2010 FX4 has 912 bulbs in the high mount brake light/cargo light and 921 bulbs in the backup lights. Same bulb, slightly different wattage, I want to say the 921 adds about 5-7 watts or so. I swapped out the 921s and 912s in the cargo lights and the backups for silverstar 921s for whiter light and they look great.

Cargo's are much whiter light and are just barely brighter with the 921s. Any reason not to use 921s in the cargo lights that anyone can point out? Seems like a good swap, heat can't be that much different, wondering why Ford went with the 912s...

 

Heat. It will eventually melt your housings if you keep the 921s in there

 

I was just thinking about that additional heat. There is a reason why they put 912's in there instead of 921's.

 

Do we know that it will melt the housing, or it potentially could? Having the bulbs in for a few days now, it doesn't even seem warm. The 2010 housing seems pretty open for heat dissipation. Or are we talking about proximity to the plastic? You'd think I would have already seen an issue. These are pretty low wattage bulbs and it doesn't seem any different than the tails.

 

Auto manufacturers do everything for a reason: cost. A 912 runs at 12.5W. A 921 runs at 18W. While that is a small margin, the 921 costs more, draws a higher amperage (therefore needing a thicker wiring harness), and heats more (thicker plastic). Without taking a look at it or doing some temp tests, I would just go ahead and say it has the ability to and over time, it might melt or begin to deform.....but the honest likelihood of that happening is small. But for an automaker producing 1,000,000+ vehicles, those little things add up in the cost department. Lastly, I could be wrong, but the CHMSL can only account for XX% of brake lamp power produced. So in summary, the above reasons are a shortlist of why Ford chose 912 over 921.

 

Good post, Raptor05121, thanks for the input. I think I'll put the 912 back in the stop lamp and monitor the situation with the cargos having 921s. Might be a different situation on a 100 degree day, versus 20 below (its coming soon). Ultimately, it might be better to put LEDs in there anyway.

 

Should you be brave enough to cut up a set of taillights, one single Philips Luxeon Rebel LED will outpower any non-halogen incandescent bulb on your truck

235lm @ 700ma

 

I haven't studied a 2010, mine's a '08, but I'd venture a guess that the change in bulb may have more to do with cost of the bulb and cost (size) of the wiring. Copper's a whole magnatitude more expensive than the plastic housing.

Unless it's been verified that the 2010 housing is significantly thinner and/or the bulbs are much closer to the housing.