but ensure you have a good connection by using your multimeter. We will call this attached wire the "Ground Tester Lead".
- Attach your Ground Tester Lead to the Spark Tester (image 2).
- Pull a Spark Plug Wire from an easily accessible spark plug, and attach the spark plug wire to your Spark Tester (image 3).
- Have a helper watch the tester, or (if working alone) position the tester so that you can view it from inside the cab by looking into the engine compartment
- Turn the key to the Start Position to test for spark.
Image 1: Multimeter connected to the battery, ensuring proper voltage. Before I compiled this list I did take my battery in for charging and testing. Without the proper amperage, a car still will not start. Make sure you're using a good battery!
Image 2: Ground Test Lead to Spark Plug Tester. I set mine up on where I could view it from inside the cab while testing.
Image 3: Spark Plug wire to Spark Plug Tester.
If you DO NOT have spark, move to Step 2 below.
For now, there is no reason to test the remaining spark plug wires, but depending on the next tests, you may may need to.
If you HAVE spark, test the remaining wires. If all wires show sufficient spark, your Ignition System is functioning as it should and Fuel and Compression tests should be performed.
Step Two: Testing Spark from the Ignition Coil
. We will check both the coil wire and the coil itself. For this step, you will be using your Spark Tester.
- Replace the Spark Plug Wire used on your previous test. It should be re-secured properly on the spark plug in which is was removed. Leave the
- Ground Tester Lead attached to the Spark Tester.
- Disconnect your Coil Wire from the Distributor Cap (keep the wire attached to the Ignition Coil, and attach the coil wire to the Spark Tester (image 4).
- Test for Spark by cranking the engine (key in the START position).
Image 4: Coil wire to Spark Plug Tester.
If you HAVE spark, test the remaining spark plug wires for spark. You most likely have either bad spark plug wires or a bad distributor cap and / or rotor. Replace Cap, Rotor, and Wires. Test truck for start.
If you DO NOT have spark, test the coil directly with the Spark Tester by eliminating the Coil Wire from your testing. If you still DO NOT have spark, continue to Step 3. If your Ignition Coil DOES have spark directly, replace the Coil wire (and probably your other spark plug wires, cap and rotor for good measure).
Step Three - Test for 12 volts at the Ignition Coil
. We're simply verifying proper power at the coil. For this step you will be using your Multimeter. (you can leave your Test Ground Lead in a safe location AWAY FROM POWER SOURCES... it may be used again later)
- Connect the Ground Lead of your Multimeter to Ground
- Turn the Ignition Key to the ON position
- Insert the Read Lead of your multimeter into the driver's side of the Ignition Coil Connector. Reference photo! (image 5)
- According to Chilton's Repair Book, proper voltage requirements are 90% + of battery voltage (12.00 volts minimum).
Image 5: Ignition Coil Voltage Test. Picture indicates a properly powered Ignition Coil
If you HAVE 12 volts, continue to Step 4. If you DO NOT have 12 volts then this is the cause of your failure to start issue. Troubleshoot WHY the Ignition Coil is not receiving power.
Step Five - ICM Switching Signal:
We will now verify that you are receiving a switching signal from the ICM and / or PIP sensor. For this step you will be using your Test Light.
- Ground the Black Lead of your Test Light (circuit tester). You can either connect the lead to the Ground Testing Lead or any other verified ground on the vehicle.
- Insert the Positive Lead of the Test Light into the passenger side of the ignition Coil Connector (image 6). Have a helper watch the TEST light or position it where viewable from within the cab if working alone.
- Turn and hold the key at the START position.
While the key is held in the START position, the Test Light should blink ON and OFF if the Ignition Coil is receiving the Switching Signal. Again... while the key is held in the START position.
Image 6: Switching Signal at Ignition Coil Connector. Mine tested a bad signal as the light did not turn ON and OFF. Instead mine remained Constant.
Now we're getting somewhere! If the Test Light blinked ON and OFF during this step of testing, then you have a faulty Ignition Coil and replacing it should resolve your no start issue.
If it does not blink ON and OFF however, then you have either a bad TFI (ICM) or PIP sensor. Step 6 will determine whether it is the TFI or PIP.
Step Six - Testing the ICM and PIP sensor:
Now we get to use the handy-dandy Makeshift LED Test Tool. To complete this test you'll need to identify the #6 wire on the TFI (ICM) six pin connector (image 7).
Connect the black lead of the LED Test Tool to the #6 wire of the TFI (six pin) Connector (image 8).
Connect the red lead of the LED Test Tool to the Positive Battery Terminal (image 9).
Have a helper watch the LED Test Tool or position it where viewable from within the cab if working alone.
Turn and hold the key to the Start position.
Image 7: TFI (ICM) diagraming the connector and wire positions. Sorry for the crappy photoshop! I did not indicate wire colors because I doubt they're the same on all Bronco's.
Image 8: LED Test Tool (black lead) to #6 wire on 6 pin TFI connector
Image 9: Led Test Tool (red lead) to positive battery terminal
If the LED Test Tool blinks ON and OFF during the testing, the PIP sensor is good but the TFI module needs replaced. Change out the TFI module and your problem should be fixed.
If the LED Test Tool does NOT blink ON and OFF during the testing, the PIP needs replaced (such as was my case). You can either remove the distributor and replace the PIP sensor, or you can replace the entire distributor (which comes with a new PIP sensor already installed). I replaced the entire distributor.
Anyway... I hope the info helps someone! If anyone knows any other tricks I missed or didn't explain right, please let me know so I can add to or edit.