Both headlights out after bulb replacement

2008 Prius with halogen headlights. Car has about 230k.

• Headlamp on the driver's side burned out. Replaced the bulb and both headlamps worked fine. Should have stopped here.
• Since the passenger side would likely burn out soon and the weather will be turning colder, proactively replaced the passenger side headlamp.
• Following replacement of the passenger side headlamp, neither bulb illuminated when switched on. Both parking lights continued to work as normal. Assumed the passenger side socket somehow grounded and blew a fuse. Socket appears a bit rough from age but no burning.
• Checked all fuses - (2) 15A HP-low RH/LH, (2) 10A HP-high RH/LH. All good.
• Checked power to both sides of the fuses. None.
• Replaced the H-LP and DIM relays as part of troubleshooting. No change. No audible/feel relay click when turning on the headlights.
• Visually checked the fuseable link for any signs of failure. All looks fine but did not check voltage. Appears that there is a 40A fuseable link that feeds the headlights. Everything else in the car works ok.
• Turning on the headlamps with the switch results in the headlamp on light illuminating on the dash. Pushing the stalk away turns on the blue high beams light and pulling the stalk toward the driver turns on the blue high beams light. Switch "feels" as normal. No sign of any issues.

At a loss and would like to avoid replacing the switch if there is something else to check first. Appreciate any insight. Thank you.

 

Brief update. I removed the clear plastic cover on the fuseable link section and checked voltages on all sections above and below each fuseable link. 12+ volts verified on all including the 40A "Head Main". I checked the base of the relays and the H-LP relay has 12+ volts on two of the 4 terminals. The DIM relay, however, has 0 V on any of the terminals. That would sure seem to be the issue but I don't know what is preventing voltage from reaching the DIM relay base.

 

Thank you for your response - much appreciated. Confirmed no 12V power at the bulb sockets with the headlight switch on. In my experience troubleshooting, I've learned to start with what changed. In this case, I changed the bulb. Nothing points to that being the problem thus far, yet it would be very coincidental if the headlight switch failed or a wire became an open at the same time I changed the bulb. I have wiring diagrams coming but if there are others who have experience, any idea is appreciated! It's our son's car which a bit rough around the edges, but it's been reliable.

 

Left both new bulbs in their sockets and applied 12V to the top of the 15A low beam fuses one at a time and both bulbs light each time. Did the same for the high beams and both high beams light. All fuses good, wiring from the relays through the fuses is good. Both headlight relays are new. The DIM relay base with 0 V present is nearly 100% the problem. But why? And why after I changed the bulbs? This points to a loose or broken wire so now to pull the engine compartment fuse box up...

 

Following up to report the solution in case anyone else runs into this issue (kind of doubtful). In my last post, I mentioned that troubleshooting pointed to a loose or broken wire. First a summary of troubleshooting and learnings to get there:

1) The Body ECU shorts the signaled output to ground. This is important to understand when tracing control wires. Following the diagram below, when the headlight switch is turned on (bottom left), the Body ECU grounds the green wire on the far left to allow current to flow through the head lamp relay coil which closes the contact and sends current through the DIM RELAY in the position shown to the LH and RH Low beams. If the brights switch is made, the yellow wire is grounded, and the DIM RELAY coil energizes to close its contact and send current through the LH and RH High beams. Applying 12 V to the relays independent of the control wires illuminated the head lights so either the switch failed or the control wires are not functioning, or the Body ECU is "bad". Lights on the dash for head lights and high beams were all working so all pointed to control wires.



Dug into the large bundle of wires that runs from the firewall to the engine compartment fuse box. Uncovered a mess. Several spliced wires and broken wires (son bought car without dad's help...) Previous owner hacked this together, so we knew we were onto the problem. Which broken wire is the one affecting the head lights? Checked continuity to ground with the head light switch off and on. This is where it's important to not check for voltage but for grounding of the signal from the switch. Turned out to be the green wire pointed to below. Soldered the broken connections with spliced new wire and heat shrink around all other exposed messes. All good now! Why did this fail when installing new head lights? I don't know, but better in our garage than in the backwoods of northern MN. Lots of good father/son teaching moments with this one. Since we had it in the moderately warm garage (>20F), we also replaced both lower control arms, both front struts, both drive axles, and repaired his broken key fob with a new one from Amazon. After an alignment, the Prius drives as well as we can hope - nice and tight and an illuminated road ahead.