# Low voltage (~9.3V) on car headlight - any recommendations? **URL:** \ **Category:** Factual Questions\ **Created:** [October 19, 2018, 4:33am UTC](https://boards.straightdope.com/t/low-voltage-9-3v-on-car-headlight-any-recommendations/823188 "2018-10-19T04:33:06Z")\ **Posts on this page:** 17\ **Page:** 1
**Author:** ![Nanoda](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Nanoda](https://boards.straightdope.com/u/Nanoda)\ **Post date:** [October 19, 2018, 4:33am UTC](https://boards.straightdope.com/t/low-voltage-9-3v-on-car-headlight-any-recommendations/823188/1 "2018-10-19T04:33:06Z")
My 2011 Ford Fusion is only registering ~9.3 V on the headlamp connector, when I’m imagining it should be up around 12. This is a problem as that doesn’t seem to be sufficient to activate the aftermarket HID lights currently installed. I’m not the best with cars but I dabble with electrical stuff, so is there anything anyone can recommend I check? Other info: I didn’t install this system, and it worked for about a year after I got the car. Also while they don’t turn on at all now, the problem started as an intermittent one, so the problem isn’t blown bulbs. I measured the low voltage on the connector, but also to the battery terminal which I gather indicates that it _isn’t_ a bad ground? ---
**Author:** ![kaylasdad99](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kaylasdad99/32/3398_2.png) [@kaylasdad99](https://boards.straightdope.com/u/kaylasdad99)\ **Post date:** [October 19, 2018, 4:36am UTC](https://boards.straightdope.com/t/low-voltage-9-3v-on-car-headlight-any-recommendations/823188/2 "2018-10-19T04:36:58Z")
Which side headlamp? Is it the same voltage on both sides? ---
**Author:** ![Rocketeer](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rocketeer/32/4437_2.png) [@Rocketeer](https://boards.straightdope.com/u/Rocketeer)\ **Post date:** [October 19, 2018, 5:00am UTC](https://boards.straightdope.com/t/low-voltage-9-3v-on-car-headlight-any-recommendations/823188/3 "2018-10-19T05:00:49Z")
If your Fusion’s battery measures 9.3 volts across the terminals with the engine not running, it’s probably bad; normal should be about 12 volts. If it measures 9.3 across the terminals with the engine running, there’s something wrong with the charging circuit (alternator); normal should be about 14 volts. If the battery voltage is normal, but voltage at only this particular headlight is low, I’d suspect a bad connection somewhere. ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [October 19, 2018, 10:16am UTC](https://boards.straightdope.com/t/low-voltage-9-3v-on-car-headlight-any-recommendations/823188/4 "2018-10-19T10:16:28Z")
Measure the battery voltage - at the battery - when the car is _not_ running. And then measure the battery voltage when the car _is_ running. Report back with the results. ---
**Author:** ![jz78817](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jz78817/32/1028_2.png) [@jz78817](https://boards.straightdope.com/u/jz78817)\ **Post date:** [October 19, 2018, 10:39am UTC](https://boards.straightdope.com/t/low-voltage-9-3v-on-car-headlight-any-recommendations/823188/5 "2018-10-19T10:39:53Z")
First thing you should do is throw that aftermarket HID junk in the trash where it belongs. The wiring in that car was not designed for it and has now been overstressed. You likely have an interim connection between the headlamp assembly and the power distribution box which overheated and is much higher resistance than it should be. Sorry, but you’re going to need someone skilled in electrical repair to trace back all of the circuits. And when that’s fixed, put the correct halogen bulbs in there and don’t repeat the previous owners mistake. ---
**Author:** ![Nanoda](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Nanoda](https://boards.straightdope.com/u/Nanoda)\ **Post date:** [October 20, 2018, 1:25am UTC](https://boards.straightdope.com/t/low-voltage-9-3v-on-car-headlight-any-recommendations/823188/6 "2018-10-20T01:25:07Z")
Thanks for the replies! The 9V is while running - the voltage _not_ running is lower - 8.4V. The battery itself is 12V not running, 14.3V running (pretty sure this is normal?). In this setup, it seems that the lights are relay-activated by the driver headlamp (the current for the HID lights themselves is provided by separate wiring directly to the battery.). The passenger side is going to take a bit to get to but I’ll see if I can measure that somehow. ---
