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

I scored on the free DW300 from Hox ( thanks again ! ) and am planning to run the DeatschWerks hardwire kit.

From their wiring instruction, seems pretty straight forward but hopefully I can confirm it here with you guys.
Chassis ground, battery power are easy. Then on my 93, the blue-red wire comes in from the COR is the 12v signal / trigger wire. COR has violet wire coming in from ST Sign fuse / main relay.

https://cimg0.ibsrv.net/gimg/www.mia...bb371e2f51.png


I assume to properly bypass fuel pump relay, we need to (1) snip the violet wire from ST Sign fuse where the blue cut mark is before COR but after the split to ECU, and reroute / extend that wire to the new DW fuel pump relay as "harness side lead".
Then (2) snip the the blue-red signal wire coming from fuel pump connector to COR where the yellow cut mark is and connect it to the new DW fuel pump relay as "power to pump".

What I'm not sure on is if I need to do anything with that green wire going from COR to AFM. I assume no, as COR is no longer controlling fuel, so any communication between COR and AFM is just redundant noise.

That DW hardwire kit is suuuuuuper cheesy for the price. China relay with a cheap plug and some wires that they have laid out to not follow the colors vs. purpose that you'd expect.

Easily duplicated on your own with not-stupid wire colors for about $10.


2/10 would not buy again.

It's just as effective, and somewhat easier, to leave the oem pump relay in place, and use the oem fuel pump wire as the coil driver for the new relay located between the battery and the pump.

This will not decrease the effectiveness of the modification in any way, and it saves the user from having to run additional wire and /or Jimmy the oem FP relay socket. Just leave everything under the hood alone, and wire the new relay's in place of where the stock fuel pump would normally be.

Thanks for the tip Concealer. Looking on Amazon, I see well reviewed 30/40 amp automotive relays with built in fuse for about $16 for 5 of them including fuse and connector. I'll need to provide my own wires but that's not an issue. I'll go that route instead. The extra relays will be nice for future projects.

Joe, you mean just splice the new relay "inline" behind ( and in conjunction with ) COR by cutting only the 12v signal line ( blue/red ) in my diagram above to use as "harness side lead" for the new relay ?

Yes. Forget about messing with the wiring up front. Cut the blue / red wire back at the tank, and hook it up to one side of the coil of the new relay. Ground the other side.

Run battery + to the NO terminal of the new relay, and run the C terminal of the new relay down to the fuel pump. This can all be mounted in the back, right by the tank.

However reliable the OEM fuel pump relay is, a 4" piece of 18awg and two spade connectors is likely an order of magnitude more reliable. A single butt connector and some heatshrink is even more reliable still.

I think the new relay for fuel makes a lot of sense. I had thought about it but my car is a bit different and the battery is up front and the kill switch next to it.
The 80amp Bosch relays are quite cheap. I bought 2 and keep 1 as a spare. Works like a champ!

https://www.miataturbo.net/race-prep...lay-fix-94026/

I also concur with others, had overheating/melting Main Relays until I installed the DW fuel pump hardwire kit. Haven't had a failure since.

This is also relevant info for those of us running the drop in RX7 100amp alternator.