Mustang Ignition Switch

Okay, so I?m stumped (again). I have a 1992 Mustang with a manual transmission that the ignition starter switch came apart (common on issue) as I was messing with some stuff under the dash. I put the switch back together only to find that it was no longer working as it had. Now, the fuel pump was no longer priming when switched to the ON position. However, it would prime for 2 seconds when the tumbler was rolled into the START position??? The car will start and run, but only for the duration of the fuel pump priming. I drove this car into the garage last week before this happened and it was running fine. This is the first time that I?ve had any electrical issues with the car.

I just figured the switch was somehow messed up, so I pulled and ignition starter switch out of another 1992 Mustang with a manual transmission and found the same results. Next I bought a new one from NAPA just to be sure, and it too worked in the same sort of fashion. I checked the ECU by installing it in another car and it?s fine, priming the fuel pump when the key is first turned to the ON position like it?s supposed to.

So, the fuel pump is working, the ECU is fine, all fuses are good, and relays seem to be fine. The big problem really seems to be that the key position is not synched with all of the stuff it?s turning on. The radio and battery light turn on when the key is moved to the ON position, but the fuel pump prime, check engine light, and a handful of other relays turn on when the key is moved to the START position.

Any ideas? Need any more information? Recommended stuff to check?

Thanks in advance.

It sounds like the wiring to the pump has been moved somehow from the ON position to the START position of the switch.

Maybe the ECU is not fine.
Assuming the ignition switch is not incorrectly timed (key/tumber to the electrical part) you might check this. I’m working off the top of my head here so I hope the wire color is correct.

Locate the underhood diagnostic connector where the codes are pulled.
There should be a tan wire with a light green stripe. Run a jumper wire from that pin to ground and you should hear the pump run and the engine should start.
If so, remove the jumper wire.

Take a voltmeter (preferrd) or a test light and attach a VOM lead or the test light point to the battery positive cable. Attach the other lead or test light wire to the tan/green stripe wire.

Turn the key to the RUN position. You should see the VOM show continuity or the test light illuminate for a couple of seconds before going off.
At that point crank the engine over and you should see the VOM show continuity again or the test light illuminate yet again.

If not, you likely have one of two problems. One could be an internal ECU fault and the other could be a lack of PIP signal to the ECU. Since your car uses the TFI-IV system which was problematic with age/heat it’s possible the ignition module could be faulty.
Checking the codes may show this if that is the case.
Hope some of that helps.

You are not using the repair manual (again), are you? It shows. The switch isn’t going to the START position. It’s out of adjustment.
The ignition switch has two parts: the switch part, and the key lock cylinder part. Which part are you talking about?
Go here, and register, and read the instructions for changing the ignition switch (NOT, the lock part): For one, did you turn the ignition key to RUN when you replaced the “switch” (lock cylinder?, or switch?). For two, the switch has two elongated holes. Are the mount screws adjusted correctly in the elongated holes? For three, see the instructions.

Tan/Green wire jumped, ignition in ON position- fuel pump kicks on, engine starts and runs.

Jumpered battery to Tan/Green wire- Key OFF: ~12.75V; Key ON: ~3V steady, no 2-second pulse; Key START (clutch not pushed in, so no start): ~11.4V steady, fuel pump goes on for 2 seconds.

As mentioned above, I tried this ECU in another car last night and it ran fine.

Any ideas from the information I provided?

I am only referring to the ignition starter switch ( However, since posting I have also R&R’ed the lock cylinder just to make sure it wasn’t doing anything strange. While I was in there I swapped in a known-good cylinder as well.

No book on this one; I used this and got to the part where it says, “Test the switch now before installing any trim to make sure everything works and that you don’t have any problems.” Although, I believe you’re right on the RUN position, not the LOCK position as they have listed. At any rate, I didn’t pull the pin on the switch until everything was in place. I have since pulled the switch back out a few more times (before I posted here) and attempted to realign it. The switch has one elongated hole and one round hole. At best you can move it about 1/16". The difference between where the fuel pump should prime and where the fuel pump actually primes is more like 1/2" off. This is at the end of the spring-return portion of the switch, which should obviously be the START position. That, and some of the accessories/lights power up when the key is in the ON position, the rest power up in the START position.

I followed the instructions as listed. On a side note, when I bought the part at NAPA, the first one they brought out didn’t have the locking pin in place, so I had them get me a different one that had the pin in place.

I’m trying to be as detailed as possible with my descriptions so that hopefully some one will note where I screwed this one up. Let me know if I need to provide any more information.

