Main Relay

If the Main Relay is the problem, it is nearly always a problem internally with some of the soldered connections between the relay and the circuit board. You may or may not be able to see the hair-line cracks in the solder connection but flexing the circuit board will sometimes make it more visible. For those that are adept with a soldering iron, many people have successfully just heated up the solder connection and re-flowed the solder on each lead using either rosin core solder or just soldering flux. However, the Main Relay is not a particularly expensive part (39400-SL0-003 is the part number for the US 1991 part and 39400-SL0-A01 is the part number for ’95-05) and is a relatively straight-forward DIY project, so many people just replace them.

Where is the Main Relay Located?

The Main Relay is located behind the panels that are on the bulkhead behind the passenger seat. The main relay is next to the ECU but it can be on either side of it and it’s not clear if the placement is determined by the year or by the model. In the early years it should be on the drivers side of the ECU and its a small square black box that says “Relay Ass’y Main” with 2 plugs going into it. In later years (or perhaps NSX-T models), its on the other side (door side) of the ECU and reverse mounted facing the firewall.

To get access, you need to first pop off the full width trim panel just below the window

and then remove the panel behind the center of the seat.

If its not on the driver’s side of the ECU, then there’s it more involved to get to it because it’s below the “B” pillar and you’ll have to remove the large curved piece of interior trim. There’s no picture of this but it’s the piece on the left side of the previous picture. In order to do this you need to remove the kick plate on the door threshold by the side of the seat; pull the weatherstrip from the floor up to the B pillar; and remove the panel behind the passenger seat. Once that is done, remove the three screws that hold the corner panel in place which should give you access to the main relay which is sort of behind another component and the ECU wiring.

The part number for the relay for early years is RZ0101 which will work in later years but may require some modifications in order to get it to mount backwards facing the firewall.

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