Higher amp main relay`
 

What is the consensus for running a higher amp main relay. My MSM relay failed over the weekend. It was the original relay. Current mods include DW300 fuel pump and FMs Crossflow with stage 2 spal fans.

The pump takes 11-13A. Speaking to FM, the Spal fans use 29A at start up and settle down to 14A. When driving the car is cycling where the factory ECU kicks on the fans at 207F and stops somewhere around 195F. Driving on the interstate in 90 degree heat 60 mph, relay fails, car dies.

So FM is selling a NA/NB replacement main relay that is the JE16-81-811 rated at 80A discussed in a few thread here. Thinking of getting two of these but not sure if running an 80A relay would affect the wiring, seems like the OEM relays are already melting down for some users. Don't want to make things worse.

Thanks for the suggestions.

Could you take some load off by running the Fuel Pump on a dedicated relay to the battery?

80A, that's beefy! Thanks for the p/n

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I didnt think the fuel pump or fans were on the main relay from the factory.

Not sure about what is served by the main relay. Pump does not prime when relay is blown, but maybe the ECU is sending that signal to the pump. Car is definitely dead though when it goes.

The guys at FM also suggested to run a direct chassis ground to the Spal fan ground wires rather than back into black harness ground wire. The feeling was that current draws should be 100% used by the fan itself but poor grounding builds up resistance causing amperage usage to go up. I like the idea of bolstering some of wiring to heavier gauge stuff in places. Seeing that the car's electrical system was never wired for many of these modified bits, adding a bit of current carrying capacity makes sense. Thanks everyone for the suggestions.

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How is upgrading the relay going to increase amperage in any way? I think you need to look up what a relay is. It merely activates a higher power circuit (entire car) with a lower power circuit (smaller size ignition switch wires). You'd want to upgrade the alternator for more juice. But this whole conversation is kinda pointless because the stock alternator and charging circuit is already 80 amps if that's really what you're after.

See pic below. 80A stock alt for NB2

https://cimg6.ibsrv.net/gimg/www.mia...060b6b27dd.jpg

The goal is to not melt the OEM main relay. Replacing it with an upgraded piece is a good way to achieve that goal. I wouldn't touch the wiring.

This is assuming it failed because of too much current. As far as I'm aware nobody has posted up conclusive evidence of that. Also, if the charging system is already rated for 80A isn't the relay as well?

Considering the factory main relay gets hot enough for the terminals to move around in the plastic...... I'm going to go out on a limb and say Mazda got it wrong.

As stated by Sav, the goal is to prevent it happening which starts with identifying the rated amps for the unit followed by defining all the components that use this relay and their amps.


There was discussion that it was under-rated but the OP in the first thread had it blowing very frequently: https://www.miataturbo.net/engine-pe...n-relay-64489/. Reverant suggested it may be under-rated in this thread and in here: https://www.miataturbo.net/general-m...failure-91002/

But you are right, I have not wired up my replacement relay and monitored current draw at highway speeds with my radiator fans cycling. Would prefer to identify the components feeding through this relay and add up their current draw. Now if someone can just identify the amp rating for the B5B4-18-811 relay.

Nope. The charging system doesn't run through the main relay. It runs through the main fuse.

In a 2001 Miata, the main relay is fed power from the 30A "FUEL INJ" fuse, which pulls power directly from the battery (one of three fuses to do so - the other two are the 100A MAIN and 40A HEAD fuses). The main relay itself provides power to:

• All four fuel injectors
• Fuel pump relay
• CDCV (charcoal canister drain cut valve)
• Purge solenoid
• EGR boost sensor solenoid
• VCTS solenoid
• EGR valve
• Cam/Crank sensors
• MAF
• ECU at 4AF

It's not hard to figure out which item on that list is pulling the most amps.

The cooling fan relay pulls power from the 30A FAN fuse underhood, which pulls power from the 100A main fuse, which pulls power directly from the battery. Ergo, upgrading your cooling fan has no effect on the reliability of the main relay.

If you upgrade the main relay to an 80A piece, I would also be inclined to add some capacity to the 30A Fuel INJ, since melting a 30A relay indicates that the 30A fuse probably isn't far behind. Having said that, adding capacity to the fuse also means the wires should be looked at, and that's a bigger job. It's easier to just bypass the OEM fuel pump relay and draw power directly from the battery for a new one. IMO, this is the correct course of action.

Hey Andrew,

By "bypass the OEM fuel pump relay and draw power directly from the battery for a new one", do you mean a new relay on a custom circuit or send power directly to FP from battery ( perhaps with emergency cut off switch in between ) ?

Both. You remove the OEM relay and install one at the back of the car which draws power directly from the battery through its own dedicated fuse, fuse and relay sized for whichever fuel pump you are using. You then delete the OEM relay and extend the trigger wire from the ECU all the way back to the new relay located at the back of the car. This will feed more voltage to the fuel pump and eliminate a substantial amount of load on the main relay as well.

Got it. Thanks for clarifying.

Andrew,
thanks for the info on the components using the Main relay. Taking the fans out of the equation was very helpful. I still do not like running the DW300 pump on the stock relay, additionally, I have not added any aftermarket fuel pressure regulation for this pump. Because I am only running stock ECU, injectors, and 10 psi boot, I would think that the DW100 pump's 165 l/h would be completely sufficient, eliminating any stress on the amperage and stock fuel pressure regulator. So this is course of action. If anyone wants nearly 2x the fuel capacity, I would be happy to send the DW300 pump free if they pay shipping. Tired of piling up stuff that I will likely go bad before it gets used. This is usually where Aidan speaks up!

Pm'ed.

I had a 450lph Walbro fuel pump installed as part of an upgrade for e85. The main relay immediately began overheating, causing intermittent and inconvenient shut downs. Bypass of the main relay, a dedicated relay and wiring from the battery to the pump solved it.

That pump is a mean son of a bitch. With the parts store relay I initially used caught on fire. I had to buy an OEM grade 45 amp delphi relay for mine.

Again, the correct course of action is to leave the OEM main relay alone, bypass the OEM fuel pump relay, and wire a new 30-40A relay and dedicated 30-40A fuse directly from the battery to the fuel pump. If you are running a DW300 you can probably get away with going through the OEM connector, but if you are running a Walbro 450, you definitely will be drilling holes in the fuel pump hangar and running new 10awg wire through isolated lugs. I did this in my build thread if you want more details: https://www.miataturbo.net/build-thr...8/#post1382215