Pull the LT HDLP and RT HDLP 15A fuses in the UHFB and then measure 10 o'clock and 2 o'clock on the 200 ohm setting.

Relay still out, All 4 fuses back in, HL SW connected to the 9 wire connector, headlamp switch full clockwise to the manual HL on position, meter back to 20VDC scale and measure all four relay socket positions for DCV. Then back probe the white wire at the HL SW connector while its still plugged in for DCV.

The last measurement is a little different but may be our last one we need if all the others make sense. Turn off the HL SW (full CC) and remove the 9 wire connector behind the switch again, meter on 200 ohm scale, black/negative meter probe removed from battery ground and measure from the white wire at the open (not connected) connector to the relay socket locations 10, 2 and 4 o'clock and tell me what the display says for each one. So one meter test cable on the white wire, the other meter test cable on the 3 relay socket pins (no battery ground involved), one at a time for 3 readings.

Question: when your HL SW is in the auto position (full CC) and its dark out, do the headlights come on?


George

 

If this last set of measurements makes sense and confirms my suspicion then I will explain everything that we have learned and what is wrong, or at least the first thing that I know is wrong and what to do about it.


George

 

01.49 ohm @ 10 o'clock
far left 1 @ 2 o'clock

12.52DCV @ 8 o'clock manual headlight postion on
12.52DCV at white wire

No ohm readings from white wire to relay sockets.

This read as your previous post in order.

No headlights in either manual or auto. Just parking lights only come on.

 

I don't know if one has to do with the other but my power locks fuse had blown, replaced it and worked fine. Until I had hit the button on the back glass hatch and it blew my fuse but the headlights came on briefly and caused the window wash motor to come on briefly as well.
But replaced again and tried to see if headlights came on manual or auto and still no lights

 

The 4 pin relay is used to switch the +12V power to the headlights on and off. A relay is used because of the massive current draw of the headlights. "Always on" +12V is presented to 8 o'clock relay position and when the relay turns on that +12V power goes to the 2 o'clock position and on to the headlights. There is switching of the ground side for the headlights as another way to change modes but your 0.6 ohm reading at 2 o'clock earlier suggests that the headlight ground is OK and not your problem. The relay has a coil which needs to be energized to turn that relay on. One side (10 o'clock) is always connected to ground and your reading of 0.3 ohms there earlier suggests that this ground is good. That leaves the 4 o'clock position which is the +12v side of that relay coil. When you turn the headlight switch to manual on (full cw position), the headlight switch puts +12V on the white wire which routes to the relay 4 o'clock position and bingo: relay on/headlights on. The computer also can put 12V on that white wire when your in auto HL mode when it gets the right signal from the ambient light sensor.

Your HL SW does output +12V at the white wire connector in manual on HL mode but it never makes it to the relay 4 o'clock position. When you tested for continuity from the relay 4 o'clock position back to the white wire at the HL SW connector you got an open circuit which confirms that this wire is compromised/broken/corroded/... The fact that other devices controlled by +12V are also acting goofy suggests an area in a wire bundle somewhere that has been damaged: Rodents chewing, frayed from chafing, corroded from water, cut, shorted, ...

So now what? There is a step that can absolutely verify my conclusions. You would rig up a fused wire connected to battery +12V, connect everything back up including the relay. Pull the relay slightly up and VERY CAREFULLY push a stiff wire or knife blade (or similar) under the relay until it contacted ONLY the 4 o'clock position and your headlights should turn on. Let me know if you need help with this. Don't leave out the fuse. The wire run from the HL switch up to the UHFB needs to be examined which may include separating wire loom protection or cutting open electrical tape wrap. If you don't find it there, you will need to unbolt the UHFP itself and carefully lift it up enough to examine the underside. One member that I helped with a similar difficult to find problem found his corrosion there after we finished the steps similar to what we have done.

Its really best to find the damage because it sounds like you are getting a growing list of critical problems. As a stop gap measure so that you can drive the truck in the dark, we can run a new wire to temporarily get your headlamps working.

Congratulations, you are becoming an automotive electrical diagnostician, the most difficult subject for almost everybody who comes here.

Let me know how I can help.


George

 

Wow this makes sense. OK you say fused wire how do I do that and what size fuse to use?

 

I did pull the UHFB and examined it underneath the other day to check exactly for any kind of corrosion or wires exposed etc. I did not see anything that looked of that nature.

 

18 gauge wire or bigger (smaller gauge number), and a 5 amp inline fuse should do it. You have to make absolutely certain that you do not contact the 8 o'clock position because that is essentially the battery positive terminal with a wire and a 175A fuse. If you short that post to anything grounded the results would be impressive, and not in a good way.

If we have done our work correctly then the headlights should turn on when fused +12V touches the 4 o'clock position.


George

 

They have these at the auto parts store:


George

 

OK I did this and sure enough they came on but only the inner lights came on like high beams.