It sure does sound that way. However, I haven’t been messing around with any wiring since it ran last week; I was just installing a mechanical gauge.

Just to clarify something. In the second sentence you mention jumpered battery to the tan/green. You don’t mean a hot wire from the battery positive post or source do you?

That tan/green should ground through the ECU for a couple of seconds. After a few seconds with the engine not being cranked over by the starter motor the ground will disappear and only reaappear if the engine is cranked over or the key is switched off and back on.

Have you checked the positive side of the ignition coil just to make sure this is not a switch timing problem? There should be about 12 volts present at the pos. side of the coil in both the RUN and the START switch positions. Red/blue wire I think.
My memory is fuzzy on this but I though the electrical part of the switch had a “T” shape notch in it and that it was possible to have the tab on the tumbler aligned the wrong direction.

If power is present in both positions and since the ECU works in another car then you need to check the codes and see if there is one for a missing PIP signal. I don’t remember the number for this code off the top of my head though.

Hope some of that helps.

This article from Mustang Mounthly has plenty of good quality photographs on replacing the 1992 ignition switch and lock:

Oops- what I meant was I put the VOM negative on the Tan/Green wire and the positive on the + battery post. No, I did not run a jumper from the + battery post to the Tan/Green.

As shown by my VOM readings, I’m not seeing the ECU grounding the fuel pump relay in the ON position.

Checked the ignition coil- Key ON: ~12.5V; Key START: ~11.8V

I didn’t try this, but it looks like you may be able to position the ignition starter switch such that the tumbler pin starts out either in front of or in back of the sliding contactor. However, positioned this way, you would be limited movement of the key and it would only move the contactor one direction, but would not be able to pull it back, since it wouldn’t be enclosing the pin.

I’m not getting anything for the self diagnostic test. I usually check it with the CEL, but the way the ignition is working now, the CEL doesn’t even come on until the key is turned to the START position, at which time it stays on solid. I don’t have a scan tool or code scanner, but I may go pick one up now to check this problem out.

Thanks for the help.

I agree- that’s the link that I responded with as well.

Have you checked to see if all the fusible links are ok?

I haven’t had a lock cylinder out of a Mustang but I’ve removed a few on a Taurus and I’ve run into the same problem you’re having. When you remove the lock cylinder there is a rack with teeth (that the lock cylinder meshes with) and it can drop slightly after the cylinder is removed. If you don’t put it back in the original postion it will be 1 tooth off, it sounds like this is what happened to you.

I not sure if the Mustang lock cylinder is like the Taurus but it’s worth looking into.

This is the removeal procedure from ALLDATA…

On models equipped with airbag, disarm airbag system as described under Air Bags and Seat Belts/Air Bags (Supplemental Restraint Systems)/Service and Repair/Airbag System Disarming.
On all models, disconnect battery ground cable and isolate with electrical tape.
On models with tilt steering column, remove upper extension shroud. Unsnap shroud from retaining clip located at the 9 o’clock position.
On all models, remove trim shroud or shroud halves, then disconnect key warning switch electrical connector.
Place gear shift lever in PARK on models with automatic trans. or in any gear on models with manual trans.
Insert a 1/8 inch diameter pin in the hole in casting surrounding lock cylinder. Pull lock cylinder out of housing while depressing retaining pin.
To install, turn lock cylinder to RUN position and depress retaining pin.
Install lock cylinder into housing. Turn key to OFF position after checking that cylinder is fully seated and aligned in the interlocking washer.
Turn the key to check for proper operation in all positions.
Install trim shroud and extension shroud if applicable.
Reconnect battery ground cable.
Reactivate airbag system

NOTE the step about turning the lock cylinder to the run position when installing. Did you do this when you installed it?

Yep. I R&R’ed the lock cylinder only after the problem started, and after changing the ignition starter switch.

I know you stated that some things work in the ON position but some don’t like the fuel pump. I assume that the ON position has more than one contact position for power be switched. If that is so have you checked to see if all the power inputs have power getting to them?

Alright- after measuring some pins on the ECU I found that Ground pin 40 is only showing ~2.4V in the ON/RUN position and ~9.4V in the START position. The CEL and ECU relay ground measure the same, so I have a ground that is making partial contact somewhere and it must be sinking something else in the START position. By jumpering pin 40 directly to ground everything is functioning as normal, so now I just need to figure out where the ground terminates and fix it. Thanks for the help!

Good work finding the ground trouble. I would suspect the ground between the battery and the body frame.