What is the purpose of a fuel injection relay?

I'm having cronic fuel injection relay issues with my miata, and I'd like to understand what it's doing before I try to track down the problem. I've gone through 3 of these relays in a year, but that's another topic.

So, what exactly is this thing doing? I know what a relay is and how it works, but why would you want to open and close the fuel injection circuit? If it's for overload protection, why not use a fuse?

A relay is used to carry a heavier current without resorting to exessive heavy wiring. Check all the connections and especially the grounds. You could be getting voltage spikes which destroys the relay.

It's not so much for overload protection like a fuse, but more for protection of inernal PCM circuits. It splits the big loads (like injectors and fuel pump and PCM power) off of the PCM itself. They are used also to take the load off of the ignition switch, etc.

If it keeps eating relays, that tells me there is probably a ground loose or a poor connection. If a part has power and ground and the ground has too high of a resistance, the amp draw goes up as a result and this will fry parts in the circuit. Check all the grounds carefully, remember things like the engine to body grounds, fuel tank grounds, etc. Poor connections at the relay socket itself can do this too, generally you'll see the blades on the relay discolored and possibly the socket melted as well.

A component that is drawing too much amperage can do the same thing. There are amp gauges you can put in the feed circuit to see how much power is being drawn, if it's way high then start unplugging things one at a time and see what makes the biggest difference. To save confusion, unplug one part at a time, record the change in amp draw, then plug that part back in and go to the next part. A wiring diagram showing what is fed by that circuit is essential to this type of test.

I've also had a relay socket where the sockets were worn, making the relay loose and causing a bad connection. I tightened them up by (carefully!) bending the socket tabs closer with a small flat-blade screwdriver. The car would intermittently refuse to start previously; 10 months later, after bending the relay socket tabs, still no recurrence of the problem.

This has been the scenerio the past 2 times:

I go to start the car (usually after an autocross), it cranks but doesn't fire. With the ignition ON (accidentally and stupidly), I open the fusebox and wiggle the fuel injection relay. I hear the fuel pump, fans etc. kick on. Car starts right up. Turn it off, but won't restart.

My first thought was that wiggling the relay with the ignition on caused arcing and consequential damage to the relay. But if that was the case I wouldn't get the car started after it made contact.

I took apart one of the fried relays, but I don't see anything bad. No signs of arcing on the legs and everything looks good inside. There's no breaks that I can see in the electromagnet, so I don't understand what is frying. If the current is passing through the magnet, the switch should close, right?

If I'm in a bind, can I zip tie the switch shut? Keep in mind this would be a temporary fix to get me home...

I bet your relay socket is loose. That's exactly what happened to my (admittedly very different) car (1990 LS400).

One way to test is to wiggle the relay while the car is running. Or use short jumper wires between the pins and the socket so you know it's getting good electrical contact.

It's always possible that I'm wrong. It does happen, amazingly enough.

I'm gonna say that since you can wiggle the relay and get powerback, you have bad contacts in the relay socket. dculberson has it right; use a small screwdriver or an O ring pick etc to bend the terminals in the socket a little bit.

See the part of this terminal at the left of the picture? Those two 'curled' pieces are what grip the male blade terminal. If they don't grip the blade tightly, the amp draw goes up and this fries the contacts in the relay. Generally, you can bend them in tighter and this fixes the problem. Sometimes the terminal has gotten hot enough to cause the terminal to lose its spring tension and the problem will keep coming back until the terminal itself is replaced.

Sounds like the relay socket isn't making good contact. Clean contact spades, try to tighten them a little (CAREFULLY) per above post.

Then look for grounds under the fusebox.

Something is loose. That's why you get fire when you wiggle stuff, and why the relays are wearing.

GRM never ceases to amaze me. You guys were dead on, one of the sockets was loose. I always overlook the simple stuff. Upon further inspection of the relay, I noticed the the plastic slightly melted around the leg that goes into the loose (now tight) socket. The melting was causing one side of the switch to move, and eventually the switch was making a poor connection when closed. I was actually able to bend it back and save it.

Thanks!

Sweeeeeeet. Very gratifying.

BTW, in a Miata the "fuel injection relay" is misnamed. If I recall correctly, it's actually the main relay that powers up the majority of the car. We're seeing more and more of these go bad (typical scenario: "I was driving the car, parked it, and it just wouldn't start up again"). If we start seeing serial failures like this, I'll keep the loose socket in mind!

Keith: You're right, the technical description from the dealer is "main relay". Also keep in mind that you can easily repair the relay if you get a failure like this. Just pop the cover off and bend the contacts back into place. Otherwise its 50 bucks from the dealer or you have a cannibalize a fusebox. If I had known they're fixable 3 relays ago, I'd be that much richer...

Stargazer wrote: If I had known they're fixable 3 relays ago, I'd be that much richer...

Dunno about the others, but I do accept PayPal.

Hmmm, something to think about. I had a '92 Civic that had the main relay fail, fixing it wasn't $50 at the garage of the "honest mechanic" I took it to. If this starts to go wrong on my current car, I'll be sure to do a thorough examination of the fuel injection/main relay system...before giving up and putting myself at the mercy of professionals.

Jensenman wrote: I'm gonna say that since you can wiggle the relay and get powerback, you have bad contacts in the relay socket. dculberson has it right; use a small screwdriver or an O ring pick etc to bend the terminals in the socket a little bit. See the part of this terminal at the left of the picture? Those two 'curled' pieces are what grip the male blade terminal. If they don't grip the blade tightly, the amp draw goes up and this fries the contacts in the relay. Generally, you can bend them in tighter and this fixes the problem. Sometimes the terminal has gotten hot enough to cause the terminal to lose its spring tension and the problem will keep coming back until the terminal itself is replaced.

I had a very similar problem on my saab, but it was a bad crimp on the RH side instead of looseness on the LH side. I made an extractor tool from a piece of paperclip to pull the terminal out of the socket, when it was out, I put a drop of solder on it, and it was good to go!