No headlights on 2000 blazer. Have parking lights that all works. Changed all relays and fuses including under the dash on drivers side, under hood, changed headlight switch and multifunction switch. Still nothing. I am at a complete loss to what it could be. Only thing I havent checked was wiring. Could the ambient sensor cause for headlights not to come on.

 

Both sides , hi and low beam do not work? No headlights regardless of the headlamp switch or button positions? Do you have a handheld digital meter to start probing some wiring?


George

 

Neither side work. Hi, low or drl all not working. I did try a multimeter on the headlight plugs and nothing reads 0.

 

You keep vampire hours like me. OK, no voltage to the headlamp sockets so let’s get you fixed up. Is your truck with or without the ZR2 package?


George

 

Yes, best time to work on a vehicle but quietly haha. Yes, no voltage. No zr2. Ls Blazer 4 door.

 

I am the diagnosis guy, we look for the point of failure and repair the root cause of your problem and then confirm the repair, I am not a parts cannon guy or trynostics guy. If your good with this I will stick with you until your fixed.

The headlamp circuit is actually quite complicated. It has fused, relay switched power controlled by the headlamp switch, BCM, relay and MFS switched controlled grounds, the ambient light sensor is involved for certain functions and there is class 2 serial data involved. To simplify things we will put the headlamp switch in the head position and not Auto and we will start with the low beams Since you have no power at the +12V side of the lamps and you have already checked all of the fuses (5) and relays (2) lets trace the power path first. Lets work backwards from the lack of power to the headlamp bulbs and ignore the switched grounds for now.

With the headlamp sw in the headlamp on position, do you have +12V into the LT HDLP 15A and RT HDLP 15A fuses in the UHFB? If yes then there is a fuse or wiring problem from here to the headlights

If no voltage then pull the headlamp power relay in the UHFB and test for 12V into the incoming relay contact from the 175A battery fuse and for 12V into the high side of the relay coil from the headlamp switch as well as gnd on the other side of the relay coil. If you have both 12V levels and a good ground then its the relay. You can look up relay contact layouts on the interweb. If you need help identifying the relay type and pinout let me know.

If no on the ground then its wiring to the block.

If no on the relay contact power then its the wiring from the 175A battery fuse.

If no on the 12V side of the relay coil then there is no headlamp on command from the headlamp switch. See if you have +12V into the headlamp switch on the orange wire from the HDLP SW FUSE 7 10A in the IPFB. If not, correct the wiring or fuse. If yes then see if you have +12V out on the wht wire out to the relay coil and BCM. If no then its the switch. If yes then its the wiring to the relay coil.


That should be enough to get the power side diagnosed, let me know if you need any help with these steps. I'll stick with you until your fixed if you want to diagnose the problem in this way.


George

 

Ok did these steps. Have 12+V to the relay under hood and 12+ on the fuse inside. No volts 0 read on both RTHDLP and LTHDLP.
175A fuse? Did I miss a fuse? Where is this located?

 

You said +12v to the relay under the hood. We need to see +12v at either relay pin 30 or 87 (probably 87) of the headlamp power relay AND either 85 or 86. That shows fused battery power to the relay for the headlights themselves and a call for power to the headlights at the relay coil from the headlight switch. I will start including diagrams to help clarify:





George.

 

A good article on relay pinouts:

https://www.12voltplanet.co.uk/relay-guide.html

Sorry, I lifted the ZR2 schematic in the previous post, use this one instead:

 

30 and 87 are 12V 86 and 85 are at 0

Here is what under the hood looks like.