Main Relay Location
 

Hi, just had the AA out 2 hours as my car wont fire (1992 Eunos).

They decided it was the main relay, but seemed very confused as they couldnt find it anywhere to check it, even looking at the scehmatics.

Looks like i'll ahev to sort it out myself...

Oh and soemone did point me towards the INJ relay (injector relay) in the main fuse box, saying this is the only main relay - but is theire a 2nd *Main Relay* if so wher is it?!

Perhaps you're referring to the "Circuit Opening Relay" which drives the fuel pump? On a left-hand drive car this is located under the dash, near the steering column. I'm not sure whether it's in the corresponding location on a UK car...

Have you been able to determine exactly why the car won't run? At the very least it'd be helpful to know whether the fuel pump is coming on, whether you get spark when cranking, etc.

Have your PM but this may be a bit hard to answer. There are 3 different relay locations and slightly different functions between US, European, and UK Miatas. I don't have a manual. Believe your fuel circuit opening relay is located still below the steering column, which is on the opposite side of the US Miatas. For this problem the only 2 main relays to be concerned about is the fuel injection and Fuel Ciruit Opening Relays (COR).

Little more info is needed. Does the engine fire or sputter at all?
Is the timing belt belt broken? Remove the oil fill cap and look to see if the cam is rotating while a helper turns the engine over. Are all the fuses in the engine compartment good?? ... the number 1 cause for your symptoms. Any error codes?

Is the fuel pump running? Strap the F/P and GND in the diagnostic box, switch ignition switch to ON, remove gas cap and listen for the humming of the fuel pump. If you hear it then most probable the COR is good .. not 100% accurate. If pump is running, use a stethoscope or long screw driver on the injectors to listen for popping while a helper cranks the engine. Listen on the cam cover first so not misled by HLA chatter. If unable to hear them clicking .. and all the other fuses are good and other tests working leads to a CAS, ECU, FI relay, or associated wiring problem.

thankyou for replies, I will try and be a bit more helpful... I'll answer the other bits when i have soemoen to help!

-The engine turns over but does not splutter at all.
-The timing belt is fine
-All the under bonnet fuses seem ok
-Error Codes: cannot be read on my Eunos (no light)


Sidenote: I dont beleive the injectors or the fusebox was getting any power according to the breakdown service, he forgot to check if the ecu was getting power.

I have pulled both the mentioned relays out of the car...


thanks

The attached file may be of help to you. It's the electrical wiring diagram for a US-spec '92 Miata, yours should be very similar.

You can determine whether the injectors are getting power by removing the electrical clip from any one of them and measuring with a voltmeter between the white/red wire and ground (earth) with the key switched to the "Run" position. You should see 12-14 volts. If you do have voltage here, that tells us that the Main Relay and the INJ Fuse are OK.

If you have access to a timing light, this would be useful to verify that you're getting spark. Attach it in the normal way, and see if it lights up while a friend cranks the engine.

There's no good way to electrically test the circuit opening relay, but as Pop said, place a wire between the F/P and GND terminals of the diagnostic box under the hood, which should cause the fuel pump to operate. If it's quiet where you're working, you should actually be able to hear fuel running through the fuel rail and stock pressure regulator, or you stick your ear to the tank.

It's possible, though less likely, that it could be something like the cam angle sensor, or of course even just a bad wire at the ECU. In my car, a few of the wires that go to the ECU actually broke right off one day- they were badly corroded from having gotten wet.

thanks for your inputs, lost without my car and the garages charge a fortune for recovrey in the UK!

Update:

- Cam is turning
- I could hear a slight clicking from the fuse box area with igiontion to "on"
- I removed spark pluglead, held it close to engine body and saw no spark. Repeated with whole spark plug coonected and saw no spark
- Connect f/p and gnd in diagonstic box, switched engine to "on" could hear no pump sound from the boot, with ear to petrol filler cap (removed) or from within the car. I take it this would be the gushing sound you usually get when turning ignition of petrol circulating?

-Didnt test injectors but am 99% positive the revoery mechanic said their was no power to the injectors.


Any further tests to do - do we think it is a blown relay? Take it their is no direct test you can do on the 2 relays?


Thankyou.

No fuel and no spark- definately sounds like the Main Relay.

Have you inspected the "ENGINE" fuse, which is a 15A fuse on the panel which is inside the vehicle, under the dashboard? This fuse supplies the power to operate the main relay, so if it has blown the relay will not close.

If you can see the wires on the underside of the main relay socket I can tell you how to test it. You may have to remove the whole fuse block from the bulkhead to do this, but the relay should still be in the socket.

You need a simple voltmeter, and unless otherwise noted all measurements will be made with the negative (black) wire connected to chassis. Also, know that +12V is just an average, it can actually be anywhere from 11.5 volts to 15 volts.

