# An interesting HID ballast failure mode.



## Bitter (Aug 4, 2022)

One of the OEM Denso ballasts in my Toyota (Ballast should be from around 03-05 year range) has had a rather fascinating failure. In what could only be described as the most Toyota way for something to fail it's gone faulty by working 'too good'. 

Earlier this year I had a passenger side HID bulb fail on me, it went pink and was fairly used up. It had been fine the fall before when I parked the car. Both bulbs were around 5yrs old at this point but had about 1/2 the normal use since it's a fair weather car only. Ok, things happen, one bulb ate it way early not a big deal. I replaced the pair of them and moved on with my life. Fast forward to roughly now and I'm doing a preventative radiator replacement since it's 20yrs old and contrary to what everyone says plastic does not last forever. I notice as I'm running the car in the garage to purge air and circulate the cooling system that the passenger side bulb is a bit more yellow than the driver side at startup and while running, it seems to start up brighter and hit full brightness faster, and the tone of hum from the ballast is lower than the other side. Knowing what I know about HID lights I make a guess and then confirm with my amp clamp, passenger side ballast is drawing 4.3 amps at around 12.9 volts for 55W input power! Driver side is doing about 3.3 amps for about 42W input power, these should be outputting 35W. Now I know why one bulb died early. Luckily I can't see any damage to the headlight reflector bowl and the beam pattern on my garage wall appears to be normal. I assume some resistor inside the ballast has failed and is fooling it into running more current than it should be. I'm getting a new pair of used but newer than mine ballasts since lights always go in pairs and I don't want to take the bumper off again this year.


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## PhotonWrangler (Aug 6, 2022)

Bitter said:


> One of the OEM Denso ballasts in my Toyota (Ballast should be from around 03-05 year range) has had a rather fascinating failure. In what could only be described as the most Toyota way for something to fail it's gone faulty by working 'too good'.
> 
> Earlier this year I had a passenger side HID bulb fail on me, it went pink and was fairly used up. It had been fine the fall before when I parked the car. Both bulbs were around 5yrs old at this point but had about 1/2 the normal use since it's a fair weather car only. Ok, things happen, one bulb ate it way early not a big deal. I replaced the pair of them and moved on with my life. Fast forward to roughly now and I'm doing a preventative radiator replacement since it's 20yrs old and contrary to what everyone says plastic does not last forever. I notice as I'm running the car in the garage to purge air and circulate the cooling system that the passenger side bulb is a bit more yellow than the driver side at startup and while running, it seems to start up brighter and hit full brightness faster, and the tone of hum from the ballast is lower than the other side. Knowing what I know about HID lights I make a guess and then confirm with my amp clamp, passenger side ballast is drawing 4.3 amps at around 12.9 volts for 55W input power! Driver side is doing about 3.3 amps for about 42W input power, these should be outputting 35W. Now I know why one bulb died early. Luckily I can't see any damage to the headlight reflector bowl and the beam pattern on my garage wall appears to be normal. I assume some resistor inside the ballast has failed and is fooling it into running more current than it should be. I'm getting a new pair of used but newer than mine ballasts since lights always go in pairs and I don't want to take the bumper off again this year.


That is interesting. Like you, I suspect a current-setting resistor has drifted or outright failed. Good choice to swap both of them out.


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## Bitter (Aug 6, 2022)

Lights always go in pairs, also the vendor I trusted for genuine used parts only sells them in pairs. I did open the cover on the ballast and eyeball the inside and I can't see anything amiss but I can only see just a little bit of the innards. It's got some very thick sticky conformal coating and I'm not digging into that jelly.


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