**Author:** ![BobBitchin](https://avatars.discourse-cdn.com/v4/letter/b/7ea924/32.png) [@BobBitchin](https://boards.straightdope.com/u/BobBitchin)\ **Post date:** [October 20, 2018, 2:17am UTC](https://boards.straightdope.com/t/low-voltage-9-3v-on-car-headlight-any-recommendations/823188/7 "2018-10-20T02:17:26Z")
I would read the manual. It must have test procedures. How do we know what voltage the lamp wants to see? Maybe 9v is right. ---
**Author:** ![excavating\_for\_a\_mind](https://avatars.discourse-cdn.com/v4/letter/e/e8c25b/32.png) [@excavating\_for\_a\_mind](https://boards.straightdope.com/u/excavating_for_a_mind)\ **Post date:** [October 20, 2018, 2:22am UTC](https://boards.straightdope.com/t/low-voltage-9-3v-on-car-headlight-any-recommendations/823188/8 "2018-10-20T02:22:05Z")
> [@Nanoda](#): > > Thanks for the replies! > The 9V is while running - the voltage _not_ running is lower - 8.4V. The battery itself is 12V not running, 14.3V running (pretty sure this is normal?). … This pretty much confirms **jz78817** ’s diagnosis. ---
**Author:** ![K2500](https://avatars.discourse-cdn.com/v4/letter/k/f6c823/32.png) [@K2500](https://boards.straightdope.com/u/K2500)\ **Post date:** [October 20, 2018, 2:47am UTC](https://boards.straightdope.com/t/low-voltage-9-3v-on-car-headlight-any-recommendations/823188/9 "2018-10-20T02:47:57Z")
> [@excavating\_for\_a\_mind](#): > > This pretty much confirms **jz78817** ’s diagnosis. Not if the factory head light wiring is now only a relay trigger, as mentioned. ---
**Author:** ![jz78817](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jz78817/32/1028_2.png) [@jz78817](https://boards.straightdope.com/u/jz78817)\ **Post date:** [October 20, 2018, 6:52am UTC](https://boards.straightdope.com/t/low-voltage-9-3v-on-car-headlight-any-recommendations/823188/10 "2018-10-20T06:52:04Z")
> [@K2500](#): > > Not if the factory head light wiring is now only a relay trigger, as mentioned. Then the relays are junk. Seriously, these aftermarket HID kits are garbage; they may have capsules from legit sources but the rest of the kits are cheap junk. Plus, the optics are all wrong for HID capsules. You should never, EVER put HIDs or LEDs in housings meant for halogens. Throw the shit out, put the correct halogens in, and call it a day. ---
**Author:** ![Gary\_T](https://avatars.discourse-cdn.com/v4/letter/g/3ec8ea/32.png) [@Gary\_T](https://boards.straightdope.com/u/Gary_T)\ **Post date:** [October 20, 2018, 3:01pm UTC](https://boards.straightdope.com/t/low-voltage-9-3v-on-car-headlight-any-recommendations/823188/11 "2018-10-20T15:01:03Z")
> [@kaylasdad99](#): > > Is it the same voltage on both sides? This is a pivotal question for any serious attempt to fix the issue. ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [October 20, 2018, 3:17pm UTC](https://boards.straightdope.com/t/low-voltage-9-3v-on-car-headlight-any-recommendations/823188/12 "2018-10-20T15:17:58Z")
FWIW - I put an aftermarket HID kit in my Santa Fe. There was no issue with voltage or current, but the beam pattern was terrible. They were bright, but had bad off-axis hot spots. I’m sticking with tungsten lamps until someone comes up with a better HID capsule design. ---
**Author:** ![K2500](https://avatars.discourse-cdn.com/v4/letter/k/f6c823/32.png) [@K2500](https://boards.straightdope.com/u/K2500)\ **Post date:** [October 20, 2018, 3:29pm UTC](https://boards.straightdope.com/t/low-voltage-9-3v-on-car-headlight-any-recommendations/823188/13 "2018-10-20T15:29:00Z")
> [@jz78817](#): > > Then the relays are junk. Seriously, these aftermarket HID kits are garbage; they may have capsules from legit sources but the rest of the kits are cheap junk. Plus, the optics are all wrong for HID capsules. > > You should never, EVER put HIDs or LEDs in housings meant for halogens. Throw the shit out, put the correct halogens in, and call it a day. Maybe so, I don’t have much experience with the product in question. I’m simply addressing the fact that it’s unlikely that the HIDS overstessed the factory wiring since the factory wire in question was used only to drive a relay(per OP). Of coarse if the relays are junk, and if the headlamp circuit wasn’t properly protected, then I could see a relay or it’s coil failure causing some problems. I’ve got led head lamps in my Harley and I like them. I can’t imagine what would make them so undesirable that I should switch back to halogens except maybe the optics you mentioned. ---
