hfanney, there are 2 connectors under the hood , on my 85 they are on right inner fender near the battery.A small handheld reader "flashes" the codes.Haynes manuels or chiltons will have a list of codes. I had too many electrical problems to fix without going broke so I'm in the middle of a 78 duraspark II "upgrade". So far for only 130.00 I've got a complete distributor and a new Carter carb. NO MORE COMPUTER!

 

The electronic box under the starter relay is the altitude sensor.
It interfaces with the computer to tweak the timing to improve performance at different altitudes

 

I have an 84 F150 I6 auto with the same TFI ignition and computer system except I have 200K miles on mine. Sounds to me like you need to go through the whole system like I just did and regroup . Start here;
· Go to the auto parts store and buy some locktite dielectric grease comes in a tube for a few dollars.
· Start at the carb disconnect the connector from the throttle actuator.
· I use a battery charger as a power source I hook a couple of test leads to it to check all the components. Put the test leads on two of the four pins on the motor side of the throttle actuator you should hear the actuator move in one direction. Reverse the leads , it should move the other direction. IF it works its ok if not replace it. Put some of the grease on the connector pins and plug it back in.
· Disconnect the feedback solenoid on the carb 2 pins hook up the test leads you should here a click each time you touch the pin with the test lead. Grease the pins and go to the next.
· Disconnect the fuel bowl vent solenoid and do the same. Grease the connector pins.
· Disconnect the altitude sensor grease the pins and put it back.
· If you have a multi-meter , disconnect the throttle position sensor, hook up the meter on ohms and move the throttle if you see a change in resistance it is probably working ok.
· Disconnect the wire to the oxygen sensor grease the connector and reinstall.
· Disconnect the connector near where the upper radiator hose connects to the engine. Grease the connector and reinstall.
· Remove the egr valve make a gasket shaped blank from a piece of aluminum flashing and install it in the flange blanking off the erg valve. (this is just a test)
· Remove the three connectors on the three solenoid valves located on the back of the valve cover check the solenoids with the test leads. Grease the connectors and replace the back two leave the front one off (that’s the egr valve solenoid).
· Disconnect the TFI module Inspect the connector if you see a lot of green or black corrosion on the connector that could cause a problem. Grease the connector and reinstall. New splice in connectors are available from Rockauto.com.
· Disconnect the spout connector near the TFI module (Yellow wire)
· Rotate the engine so that the #1 cylinder is at 10 BTDC ( remove the distributer cap an ensure the rotor is facing the #1 plug wire.) Replace the cap.
· Loosen the distributer hold down nut. Remove the #1 plug. Ground the plug to the engine, Hook up the plug wire to the plug, turn the ignition on rotate the distributer while watching the plug adjust the distributer until the plug just fires. Lock down the distributer making sure not to move it.
· You have just verified initial timing. You should really check it after you have it running and warmed up with a timing light but I have never had to readjust it using this procedure.
· Reconnect the spout connector.
· Go under dash and disconnect the computer connector. It has a bolt that you just back out which disconnects the plug . Grease and reinstall.
· Inspect all of your vacuum lines and tree plug under the carb make sure you don’t have any lines off or broken.

Start her up and see what you have. After it is warm set the idle the procedure is on your vacuum hose diagram at the front of the truck nead the radiator on the left.

 

The box under my starter relay controls the firing of my plugs, or so I was told.

 

If its a small black module about 2x3" with a connector plug in one end screwed to the fender wall, Its the altitude sensor. It will send a signal to the computer and the computer will adjust the timing as necessary for the altitude you are at. Should be very close to the diagnostic connector.

 

Not on mine sail2244, I don't have a computer, heck I only have one relay in the whole engine compartment. Thas why I love my '83, it has the bare minimum.