OptionXIII

I got lucky the other day when my fuel pump relay failed at just the right time - while I was working on and diagnosing the fuelling system. I isolated the issue to the fuel pump relay by unplugging it and using a jumper wire between power in and power out. The fuel pump relay failing is a fairly well known problem with some off the shelf solutions. They can be pricey for what you get. Here's my cheap fix that should not only fix the issue, but improve the general reliability and ease of repair in the future.

First, let's understand the stock layout. Below are the wiring diagrams from the Mellens Miata manual for my year, 2001.



Following the flow of electricity to the fuel pump, the 12v feed goes from the battery to the 30a Fuel Injection fuse, to the Main Relay. Once that relay is triggered, power can then go to the fuel pump relay. The same power out wire from the main relay feeds to both the 12v power feed of the Fuel Pump Relay, and the relay trigger circuit. The ECU also has fuel pump control via controlling the ground of the fuel pump relay.

Failure of both the main relay and the fuel pump relay is not uncommon on NA/NB Miatas and seems to get worse with every later revision. The factory relays just do not seem to be rated well enough to hold the current asked of them. Worse yet, they're pretty expensive and hard to find when they do go out, so most people worried about failure carry a spare. As per this post from Savington, the fuel pump is far and away the largest draw on the Main Relay.

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Higher wattage rated relays are available in the stock relay style for both the Fuel Pump and Main Relay, but $100 for a 12v relay is absurd. While the Main Relay format would be difficult to change due to its location in the engine bay fuse box, the Fuel Pump relay is on its own electrical connector, hanging in open space, and uses blade style terminals just like a common Bosch style square relay. Those are dirt cheap and readily available in comparison to the stock units. The quick fix is obvious here.

I depinned the stock connector and repinned it into a Bosch connector, except for the 12v power feed wire (one of the two White with Blue Stripe wires). That was insulated with electrical tape, tied back to the rest of the harness, and replaced with 12ga wire that went direct to the battery.


Confirming that the terminals will fit the Bosch relay pins.


Unfortunately, Mazda reinvented the wheel and did not include these easy to depin tabs on the stock terminals. I tried a bunch of depinning tools and still struggled with it. Additionally, they won't lock into the new connector body I sourced for this project. The proper thing to do is replace the wire terminals. As a test run, I just crimped the wires tighter and pushed them on anyways.


Here's the new 50 amp rated Bosch style relay, hanging in the stock location using a carpet pin. The thick blue wire is my new 12v feed that goes direct to the battery.


I ran the new wire along the stock wiring harness along the drivers side sill, up the quarter panel, in front of the fuel tank...


...and back to the battery. When I have more time I'll replace both of these fusible links with a proper standalone fuse block. I used a 30 amp fuse to match the stock unit. It will have a bit more headroom due to the removal of all the other powered devices on the main relay.

Here's the pinout for the common Bosch relay.



Pin 30: 12v feed direct from battery. This is the new wire you run. On the stock layout this is one of the two wires that is White with a Blue stripe.
Pin 85: Relay trigger, ground side. Connects to the ECU. Wire is Red with White stripe, thin gauge.
Pin 86: Relay trigger, positive side: Connects to Main relay. Wire is White with Blue stripe.
Pin 87: Power out to load. Connects to the Fuel Pump. Wire is Red with Blue stripe.
Pin 87a: Not applicable

Pros:

• Easy and low cost. A few feet of wire, one fusible link, a 30a fuse, some terminals, a new 30a or higher rated relay and matching connector block are all you need.
• Bosch style relay is cheaper and readily available at higher power ratings.
• Removes the main relay from the Fuel Pump power feed circuit, reducing the load on it and improving its reliability as well.
• Cuts no wires and is reversible.

Cons:

• Extra wire length adds weight and electrical resistance. The stock Red/Blue power out wire to the fuel pump is still going to have higher than necessary resistance.
• Removes the main relay from the power feed circuit (yes, its a positive and a negative). I believe the stock setup is designed to provide two separate methods of shutting off the fuel pump - ECU control via the ground, and the main relay (car key activated) control of the power feed to the fuel pump relay. So on a stock car if the Fuel Pump relay fails closed by welding itself in place, the Main Relay would still be able to disconnect unless it also welds shut at the exact same time. As I have it set up, if the relay welds shut it will continue to supply power until the fuse is pulled. I've never seen a relay fail like this in a 12v application. A standalone switch or the master cutoff switch would eliminate this concern on a real racecar.

I'm very pleased with how this turned out compared to a $100 upgraded relay or a $100 Trackspeed rewire kit. Since I don't really need more power to the pump, this checks all the boxes for me. Cars with a massive fuel pump should have the entire circuit rewired to have the relay as close to the battery as possible, as the Trackspeed kit does.

TL;DR I'm pretty sure I just replicated the V1 Trackspeed kit, but with the relay in the stock location instead of by the gas tank.