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Re: check engine light. 91 ford F250 5.8
Yes you can use a piece of wire on the Diagnostic Link Connector and the Self Test Input connector.
The DLC may or maynot be labeled, but it is the top right position you connect to the STI to get the codes. Can be read with the Check Engine Light. I would suggest getting a service manual from a parts store to help with this diagnostic, it has the codes list.
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Only way to be sure what is wrong is to have the computer scanned with a code reader or scanner to retrieve the trouble codes. They will direct you or a technician to the proper sensor, control, or circuit that is malfunctioning. Many national parts stores provide code reading for free.
Under the passenger side kick pannel, there is a pair of wires with a blue connector at the end. You might be able to find it without remove the panel, but you will most likely have to take it off (just pull it straight out). Once you find it, take a paper clip or some other piece of wire and short the connection (put one end into each hole of the connector). Put the key in the ignition and turn two clicks. The check engine light will begin to blink out the code; long blink=10, short blink=1. Just add up the numbers to get your code. Examples
2 long, 2 short = 10+10+1+1 = Error code 22
4 long, 1 short = 10+10+10+10+1 = Error code 41
If your car is running fine and your check engine light is on, you will most likely get code 41, which is the heater circuit in your O2 sensor. You will have to replace the sensor to fix it, but your car will still run okay without doing so. You can usually do this yourself with a simple socket set and renting an O2 sensor socket from your local auto parts store.
The Check Engine light will light when the ECU stores a trouble code.
Counting the Flashes
To retrieve trouble codes, turn the ignition key to the "on" position with the engine OFF.
The Check engine light will flash many short blinks. The total number of blinks equals the code number. For example, flash,pause, flash,flash = code 12.If more than one code is stored, the Check engine light will flash the codes in sequence (lowest to highest) with a pause between each code. The codes will repeat until the key is turned to the OFF position.
Transaxle codes present
Diagnostic check only
Oxygen sensor circuit
Coolant sensor/high temperature
Coolant sensor/low temperature
PCM fault - pull-up resistor
6X signal fault (92-95 models)
Throttle position sensor-voltage high
Throttle position sensor-voltage low
IAT circuit low
VSS circuit-no signal
IAT circuit-Temp out of high range
Quad driver output fault
EGR system fault
MAP sensor - voltage out of range high
MAP sensor - voltage out of range low
IAC - rpm out of range
Ignition control circuit-open or shorted
Bypass circuit - shorted or open
IC control circuit grounded/bypass open
Knock sensor circuit shorted or open
O2 sensor indicates lean condition
O2 sensor indicates rich condition
Power steeringpressure circit(91 models)open or shorted
High idle indicates vacuum leak
PCM memory error
System voltage error
trans fluid temperature too high
Trans fluid temperature too low
A/C pressure sensor
ABS message fault
PCM internal communication fault
The problem you describe has NUMEROUS causes. Most common of these is a COIL is going bad. The RPM gauge gets its signal from the ignition coil/crank sensor circuit (could be either one OR could be the wiring between the engine control module (ECM) and the engine components. Does the Check Engine light come ON when this happens? Whenever the "Check Engine" light comes on, there is always a DTC (Diagnostic Trouble Code) stored in the ECM (Engine Control Module). You'll need an OBDII (On-Board-Diagnostics) scanner to retrieve any codes recorded/set in ECM. Once you do this and let us know what, if any codes are set, then we can go from there.
unless you know the nuts and bolts of your truck, you are better off taking it to a garage, go through the yellow pages and get prices, I charge $20 to get the codes and turn off the light, others may charge more depending on the equipment they have.
I don't suggest jumping at the data link, if you are wrong your ecu is now toast, and that will cost you a lot more than a scanner hook up
What year is your truck, the pre-OBDII ford do not have the capability of storing a lot of the codes nor will they show live data that well.
What are the symptoms you are having?
CANT HELP W/ AC BUT THE TROUBLE CODES CAN BE READ THROUGH THE CHECK ENGINE LIGHT IF YOU JUMP THE DIAGNOSTIC TERMINAL W/ THE GROUND. YOUR CHECK ENGINE LIGHT SHOULD FLASH IN SETS(3 FLASHES THEN 2 FLASHES) WHICH IS YOUR CODE. THE FIRST CODE IS USUALLY A DIAGNOSTIC STARTED CODE FOLLOWED BY THE ACTUAL PROBLEM CODE. KEEP IN MIND THE SENSOR THAT IS TRIPPING IS NOT ALWAYS THE PROBLEM. MANY TIMES THE SENSOR IS JUST DOING ITS JOB OF TELLING YOU THERE IS A PROBLEM RELATED TO THE OPERATION OF THAT SENSOR.(IAC VALVE JUST LETS YOU KNOW THERE IS NOT ENOUGH AIR)