Fox Body Mustang Restoration

Sunday, July 11, 2021

Troubleshooting Electrical system

Recently I had some issues with the 1979 Ford Mustang. Engine would warm up and then it would misfire.

After contacting Holley I was able to get ohm specs on the MSD coil I had installed. I did end up replacing the coil as it failed secondary test, but after I replaced the coil with a new one I noticed the issue come back. Engine would idle fine then it would misfire after warmed up. Timing was right on, so I decided I was going to pull off the high performance ignition system and put the car back to a factory Duraspark II system and test things out.

In the last post you saw me make a coil relocation bracket which worked out great.

After installing the original harness back on and disconnecting the MSD components I installed a brand new Duraspark II Ignition Control Module and new Super Stock 12v Ignition Coil.

On first attempt to start the engine it failed. Instantly, I could tell there was no fire. First step was to check voltage at the coil. To my surprise I only had about 6 volts where I should have 12.

Ah hah, I found another problem in this old car. At this point I decided I am going to disconnect the harness for the Ignition Control Module, Distributor, and Ignition Coil.

Next test I wanted to perform was testing the male side of the connector that is connected to the harness going into the car.

I made a diagram of the plug on my old trusty Army notepad and traced back the ignition power wire to find the correct pin to test. I turned the ignition key to the run position and verified that I had just over 12 volts, which I did. This was a relief because it meant the harness leading into the car was completely fine and the source of the problem was in the engine harness. I sourced myself some terminals. That took a bit to figure out what exactly they were. Dorman makes an assortment pack, but for this plug they are called Ford Wedge Lock terminals. I found out that Summit Racing also has these terminals. No one Sells a pigtail version of the this plug, so if issues are bad you need to rebuild it.

This is a handy Lisle terminal tool makes life easy safely taking apart this plug. Take care when doing this because these are very hard to come by nowadays. I've tried finding in salvage yards no many of the early foxbody mustang exist anymore.

https://www.pepboys.com/lisle-terminal-tool/product/669946

Pop the red block in the center before attempting to release any wire terminals.

The red block is a lock. REMEMBER TO TAKE CARE WHEN RELEASING TERMINALS!!!

Next step I tried was to clean corrosion off of the terminal and try another power test.

Even with it polished up the test still failed. I was still getting just a little over 6 volts, so I made the decision to replace the hole Ignition Coil hot wire giving us issues.

This crimper replicates the factory crimp perfectly.

Crimped new terminal to new wire. I would like not I decided to step it a bit on quality of wire. I sourced some marine grade tin coated copper wire which holds up better to the elements and harsh conditions.

With the new wire connected to the correct pin. I decided to replicate test performed earlier.

We have a winner. Over 12 volts coming down the new ignition coil hot wire (Positive Wire).

Mustang Coil Relocation

Fabricating and making your own ignition coil mount.

Pending on your year and the modification you have done to it you may need to relocate things. Here was my solution. I've been having some issues with ignitions system, so as a troubleshooting step i figured I would rollback to the duraspark II system which was original equipment. I didn't want to remove everything for now, so I created a new mounting point for the classic style coil. Also needed to be closer to the distributor because no one had in stock the performance wires that would stretch the distance to the left hand side of he engine compartment. My solution was to create the new mounting point that allowed the coil to be closer to the distributor while still reducing heat. I made this nice aluminum bracket from spare scrap aluminum laying around in the garage.

First creating a template.

Tracing out on the aluminum piece

Using the vice as a metal break for bending to desired dimensions.

Trial and error test fitting and drilling mount point holes.

I decided to use a carriage bold to mount the coil mount to the aluminum bracket.

First, I drilled the whole to get close to dimensions required then I used an air file to square up the hole for carriage bolt to fit precisely.

Perfect square for carriage bolt. With this design you wont need two wrenches for removal.

Since this is aluminum I decided to polish it out to match the rest of the aluminum under the hood.

Here I needed to indent the aluminum for the hole location under the coil. This will prevent the coil mount from spinning or pivoting when mount is tightened down to the bracket.