Here's the procedure to do:

1. With the ignition key off, you should see +12V on the white / green wire. This tells us that the path from the INJ fuse is ok.

2. Now, with the ignition key on, you should see +12V on the black / white wire. This is the signal coming from the ENGINE fuse which tells the main relay to turn on.

3. Once you have +12V on the black / white wire, then you should also see it on the white / red wire. This is the output of the main relay.

4. If you have +12 on black / white but not white / red, turn the key back off and switch your meter to resistance mode. Measure between the black wire and chassis. You should see close to 0 ohms. (full continuity.)

5. Still in resistance mode, measure between the black / white and chassis. You should see perhaps between 100 and 1000 ohms. Remove the cooling fan relay and re-measure, and you should see a slightly different value, but still somewhere in that range.

Let me know if any of these tests fail.

Hi, just got a voltmeter and doing the tests.

Originally someone told me the yellow / black box under the dash was the main relay - it has 5 wires going into it and these do not match the colours you mentioned. This is bolted to the chassis.

Next to this is a grey / black box with 7 wires entering it. Is this the main relay? This is also bolted to the chassis.

I'll let you know results shortly, thanks

Ok, seems as though the UK wire colours may be different?

I tested all the wires (both relays) on ignition on/off and took a pic of each relay. The 15a fuse looked fine undre the dash.

Results are as follows...

Grey/Black Relay---

Ignition:
Orange = 12v
Red/Black = 12v
*same in ignition off position*

Yellow/Black Relay...

Ignition:
White/Red = 2v
Green = 2v

Off:
No readings

http://www.yamr6.plus.com/Cars/yellow.jpg

http://www.yamr6.plus.com/Cars/grey.jpg


Sorry for being so clueless on all this! Im learning alot about car electronics here.

The Yellow box you've shown is the "Circuit Opening Relay", which basically switches the fuel pump on whenever the starter is engaged or the switch in the airflow meter is closed. The fact that you're seeing only 2 volts going into it (on the white/red wire) is bad, but I don't think that the COR itself is the problem.

The grey box is just the turn indicator control. Pay it no mind.

The Main Relay is in the fuse box under the hood. On a US-spec car it's on the intake side of the engine compartment, between the shock tower and the firewall. I've attached pictures which should help you locate it if UK cars are different in this regard.

At any rate, this relay takes in power on the white/green wire from the 30A INJ fuse (hot at all times), and when closed puts this power out on the white/red wire. This feeds several critical circuits in the car, such as the injectors, the CAS, and of course the circuit opening relay.

So, a quick check would be to unplug the main relay, and measure the voltage from the terminal I've indicated (which is the white/green wire) to ground. You can do this with the key off. If you see 12V here, the main relay is probably bad because it’s not passing it to the output. If you see only 2V here, then check the INJ fuse, and the associated wiring.

thanks for the pictures. Good news the terminal indicated is showing 12v!

So i guess it is definitely this relay that needs replacing then...

Update:

After some further valuable advice from Perez I replaced the Inj Relay and now have 12v power at the Circuit Opening relay. I can now hear hear the fuel pump come on by jumping the F/P to Gnd in the diagnostic box.

However the car still does not start. I dont seem to be getting a spark, but dont have any equipment to test this, their was a singular spark usually when turning the key to off position, not when turning over.

The inputs on the coilpack was: Red - 12v, White - 2v, Black - 0v. (note the red wire from coilpack was a blue wire before the plug).

Could this be coilpack, ECU, ecu fuse, CAS?

OK looks like i fixed it!

Couldnt work out why after replacing faulty Inj Relay it was not starting...

turns out the damn AA (breakdown servide i called 2 weeks ago) forgot to plug the ECU back in. Plugged it in and it now starts - however its shuddering like hell before stalling.

Take it this is something minor, due to trying to start without the fuel supply?

thanks for all your help!

Dumb question perhaps, but it's got me stopped in my tracks.
How do you remove the main relay from a 1993 Miata?
I don't see any fasteners holding it in place. When I try simply to pull it out, it moves 1/8 inch and stops. I afraid that if I try to force it, disaster will follow.

Ladies and Gentlemen, a new MiataTurbo record for thread necromancy, and one that is completely relevant, on-topic, and exhibits evidence of having searched deeply:

You know, I wish I could remember. I'm quite certain you don't need to get to the underside of it, I just can't recall whether there's a "squeeze here" sort of fastener on the side, and I'm 700 miles from my car just at the moment.

Are you referring to the INJ relay? If so there's a 10mm bolt (nut?) holding it in.

Now that I think about it, I do believe that you are correct.

(Disregard my last post- it's been many years since I had to change one of these.)

The fuse box does have to be removed from the car and turned upside-down.

Protip: disconnect the battery first.

Oh. Thanks.
Seems unnecessarily troublesome to me, but not a big problem.
I'll give it a try this weekend. (and yes, I was referring to the inj relay)