I have no headlights in either switch position Auto or On and no fog lights. I have running lights, Flash to pass works as long as it's held in. All other lights work, dash lights dim when in the lights on position, but no headlights.
I've tested all fuses and bench tested all relays in the top of the dash (5) above the glove box. Everything tests okay.
When I put a tester on the 2 10amp fuses for low-beam and the 5amp switch fuse there is no power but the fuses are good.
Every once in a while after pulling relays plugs switches etc., the lights will work, then they won't!!!
Is there a test for the switch to determine if that's the problem? I don't want to lay out the cash for a new one only to find that isn't the problem.

Any ideas?
Thanks

 

Set your meter and ohm it out. See what it says.

The headlight/foglight switch isn't anything other than a mere switch.

 

What am I ohming out??? The switch? There are 9 pins in the larger block and 5 in the other and I don't know what he pin positions are for which function.

 

Courtesy of SSCULLY...

 

This is a pic of my switch, I can't relate the diagram you posted to my switch.

 

Is the "Bambi mod" done on the switch ?
- This is where the ramp is recut and the circuit changed so the fog lamp switch will pull out in the park position as well as the fog lamps will stay on with the high beams.

did you test the fuses while installed in the fuse panel, with the test pin on the back of the fuse ?



You need to check for + VDC from each pin to a known good ground.
- That is one lead on ground & one lead on one pin, and then to the other pin in the test case.

Turn on the main headlamp switch to the headlamp position.
Make sure the MFS is pulled back, so the low beams are on.

Test Central Junction Box ( CJB ) fuses # 26 & #28 for + VDC on both pins.

If there is + VDC at each fuse, next test for + VDC at the headlamp bulb connector.





Relays have zero to do with the operation of the low beams, and only one comes into action with the high beams if the Daytime Running Lamps option is installed, and that relay is not in the CJB.

 

The switch has not been modded. The fuses are all good, tested. With the fuses in place there is no power to 26, 27, 28.

 

Just checked for VDC on 26,27,28 again and they all test good and lights are working. But this is typical that they work intermittently, next time I try it they might not work.
The only thing different this time is I had the switch out overnight sitting on my desk, went out this morning, and connected it and tested the fuse locations.

 

Did you tilt the steering wheel while removing the switch ?

The next step is to check that regardless of the steering wheel tilt that you have continuity from Battery Junction Box ( BJB ) fuse #3 to CJB fuses 16, 26 & 28.

You need to have the main headlamp switch in the on position, and when testing continuity from BJB #3 to CJB #16 the MFS needs to be forward ( high beams ) and BJB #3 to CJB #26 & #28 the MFS back / low beam position.

Pull BJB Fuse #3 when testing ( that is hot all the time )

Check the continuity / resistance with the steering wheel in different positions.

This could be a case of the wiring harness pulled tight and it is breaking the connection from the MFS to the truck's wiring harness.

Usually this exhibits the problem of no rear turn signals / brake lamps while the front works fine and the CHMSL / 3rd brake lamp works fine.