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Cracked solder in your EFI Main Relay: Small problem, but a big headache!


As stated earlier, the Main Relay gets fairly hot in use. When you shut the car off, it cools down again. Since the circuit board, the solder, and the pins on the components all expand and contract at different rates, it's inevitable that the weakest point will fail, and that's the solder.

When the solder cracks, it's called a "dry joint", and  causes either a complete failure to conduct electricity, or intermittent conductivity. Often, a joint may be fine when cold, and become non-conductive when hot, or vice versa, leading to confusing symptoms that occur only when the car is hot, or only when cold, but not in both circumstances.

Below, taken from this excellent page, is a picture of a dry joint on a Main Relay that was installed in a Rover 216, a Civic built by the Rover Group in Britain. Look closely at the dark ring around the pin. That's the usual appearance of cracked solder. If you can wield a soldering iron, it's childishly simple to fix this by simply remelting ("reflowing") the solder, which closes the cracks up and makes the relay literally as good as new.


Cracked solder

James's Main Relay cracked solder
James Start has submitted this excellent photo of the dry joints on the Main Relay of his '93 Accord.

When you know what to look for, it's really obvious.



The Main Relay has five circuits:
The Starter circuit only carries power for brief periods, and does not usually go bad. The others are the trouble spots. Cracked solder joins may occur singly, or in combination. The images below show each problem singly. If I included representations of every possible combination, I'd be here for a week writing it all up. Forget that.

NOTE: The graphics below show the Main Relay for Multi-Port FI (one injector per cylinder).
Dual-Point FI (two injectors in the throttle body) Main Relays differ in two respects:

1) The injectors get their power from the line that runs between Termnal 7 and the fuel pump, and
2) Terminal 3 ONLY goes to the ECU, and has no connection to the injectors.
Also, as of about 2005, Honda separated the Main Relay into two assemblies that plug into the under-dash fuse box. These graphics do not cover that design.


Battery circuit failure
This is the most common failure, and usually (but not always!) happens in hot weather.

The crack occurs somewhere AFTER the injector feed.  The THIRD CLICK is normally missing, which is a smoking gun. The relay may be cool enough to close the second relay initially (the first click), charging the fuel rail during the two seconds the Check Engine light is on, but then may or may not allow the second relay to close while the starter is engaged. If the second relay does not close when the starter is engaged, the third click will definitely be missing.

Since the Start circuit bypasses the Battery feed and powers the second relay on its own at full voltage, the pump and the engine may run as long as the starter is engaged, or may run off residual fuel rail pressure, but as soon as you let go of the key, it stalls.

Another smoking gun for this one is found when the car starts fine if you let it sit for ten minutes or so, which allows the relay to cool off. Confusingly, a bad igniter can give the same symptom!

Once the car does start and run, the relay seems to be able to maintain the connection even with cracked solder, and keep the car running. I wasn't sure why. Michael offers an answer...

Reader Michael is a retired pulse power engineer. He offers this very convincing explanation for the continued operation after a successful start with a bad Main Relay:
"A complete crack in the solder surrounding a lead does not form abruptly. Local microcracks first appear at highly stressed locations in the solder; current can still flow in the solid solder between these microcracks so the car still runs fine.
"With continued stressing of the solder, these microcracks grow and ultimately join up together to form a complete crack. When a complete crack forms, no current can flow across it and the car dies but I don't think it ever gets to a complete crack in reality.
"As a crack almost but not quite completely forms, the current is forced to flow in the last few tiny remaining bits of conducting solder between microcracks. Since the available area for current flow is substantially reduced, current density goes sky high in--and a large voltage drop occurs across--those last few tiny remaining bits of conducting solder.
"The almost-failed solder joint turns into a resistor and results in a loss of voltage to the ECU and fuel pump which will kill the engine. Electrical power will also be deposited into the almost-failed solder joint which will cause it to heat up, perhaps to the point of local solder re-melt. The whole joint does not re-melt; it may only occur at a few local spots where current density is very high. At these spots, re-melt heals nearby microcracks and improves current conduction -- that's why the car will run after a soft kill and restart.
"Even if the solder doesn't actually re-melt, solder does get softer and more easily deformed at high-but-below-melt temperatures and it then becomes easier for vibration to deform the solder and force nearby cracks to close, again improving current conduction to the point where the car will run after a soft kill and restart. The whole cycle repeats so one ends up with a car that soft-kills every now and then but mysteriously restarts and works for a while."

A "soft kill" by the way, means the engine dies as though the key was turned off. The car will eventually restart.

More from Michael is here.

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Ignition circuit failure
You will hear one click (as the starter is engaged).

This failure will result in a non-start, since closure of the first relay requires that the Battery circuit be grounded at Terminal 8, which won't happen if Ignition power cannot make it to Terminal 2. Since the first relay stays open, the injectors will not get power. However, the fuel pump will get power while cranking, as the Start circuit will still be able to close the second relay.

