Project Lexus SC300 Road Racer: Part 11 – Engine Bay Wiring


A Littelfuse mini power distribution module (HWB18).

While I don’t really believe it, the specifications for the cooling fans suggests that each fan draws more than 20 amps. That’s potentially inrush current when the fan is first started, but, either way, better safe than sorry. I probably could’ve hooked it up to a meter and measured it, but this was an opportunity for learning.

As the Racepak only has 20 amp outputs, and I wasn’t about to blow several of them just for some silly fans (pun intended), it was decided to use some “old school” fuses and relays. However, this Littelfuse unit is pretty modern.

The HWB18 is an IP67 rated assembly that uses both mini 280-style fuses and relays. It can handle up to 30 amps per “circuit” and has 18 cavities that can be used. It also uses Delphi Metri-Pack 280 terminals, which are very common.

Ultimately this little guy is going to have two separate circuits in it, with one powering each fan. A signal from the Haltech ECU will activate the relay coils, which will then power the fans.


The Mini relay and fuse.

These relays are available in a number of configurations and current specifications. In fact, this style of mini relay and fuse panel has become popular for wiring entire cars. Cooper-Bussmann makes larger panels that support dozens of relays and fuses. A power distribution system like the Racepak Smartwire is infinitely more configurable and usable, but, for a simple solution, you can really do quite a good job with these little fuses and relays.


Here the incoming signal from the Haltech Elite 2500 is spliced out to two terminals, with one for each fan circuit.


A much bigger 16-gauge milspec wire is used to provide power to both relays and to ultimately power the fans.

We could have used several 20-gauge wires, but that would look ghetto. H Craft had the 16-gauge laying around, so we used some.



The relays actually can only be inserted in one way in the HWB18. Don’t make that mistake! Although it’s easy to correct.

Leave a Reply

Your email address will not be published. Required fields are marked *