Hey, so I just inherited a 91 ranger with the 4.0. It hasnt run in two years. A year ago we determined it to have either no spark or very bad spark. It gets fuel and if you use starting fluid it would pop every once in a while. I read engine codes, I dont remember them. I do know we replaced the coolant temperature sensor because it was bad, but the code still shows for it.

I was wondering if anyone knows a way to test the computer or how hard it is to check the crankshaft sensor.... because those, to my knowledge, are the only two other possibilities.

 

Computer doesn't run the spark on a 1991 4.0l Ranger

1990-1994 4.0l used an EDIS-6 module for spark, it was located on Rad support, back OR front side, my 1994 is on front of rad support
Picture here of complete Spark system on your 1991: EDIS-6Large.jpg Photo by BRAAPZ | Photobucket

3 pieces that are connected together, computer is not use/needed for start up spark
EDIS-6 module
Coil Pack
Crank sensor and Tone wheel

Crank sensor starts the ball rolling for spark AND for fuel injection
EDIS module sends Crank sensor pulses to Computer so it can time injectors

Crank engine a few times and then check a spark plug tip, it should be WET with fuel if Crank sensor pulse is getting thru to Computer.

Turn key on and test for 12v on red wire at coil pack, same 12v should be at EDIS module and at each fuel injector.
This power comes from EEC-Relay in engine fuse box
If CEL(check engine light) comes on with the key then EEC Relay is working, EEC = Computer, so when EEC Relay closes computer, EDIS, coil and injectors all get powered up.

Crank sensors rarely fail, not never, but almost last thing you would check, if spark plugs get WET then Crank sensor is OK and most likely EDIS module is to

Crank sensor is a VR(variable reluctance) sensor, it generates its own AC voltage
You can not test with ohm meter, but can test with Volt meter set to AC Voltage
Hook up volt meter to sensor's 2 wires, its AC volts so there is no +/-, crank engine and watch voltage, should be .5-1.0vAC, the faster the tone wheel spins the higher the voltage

Crank tone wheel has 1 tooth every 10deg, so 360deg circle would have 36 teeth, this tone wheel will have 1 missing tooth, this IDs Top Dead Center(TDC) for #1 piston, make sure all the teeth look OK, and only 1 "missing" tooth, if a tooth gets damaged then EDIS module won't know when #1 is at TDC so can't start spark

 

So in a few days I will get down and dirty and get under the hood... but until then, what kind of work is required to get at that crank tooth thing? And how do I identify? I will probably do crank and check for wet plugs first. If I do get wet plugs, I would check that tooth thing?