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1991 Mustang Lx 5.0

New User
Posts: 2
Joined: 07/12
Posted: 07/31/12 09:37 PM

i am having a little bit of a issue with my mustang i am throwing a code for DTC 67,31,41,96 then i cleared to see what came back and now its trowing 67 again and now 11 can anyong tell me how to fix please  Confused  

Post Reply
Posts: 96
Joined: 04/12
Posted: 08/01/12 07:23 AM

there are a bunch of things involved...

first thing to check...

look at both ends of the radiator core support...   there are ground wires attached....  how is the condition of the wire into the crimp terminals...  i have found at least one wire broken and frayed on almost every car the first time i open the hood..   you will want to take them off and wire brush the ring terminals and the under side of the bolt head..   there is another ground connection on the inner fenders somewhere between the shock towers and the back of the headlights..

you will want to do this test before you start cleaning. and after you finish cleaning the ground connections..   you may also have to do work at the battery cable terminals...

print this image.. take it to the car with you and run the whole test the first time.. then the 4 part test every time after that..  the second part can be tested in under 2 minutes.

VOLTAGE Drop Testing

you can see what had to be done on one Tbird...   the wiring was all frayed and broken.. out of an 8 gauge cable.. with 40 strands.. there was only 3 strands still connected.. the voltage drop was horrific.. there was not enough electrons to power the electric cooling fans and it over heated.. but it also cut down on the voltage to the computer system and started throwing codes..


Post Reply
Posts: 96
Joined: 04/12
Posted: 08/01/12 08:01 AM

code 67....   i bet you have a 5 speed car....   and you did not run the tests with your foot held to the floor on the clutch....

it shows that the clutch interlock switch is not sending the proper clutch depressed signal to the ECM...

96 is showing there may be an issue with the fuel pump circuit feedback voltage...
you could have a fuel pump relay or even a main relay starting to fail...
you could also have a bad ignition switch.. those 3 things are cheep.. and do wear out..
also the inertia switch.. but those rarely fail...

41 is fuel control..  clean those grounds.. make sure you have the proper thermostat..
there is a BUNCH of other stuff that could cause this..

31 is the EGR valve position sensor circuit.. there are a lot of things that can cause this also..

please... DO NOT change parts unless you verify they are bad...

except for the known failure issues of the main relay.. the fuel pump relay and the ignition switch...   AND GROUNDS..   bad grounds.. you won't have enough electrons to build the voltage the computer needs to see from the sensors.....

pass 11 shows that all the other circuits are working...

oh.. and a HUGE tip...  

the EFI main relay DOES NOT HAVE A DIODE in the drawing on the side..

the FUEL pump relay DOES have a DIODE in the drawing on the side...


the main relay is controlled by the ignition switch... comes on when you turn on the key...

but the fuel pump relay... is controlled by the ECM...

i am working from a 91 pickup and van emission manual..  my domestic car manuals seem to have grown legs except for my 88.. which is a little early..

i guess you can tell what i do when i am not answering questions...   fix electronics on cars.  

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New User
Posts: 2
Joined: 07/12
Posted: 08/01/12 10:14 PM

i held the clutch all the way in and still says it would you say to changed my clutch switch also where is the fuel relays located can i test the relays to see if there any good  

Post Reply
Posts: 96
Joined: 04/12
Posted: 08/02/12 06:02 AM

run the voltage drop test first.....

then  test the ignition switch like this..  ignition switches are known failure places.. and they are cheep.. not hard to replace..  have a medium length screwdriver with a nice square tip.. knock some dents in the sides of the tamper proof torx screw heads... then change the angle of the screw driver and small hammer and use it to knock the screw outer head to UNSCREW them.. use some needle nose pliers to completely unscrew them..   there are short ratcheting box end wrenches with a really short tamperproof torx installed to remove these hard to get at screws.. but its about 20 or 25 bucks.. and thats almost twice what the switch costs..  

do you have a circuit test light...  stick the probe end into the positive wire cavity on the coil... ground the clip...   get in .. slowly rotate the ignition key on.. go all the way to crank and start the motor..  then turn the key off..    you should have constant power as soon as you turn the key to the ON position through crank and back to run...  without the light blinking out..

the fuel pump and main relays are usually under the hood... i don't have a component locator for a 91 in front of me right now...    usually they have a green and a brown relay socket...  could be tucked into a little black plastic cover box...  where they are latched inside by the side ..

this is the clutch interlock switch... about 25 bucks at AZ..

one pair of contacts is for the cranking circuit.. to make sure that you have your foot on the clutch or it won't start...   saves the bumpers and garage walls from damage..

the other pair are between pin 46 that powers a lot of the sensors... and pin 30.. where the voltage is switched to let the computer know you have your foot on the clutch..

the computer needs to know this info if you have your foot on the clutch or not...

each time you push the clutch pedal down.. the contacts move across each other..  they can and do wear out..  they get dirty...  

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