Build and Install a Subaru ECU Box

Here's how you create a water-proof protective box for your Subaru ECU, so you can safely put the ECU in the engine bay.  If you go this route, you need to have it protected from the elements and this design is rock solid in that respect.  This page covers the items you need to buy, the tools to do the job, and steps to get it done.  Have fun and good luck!

Things You Need

1. Water-proof box (http://www.polycase.com/yq-series)
2. Bracket(s), screws, bolt, and washers to secure box to van
3. Diagnostic Connector for OBD2 port
4. Seal-All - water-proof, gas & oil resistant adhesive
5. Large grommet (big enough for the wiring harness to fit through)
6. Self-fusing Silicone tape - to close up anything you feel the grommet missed  (http://www.delcity.net/store/Self!fusing-Silicone-Tape/p_77310)

Tools

1. Dremel and a bit to cut metal/plastic

2. Pliers

3. Hand Drill

4. Metal drill bit
5. Hole saw bit, approx. 3-1/4"

Choosing the Right Box

Protecting your ECU starts with the proper enclosure.  Water-proof, air-tight, and made of strong plastic, so it is easy to modify.  I chose the YQ Series box from Polycase at approx. $65.

http://www.polycase.com/yq-series

The size is 8" x 10", which fits in the rear passenger tail light compartment with minor modifications.  This box size is required to fit 1999-2004 Subaru ECUs.  Later years can use an 8" x 8" box, but for my 2001 Subaru Legacy ECU, I needed the 10" length.  Besides being water-proof, it has high temperature ratings, and has the option for a clear cover, which I opted for, so I can see inside my enclosure for condensation or even put in a thermometer to spot check heat.

Japanese-manufactured ECUs don't really overheat nor require air flow.  Subaru installed them under the floor (e..g, no air flow) and right by the floor board heat vents (hot!), so honestly, I am not worried about the ECU overheating or a lack of air-flow.  But, for $5 more, why not get a clear cover?

OBD2 Connector

One of the benefits of having the Subaru ECU in the engine bay is the easy access you have to the OBD2 connector to scan & reset codes.  You need to decide how to access the OBD2 connector.  You can run the wires and connector out of the box and leave it exposed in the engine bay.  However, I chose to keep the OBD2 wires and connector inside the box, protected from the elements and put a diagnostic port on the box. Here's how I did it.

Use a dremel and cut opening for Subaru OBD2 Diagnostic Connector

Cut the Diagnostic Connector to fit the hole and glue with Seal-All

Subaru Diagnostic Connector cut out of dash from donor car

OBD2 diagnostic port on box

OBD2 connector snaps into place once the port is in the box.

View of port from inside the ECU box

Engine Bay and Box Modification

To fit in the rear passenger tail light compartment, both the box and the van need minor modification.

Remove the tail light for access to the compartment.  This exposes the fin you need to modify and the bottom where you will be drilling a hole for the bracket that secures the box to the van.

Use a dremel to cut the fin and then bend it back with a pliers.  This will make room for the box to slide in better.  Be careful not to cut the tail light and turn signal wires.

At this point, if the box does not fit, also use a dremel to cut the side of the box to shave off enough for it to fit in the compartment.

If you haven't already, drill a hole into the box to fit the wire harness through and put a grommet in the hole to ensure the box remains water-proof.  The hole size also depends on the size of the grommet and harness.  I got a grommet from a salvage yard.  Get a large one like from a truck steering column - where the steering column goes through the firewall.  This will be large enough for the Subaru wiring harness to fit through the box. Wash it, then put Vaseline on it and the grommet will come back to life like brand new.

3 - 1/4" hole saw bit used for my box & grommet

Install the footings on the case and drill a hole as shown (where my finger is pointing :) to secure the front with a self-tapping screw.  If needed, add a washer to make it secure based on the size of the footing hole.

Wire harness grommet (on left) & footings (bottom) for securing box to van

From the rear tail light opening, drill a hole for the "L" bracket in the van. 

Drill a hole into the box for a bolt for the "L" bracket.

Secure the "L" bracket to the box with a bolt/nut and then use the Seal-All, to ensure the hole and bolt are properly sealed from the elements.

Don't secure the box until the wiring harness is installed and the box is finally ready to be installed.

Finally, if you are super paranoid, you can finish up by using sealant tape to wrap the wire loom coming out of the grommet on the inside and outside.  This self-fusing tape will form an extra 100% water-proof, dust-proof layer against the elements.

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