Don't know if this will be useful, but here's a story about burning up main relays

Burnt through my first main relay a year ago. Thought it was because my fuel pump connector was burnt to **** on the fuel pump assembly, so replaced with NB fuel pump assembly and DW200(didn't remove check valve and I'm running a '95 NA = nowhere for fuel to return). Ran for 6 months and rebuilt the motor with 70psi constant pressure on the fuel lines. Fixed fuel pressure issue by removing the check valve inside the fuel tank from the NB fuel pump assembly, so now my fuel can return.

Second main relay burns out just after. Swap new one in and continue driving for 3 months, do a track day, race in the mountains, and the whole shebang.

Third main relay burns out. I start measuring resistances across my wires looking for any evidence of corrosion. Cutting into the harness sheath looking for anything that could be causing my excessive current draw(car apparently had rats at one point in its life, so gnawed wires wasn't out of the question). I check all of my grounds, look at obvious connectors, and reference the wiring diagram looking for something missed. By this point I'm going crazy, because everything is perfect. Grounds are perfect. Voltages are good at the proper points, and resistance along wires is near zero(>0.1ohm according to my multimeter). So I begin doubting my multimeter and order a good one. While waiting for that I drop by Rspeed telling my woes and looking for incite. They just say "We've been seeing a lot of ignition switches causing strange issues lately." I peer into the engine bay of one of the cars in the shop making sure I have grounds in the correct place only to conclude nothing is different from my car. Go home, check under the dash and boom. There it is. Burnt holes through the plastic of both the receptacle and connector.

This has got to be it right?? I'm not running anything additional. I have 2 O2 sensors, but the heaters shouldn't draw enough current to melt an 80A rated relay. As far as I can tell the DW200 isn't drawing anything much higher than the stock pump.

Sharing my story because I'm an idiot who wasted a good chunk of time chasing corrosion issues only to find my ignition switch was burnt to ****. Will update later this week with final results

 

Yeah, you don’t need added load to burn things up, resistance due to corrosion will do it too.

if you look up YouTube for “Miata Ignition Switch cleaning”, there’s a good vid. the work isn’t hard if you have a knack for small, fidgety things. I’d recommend it for anyone doing a fixer upper project or 100k maintenance. Easy afternoon project in front of the TV.

 

any news on what fixed this? Having the same issue. Looking at cleaning my ignition switch as a last resort

 

@bigcat Yea I had to put the fuel pump on an isolated circuit.
Looked at the wire going to my main relay through a thermal cam and found it to be at ~140C after just a minute or two of the car being on. The wire itself could handle the heat, but the plastic on the ignition switch and main relay could not. After giving the fuel pump it's own dedicated relay directly to the battery I have not had a main relay fail.

Stock wiring can handle ~13A. Stock operating amperage is like 10-12A. DW200 is gonna push that over + the age of the wire increases the resistance which decreases the current limit. Boom hot wire.