Boys and Girls,

I'd like to share an easy method to test your truck's ignition switch without any need for removal first. As lazy as I am, I want to know for sure the switch has failed before I gain access to it under the dash.

On my '84 F250, I'd notice sometimes the HVAC fan seemed to be running slower than normal. On occasion, right after starting the engine, the fan didn't always run unless I wiggled the key, and then the fan would seem okay. I suspected the ignition switch was wearing out.

Here's a wiring diagram of how the ignition switch feeds the fan, fuse #9. The switch has 4 poles, and the second from the left seems to be the problem child. I've added an extra red line inside that second pole, showing where the switch would be in the run position:




Note how 12V power (unswitched, hot from the battery) enters at the upper left corner. From there, it passes through splice S208 and then feeds 3 of the 4 poles in the ignition switch. In the picture above, I've also added two red dots to show the ideal points to connect your voltmeter leads to test the switch.

Notice I said "voltmeter", not "ohmmeter". I'm going to have you run a voltage drop test across that suspect switch contact. Verbose, long-winded details are here for what is possibly the world's simplest electrical test:

https://www.ford-trucks.com/forums/1...w-starter.html


Please remember the voltage drop test only works on a circuit under load. You'll need to turn the ignition to RUN and set the fan to HI to load up the circuit. Put the red voltmeter lead on wire 37, the upper red dot. Put the black voltmeter lead at switch terminal A2, wire 687, the lower red dot.

Hopefully by now, you'll be saying, "That's no lazy man's method! You've got to disassemble part of the lower dash and drop the steering column down for access. You've lied to me!" You'd be right. That was a trick to see if you were paying attention.

Rather than do all that work just to test the switch, is there a true lazy man's method? Funny you should ask, because yes there is. What if I told you that you could run the same exact test across the switch, but instead could run it from an EXTREMELY convenient access point? Well, you'd better sit down first. Look at the same diagram, only with the red dots at different points:




Put the red test lead at fuse #1. There's no need to remove the fuse, just use either of the little recesses on the top of the fuse. Note how fuse #1 is connected to splice S208 and then on to the inaccessible test point I tricked you with earlier. It's as if that run of wire is an extension of the test lead. Put the black test lead on fuse #9, the one feeding the heater. Once again, the wire run from there to the switch is like an extension of the test lead.

Voila! With barely breaking a sweat, you're running a rip-tooting voltage drop test of the highly inaccessible ignition switch. For a voltage drop test, remember the circuit must be under load, so set the ignition switch to RUN and the fan to HI. Up to 0.5VDC is an acceptable limit, but the closer the meter shows to zero the better. Ignore any readings when the circuit is not under load.

Here's my ignition switch being tested. The meter is showing over 0.9VDC, almost twice over the limit:





After replacing the switch, the same test showed roughly 0.1VDC, a huge improvement.

Here's a diagram of the fuse panel. For the second pole of the switch as tested above, I used fuses #1 and #9:






To test the first pole, keep the red lead on fuse #1 but put the black lead on fuse #15 or #18. This pole doesn't carry much current, so it will be a little trickier to load it up. Probably the best you can do is make sure the seat belt warning light is illuminated.

To test the third pole, continue to use fuse #1 for the red lead but now put the black lead on fuse #6, #11, or #14. If you have power windows, run them up and down to load up the circuit. Take your reading while the window is in motion. If no power windows, run the wipers and crank the radio.

In my situation, I only tested the second pole. After confirming it had failed, I didn't bother testing the other poles. I would suggest testing this pole first, as it carries the heaviest load and is likely to fail before the others.

If your ignition switch has failed, here are directions for replacement, courtesy of Gary:


Ignition - Gary's Garagemahal (the Bullnose bible)


Replacement isn't that hard. I think it took me only about 45 minutes, but it sure is nice to confirm it's failed before digging into it.

 

I just changed out my Ignition switch , was having the same problem .Seems it has worked great ,:but now notice I have to play with my selector switch for my AC, at times .Moving it from max to ac it comes on.TR