Jim Grant's Tech Tips
'91 Ford Mustang, DIS Ignition Woes
Q: I have the distributorless ignition system with two coil packs on my 91 Ford Mustang with a 2.3L engine. The aft coil pack does not spark; I switched the front & aft coil packs and had the same problem. The front coil pack does not fire on all four plugs but if I pull one ignition wire away from the coil pack then all four plugs fire. The aft pack remains dead. Just in case, I added a new ground for the DIS module bolted on the intake manifold. Oh yeah, no tachometer movement. Could the DIS module be dying? Any suggestions?
A: The ignition system in your vehicle has a unique feature that you may not be aware of. The computer can turn off one of the coil packs. This is called the Dual Plug Inhibit (DPI) mode. Under certain conditions, the programming in the computer will turn off the left coil pack and allow only the right coil pack to operate. Before replacing parts I would suggest checking the computer to be sure that it is receiving the information it needs to operate correctly. If the computer checks out then monitor the control signal from the ignition module to the coil packs. Under normal conditions, both coil packs should be receiving the same signal. If one coil pack is not and the computer checks good the ignition module may need replacement.