What do we need it for and why can't you buy a new one?

Long story: A friend bought a low mileage 1960 Miata that had been sitting for years and asked me to get it going for him, which I did. New battery, fuel pump, clutch master and slave replaced, new top and many other minor things completed. Started and run for a while, disconnected the battery and left it sitting in the garage for three months.

Reconnected the battery after sitting for three months and it turned over fine but wouldn't start. No power to the fuel pump. Jumping FP to ground in the diagnostic box did nothing and the main relay worked fine, so there was something wrong at the circuit opening relay. My God it is a chore for an old guy to get this relay out.

Went looking for a schematic of the relay to try to check it out and could find only one that seemed to fit, but wasn't sure, so I started to open the relay, and the degraded plastic fell apart. Traced everything visually and found nothing to be evidently wrong with the relay, but now it had no case. Plugged it back in and the car started. Evidently a bad contact somewhere at the relay, but now I have an open relay hanging there.

Went looking to buy a new one, and all I can find are used ones on Ebay, so I started wondering of its' purpose. Why do we need it? Can anyone give me a good reason not to bypass it?

 

If you saw my pre-edited reply please disregard, I went back and studied the wiring diagrams and I was very incorrect. I should know never to trust my memory alone.

The Circuit opener relay essentially prevents the fuel pump from running if the air meter is unplugged, and also gives the fuel pump power from the starter signal redundantly during cranking. Maybe similar to how old cars had a jumper to the coil off of the starter solenoid, for a voltage boost during cranking.

That said, they're remarkably reliable, I'd think replacing it would be the simplest and easiest route. Or hell, just concoct some sort of cover for yours made of something reasonably robust non-conductive (and ideally non-flammable.) I'll take a look under my dash, I did a lot of rewiring when I switched to a Haltech, if mine is unused you're welcome to it for the price of postage.

 

Normally, the fuel pump would run off the ECU, I'm not sure why the ancient 90 ECU couldn't do this. Since it doesn't run the pump, here's what Mazda has done. First of all, this relay is non-standard, as it can be driven both with a 12V feed and with a ground. Normally, it is grounded by a switch in the flap (AFM) assembly, ie as soon as there is sufficient air flow to open the flap, the switch makes contact and triggers the relay. However, some times, the flap may not open enough during cranking; in this case, the relay is activated from the 12V that goes to the starter, through the STSIGN fuse.

 

Ahh, that makes sense. I was puzzled at the diode in there, but now see why it’s needed. Maybe during cranking the flapper door can oscillate and wont give constant power to the pump? Agreed odd that the ECU wouldn’t run the pump directly, since it switches ground for plenty of other relays. Thanks for clarifying!

 

I pretty much understand what makes it work, but why? If the engine stops running in an accident and fuel lines are severed with the key still on, I can see the problem, but that is about it.

This would not apply to those of us that have installed a turbo system, removed the AFM, and jumped FP to Gnd in the diagnostic box. Is there any other reason for it?

 

No, I'm 99.9% sure its just a safety thing to prevent the fuel pump running without the motor running, just a kinda overly complex analog system in place of what rapidly became an ecu controlled one--probably was already ECU controlled for other cars of its day. Obviously when they went to 1.8 the pump became wholly ECU controlled.

 

My best guess, for what little it's worth, is that it's there so your fuel pump isn't running in the key-on, engine-off situation. The pump would be a significant draw on the battery in this case. With a ~32 amp-hour battery from the factory, there's not a lot of extra room for such shenanigans. Most people might not notice the sound of the constantly-running pump if they left the key in ON instead of ACC.

 

My guess is that the cost of the circuit to control the relay would have been too high in the 1980s. The '90 ECU doesn't actuate too many 12v circuits - two fuel injectors, the idle control valve, and the purge solenoid valve are all I count. I can't tell from the diagram I'm looking at, but I think that even the A/C fan circuit is an input to indicate idle-up, not the ECU controlling the fan.

I'd guess the computer was originally designed with a mechanical fuel pump in mind, but that's an unsupported hypothesis.

 

I don’t think there was a mechanical pump capable of supplying adequate pressure for EFI systems of the time, so I don’t think that’s the reason.

Perhaps they had doubts as to the duty cycle of the circuit board switches back then? Everything else the ECU controls is a much shorter duration, and cycles on and off during use. Fuel pump is the only one that would need to be 100% duty every time the motor is running.

 

The cost of one transistor is negligible. The issue is the microcontroller they could use, and whether that microcontroller had enough I/O to do the job. The A/C clutch is controller by the ECU so it can cut out at full throttle and high/low RPM, and then you have the CEL as well.

 

Am I the only one who noticed "1960 Miata"? I realize it's just a typo, but holy cow, can you imagine what the muscle car era would have been if there was a Monster Miata back then?

 

Some of us are more observant than others, not including the OP who never realized what he typed.