**Author:** ![Nanoda](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Nanoda](https://boards.straightdope.com/u/Nanoda)\ **Post date:** [December 19, 2018, 3:50am UTC](https://boards.straightdope.com/t/low-voltage-9-3v-on-car-headlight-any-recommendations/823188/14 "2018-12-19T03:50:34Z")
In case anyone besides me cared, just to wrap this up: it was the smart junction box ([random purchase link](https://www.autopartscheaper.com/products/2010-2012-ford-fusion-fuse-central-smart-junction) with photo). It’s like your car’s motherboard, and lives under your dash with 1/2 a dozen big cables going to it and holding 1/2 the fuses for your car in it. It does things like control how your interior lights dim fancily as you start it, locks your doors when you start driving, and turns your headlamps on and off according to your switches, etc. I got a used one from a U.S. ebay dealer with exactly the same model #s on the sticker for 25$ and other than feeling like I’d done a Cirque du Soleil routine after spending at least an hour upside-down under my dashboard, it went pretty well. (Though as per the cautions above, I _did_ also replace the interface module to the HID lights with an [updated one](https://www.theretrofitsource.com/canbus-morimoto-standalone.html)) Thanks again for all the tips! ---
**Author:** ![bob\_2](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bob_2/32/3341_2.png) [@bob\_2](https://boards.straightdope.com/u/bob_2)\ **Post date:** [December 19, 2018, 10:36am UTC](https://boards.straightdope.com/t/low-voltage-9-3v-on-car-headlight-any-recommendations/823188/15 "2018-12-19T10:36:15Z")
I don’t know what the rules are in your state, but in the UK, fitting HID lamps into a headlight assembly not designed for them is illegal and the car would fail its annual test. Not to mention annoying other road users by dazzling them. > [@](#): > > In the Department for Transport’s (DfT) view it is not legal to sell or use after market HID lighting kits, for converting conventional Halogen headlamps to HID Xenon. If a customer wants to convert his vehicle to Xenon HID he must purchase completely new Xenon HID headlamps. The reason for this is that the existing lens and reflector are designed around a Halogen filament bulb, working to very precise tolerances. If one places a HID “burner” (bulb) in the headlamp, the beam pattern will not be correct, there will be glare in some places and not enough light in other places within the beam pattern. ---
**Author:** ![ohiomstr2](https://avatars.discourse-cdn.com/v4/letter/o/82dd89/32.png) [@ohiomstr2](https://boards.straightdope.com/u/ohiomstr2)\ **Post date:** [December 19, 2018, 11:36am UTC](https://boards.straightdope.com/t/low-voltage-9-3v-on-car-headlight-any-recommendations/823188/16 "2018-12-19T11:36:26Z")
[![](https://img.youtube.com/vi/B72cwOYhe9E/maxresdefault.jpg "Ford Truck - One Headlight Not Working") ](https://www.youtube.com/watch?v=B72cwOYhe9E) “Test don’t guess” ---
**Author:** ![jz78817](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jz78817/32/1028_2.png) [@jz78817](https://boards.straightdope.com/u/jz78817)\ **Post date:** [December 19, 2018, 3:42pm UTC](https://boards.straightdope.com/t/low-voltage-9-3v-on-car-headlight-any-recommendations/823188/17 "2018-12-19T15:42:33Z")
> [@bob\_2](#): > > I don’t know what the rules are in your state, but in the UK, fitting HID lamps into a headlight assembly not designed for them is illegal and the car would fail its annual test. Not to mention annoying other road users by dazzling them. it’s illegal here too, but never enforced.