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Dumping The computer diagnostic codes on 86-95 Mustangs

Revised 19-May 2009 to add information to Dumping codes with no check engine light drawing.

Dump the codes and see what the computer says is wrong…Codes may be present in the computer even if the Check Engine light isn’t on.

Here's the way 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. You watch the flashing test lamp or Check Engine Light and count the flashes.

Be sure to turn off the A/C, and put the transmission in neutral when dumping the codes. Fail to do this and you will generate a code 67 and not be able to dump the Engine Running codes.

Dumping the Engine Running codes: The procedure is the same, you start the engine with the test jumper in place. Be sure the A/C is off and the transmission is in neutral. You'll get an 11, then a 4 and the engine will speed up to do the EGR test. After the engine speed decreases back to idle, it will dump the engine running codes.

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. You watch the flashing test lamp or Check Engine Light and count the flashes.

See Troublcodes.net Trouble Codes OBD & OBD2 Trouble Codes and Technical info & Tool Store. By BAT Auto Technical





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.



The STI has a gray connector shell and a white/red wire. It comes from the same bundle of wires as the self test connector.

89 through 95 cars have a working Check Engine light. Watch it instead of using a test lamp.



The STI has a gray connector shell and a white/red wire. It comes from the same bundle of wires as the self test connector.


WARNING!!! There is a single dark brown connector with a black/orange wire. It is the 12 volt power to the under the hood light. Do not jumper it to the computer test connector. If you do, you will damage the computer.

What to expect:
You should get a code 11 (two single flashes in succession). This says that the computer's internal workings are OK, and that the wiring to put the computer into diagnostic mode is good. No code 11 and you have some wiring problems.

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.

Alternate methods:
For those who are intimidated by all the wires & connections, see Actron® for what a typical hand scanner looks like. Normal retail price is about $30 or so at AutoZone or Wal-Mart.

Or for a nicer scanner see Equus - Digital Ford Code Reader (3145) – It has a 3 digit LCD display so that you don’t have to count flashes or beeps.. Cost is $30.

87redgt said:

I found the connector it says EEC something(cant remember exactly) looked pretty much like the one in the picture, but it was a little bigger and it had like 3 wires plugged into it. Do you know if those code readers will work on cars witthout a check engine light?

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All of the 5.0 Mustangs from 86-95 used the same diagnostic connector and scanner/code readers. The presence or absence of a check engine light was not a concern when using the correct scanner/code reader.

If you will re-read my previous post, you will see that the red jumper wire and the automotive test lamp are not present until you install them.

Erratic RPMs, especially in hot weather is commonly caused by a failing ignition module (on the side of the distributor). Usually, it will start out intermittantly, and if you shut the car off and let it cool down, the idle will smooth out. Then when it heats up again, it will start running rough again.

bradleyem said:

Erratic RPMs, especially in hot weather is commonly caused by a failing ignition module (on the side of the distributor). Usually, it will start out intermittantly, and if you shut the car off and let it cool down, the idle will smooth out. Then when it heats up again, it will start running rough again.

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I agree. That makes a lot of sense. Now that we're getting into the hot summer months, ignition components that are prone to heat soak (ignition module and PIP sensor on Fox Body 5.0's, PIP sensor alone on SN95 5.0's) will be more likely to fail.
In this case I'd say it's the ignition module because of the erratic rpm before the engine died. A bad PIP sensor will usually just cause the engine to die suddenly as if you tripped a kill switch.

Yep, still sounds like the ignition module.....at least thats where I would start. It'll probably get worse as the weather warms up too......

Plus its a pretty cheap part....around $35 at VaderZone.

Dump the codes. A bad PIP sensor will often set code 14. Other codes may be present that will help you find the source of the problem.

The PIP sensor (Profile Ignition Pickup) is a hall effect sensor that is located inside the distributor underneath a metal cap right below where the rotor is. It is what senses the rotor position and generates the ignition pulses to fire the ignition. Typically, if this sensor is bad, the car will not start at all, or it will just die when the sensor heats up. It typically doesn't cause the surging idle problem that you're experiencing. Reading the codes is a good suggestion, but keep in mind that you may or may not get a trouble code. When my ignition module went bad, it never threw a code.

Also, two other things that can cause rough idle/surging are the IAC (Idle Air Control) valve and the EGR valve.

The IAC valve can get carbon build up that will cause rough idle, and the EGR valve can stick, and cause similar symptoms. BUT, usually these will only cause a rough idle, not surging at higher RPMs (when you're on the gas) and they usually will not cause the car to die.