This one should turn the Check Engine light on while cranking, with a Code 16 in pre-'96 cars, or a P0xxx code in post-'95 cars.
Often misdiagnosed as bad injectors or a bad throttle body, because that's what the Service Manual says Code 16 is. Honda apparently did not anticipate this problem.
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Pump circuit failure
Another no-start.

You'll get all three clicks, but you will not hear the pump run while the Check Engine light is on. The engine will crank and crank, but never fire. If the car has recently been shut off and fuel rail pressure is still sufficient, the car may fire, run for a second or two, stall, then not fire again. The connection may be restored and the pump may operate once the relay cools off.

Can be confused with a bad fuel pump check valve if you don't know to listen for the pump running while the Check Engine light is on.
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Injector circuit failure

You'll get all three clicks, but the Check Engine light will illuminate while the car is cranking. This one should set a Code 16
in pre-'96 cars, or a P0xxx code in post-'95 cars.

Often misdiagnosed as bad injectors or a bad throttle body, because that's what the Service Manual says Code 16 is. Honda apparently did not anticipate this problem.



More from Michael...
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I had to post Michael's entire email to me, just because it contains so much great information.
     "Had experienced 2 inexplicable soft engine kills on my 92 Civic (as if the ignition key was turned off) on hot days. Had replaced the distributor not too long ago and decided the fuel pump relay was the culprit. Was surfing the net for a replacement when I stumbled upon your fix. Your description of the problem and solution, and images, on tegger.com are excellent. I did as prescribed and soft kills haven't occurred again; the resoldered relay is working fine at ~195 kmiles. I hope to take it to the moon.
     "What I liked least on my relay were the excessively large holes for some of the leads; had to add liberal amounts of solder to those joints. Some leads were not well wetted with solder. Though harmless, I didn't like that flux was left on the PC board.
     "You state "Once the car does start and run, the relay seems to be able to maintain the connection even with cracked solder, and keep the car running. I'm not sure why." I offer this explanation as a retired pulsed power engineer. A complete crack in the solder surrounding a lead does not form abruptly. Local microcracks first appear at highly stressed locations in the solder; current can still flow in the solid solder between these microcracks so the car still runs fine. With continued stressing of the solder, these microcracks grow and ultimately join up together to form a complete crack. When a complete crack forms, no current can flow across it and the car dies but I don't think it ever gets to a complete crack in reality. As a crack almost but not quite completely forms, the current is forced to flow in the last few tiny remaining bits of conducting solder between microcracks. Since the available area for current flow is substantially reduced, current density goes sky high in, and a large voltage drop occurs across, those last few tiny remaining bits of conducting solder. The almost-failed solder joint turns into a resistor and results in a loss of voltage to the ECU and fuel pump which will kill the engine. Electrical power will also be deposited into the almost-failed solder joint which will cause it to heat up, perhaps to the point of local solder re-melt. The whole joint does not re-melt; it may only occur at a few local spots where current density is very high. At these spots, re-melt heals nearby microcracks and improves current conduction -- that's why the car will run after a soft kill and restart. Even if the solder doesn't actually re-melt, solder does get softer and more easily deformed at high-but-below-melt temperatures and it then becomes easier for vibration to deform the solder and force nearby cracks to close, again improving current conduction to the point where the car will run after a soft kill and restart. The whole cycle repeats so one ends up with a car that soft-kills every now and then but mysteriously restarts and works for a while.
     "As to why cracking occurs in the first place, I think it is vibration and not temperature cycling. Though, once formed, it's possible for temperature cycling to lengthen/widen microcracks. I fixed a pager for a colleague in my prior life by resoldering all cold-looking joints and it worked. The pager was never exposed to temperature cycling but had been banged about, dropped, etc., as he wore it daily, clipped to his belt for many years. He began receiving everyones' pages and it drove him crazy. He sent to our electronic shop twice; they couldn't fix it. I only offered to look at it as a last resort. The only problem was everyone began giving me their pagers to fix.
     "If temperature cycling is the root cause, one would expect other relays in the car to have this problem as well but only the fuel pump relay has it. Don't know how the relay is mounted in your car but I think the mounting method used in the 92 Civic greatly contributes to mechanical shaking of the relay. The relay is bolted on the far end of a stick-like metal bracket like a baby's rattle and gets shaken like one under normal driving conditions. Potholes must be murder. Am guessing other relays must be mounted differently and hence subjected to less violent shaking. Just loosely hanging the relay with plastic tie wraps like a hammock (or any "soft" attachment method) may eliminate the problem for good but I didn't do it as it'd worked for ~190 kmiles and I've only another ~45 kmiles to go on this moon shot."

Last
update: Nov21/07