The 88 and eariler modles did not have a functional check engine light. The socket and markings are there, but the wiring isn't.
Copied from mustang88GT:
Adding a "Check Engine" light
Another difference between the speed density and MAF cars is that MAF cars have a "Check Engine" light in the dash to alert you to possible serious problems and it makes reading out self test codes much easier. The MAF cars have the "Check Engine" light located in the light panel on the lower right of the instrument cluster. Speed density cars don't have this window, but there is a "Check Engine" position for a light in the tachometer face, though there is no bulb installed. The wire from the EEC to the bulb (via the flex connector) is also missing.
In the '88 harness (not sure of other years), there is a tan and a black/blue wire connecting up to the 'Check Engine' circuit on the instrument cluster flex circuit. For MAF models, the tan wire hooks to the EEC STO/MIL line and the black/blue is for the lamp test out when you start the car. I don't know what these are connected to in an '88, but the test out feature does not work, and the STO/MIL line at the EEC-IV and self test connector is yellow/black. The wires connected to the check engine lamp probably end at some open connector somewhere, but I couldn't track it down, so I cut the tan and black/blue wires off the flex connector and ran a new wire to the STO/MIL wire on the self test connector under the hood (you could also tap in by the EEC-IV, but it was easier for me to go through the firewall. You can get a bulb socket from the HELP! rack at the local parts store, put in a N194 bulb, and voila, you now have a working check engine light.
How to dump codes with no check engine light
Here's the link to dump the computer codes with only a jumper wire or paper clip and the check engine light, or test light or voltmeter. I’ve used it for years, and it works great.
See
http://www.troublecodes.net/Ford/
OR
See
http://www.mustangworks.com/articles/electronics/eec-iv_codes.html
IF your car is an 86-88 stang, you'll have to use the test lamp or voltmeter method. There is no functional check engine light on the 86-88's except possibly the Cali Mass Air cars.
Codes have different answers if the engine is running from the answers that it has when the engine isn't running. It helps a lot to know if you had the engine running when you ran the test.
Trouble codes are either 2 digit or 3 digit, there are no cars that use both 2 digit codes and 3 digit codes.
For those who are intimidated by all the wires & connections, see
http://www.actron.com/product_detail.php?pid=16153 for what a typical hand scanner looks like. Normal retail price is less than $30 or so at AutoZone or Wal-Mart.
Or for a nicer scanner see
http://www.midwayautosupply.com/p-7208-equus-digital-ford-code-reader-3145.aspx– It has a 3 digit LCD display so that you don’t have to count flashes or beeps.. Cost is $30.
