The low beams are fed by the “HDM” fuse (fuse #14) then through the “HDM Relay” (relay #47). The HDM relay is pulled in by a 12vdc signal from the Body Control Module. Ground for the HDM relay is G101 on the schematic.
Take a look at your underhood fuse box and locate relay #47. There is probably a second relay in the fuse box with the same part number on its case...... if so, swap them to see if it fixes your problem.
If the relay and fuse are good, you may have a problem with the multi-function switch or wiring to the Body Control Module. The multi-function switch failure would be rare and a problem with the BCM Evan more rare..... let’s see what you find in the fuse box before we jump off the building.
Very good suggestions - test fuse and then relay. Unfortunately this relay is an oddball and there is no other one like it that I am aware of. See Picture #1
I will try to explain and show how to test the circuit - keep reading) Advance autoparts has it list as Low beam relay part #R6100 and is $29.99 -- fuse box labels it HDM relay. I believe its odd and pricey because it is a solid state relay and uses a transistor not a normal set of contacts. I think it is because of fuse box design and on the trailblazer this relay is modulated to provide a reduced current for DRL operation and they had a lot of problems and even a recall on trailblazers. I know, I have diagnosed more than a few of these.
I've replaced my HDM relay twice when the low beams went out and the drls did not work either when the relay went out. Its very odd that the drls would still function if both low beams are non op.
The current supply for the DRL is from the 2 separate DRL relays (one for each side) that then feed lower current to the headlights after the HDM relay.
Marked in the diagram on picture #2 in the block that says DRL controls. I don't believe wiring diagram is very accurate (not uncommon) or the HDM relay should not have caused inoperable low beams.
Testing with black lead of voltmeter (Or test light alligator clip) connected to good ground (alternator bracket is good place) perform the following
With the headlights turned on to manual low beams
1. touch the red lead (or test light probe) to the top of both sides of fuse #14 (in highlighted box next to relay 47) - if voltage (or a light) on only one side fuse is blown, if 0 volts or no light check other fuses (or touch the battery wire bolted to back of alternator) to confirm you have a good ground, if no light move the ground connection around until you get voltage. Then try again --- If there is voltage or a light on both sides go to the relay next
2. Remove relay #47
3. Insert a small male terminal into the following slot (cavities) and test as I will explain. You can use a small cotter pin or even a needle as long as if fits into the slot without force but DO NOT try to force your voltmeter lead or test light probe into the slot - it might not go in far enough or it might distort the female terminal.
4. With voltmeter still connected to ground check the slot labeled 40H (power from the fuse) and 1970 (power from the body control module) on the diagram in the highlighted box. Voltage at both locations indicates fuse box circuits and multifuction switch to body control module circuit is ok.
5. Now with the red lead still connected to slot 40H move the black lead to slot 450 (this will require another test terminal) If test light lights your ground is good and if it passed previous tests you can be confident if the wiring diagram were correct your relay is faulty. If the test light does not light on this last test you need to find and repair the ground wire.
OR You could just go spend $30 and cross your fingers --- not the way I do things or teach. As a customer if a technician did this and charged you for a relay (because he might not be able to return it) you would say they didn't know what they were doing.
if you find results other then what I listed please list your test results and I will go the next steps with you.
If you plug in a new relay and it doesn't fix it you will have to perform these tests before I can be anymore assistance other than just guessing and which part to replace next.
I hope this helped
Good luck -and let us know