main relay vs ignition switch while driving

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Hi,
When there is a problem with the ignition switch the car can stop even after the engine has ignited. (I 've experienced that already!)
Can the same thing happen when the main relay has gone
bad?
I am afraid my Honda Accord has a bad main relay. After (if!) the engine starts, is it dangerous to drive it? Can the car stop suddenly even while running, or a bad main relay basically affects only the ignition process?
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snipped-for-privacy@gmail.com wrote in

Generally,ordinary body vibrations keep the main relay working once the motor has started running.(and that the car's interior is usually cooler once you have it running)
A bad relay affects the fuel pressure available;that's why the failure moded is often "starts then immediately quits,then only cranks,no startup,until the car sits for several minutes". The motor uses up the stored fuel pressure,then starves and quits because the fuel pump is not running,because the relay is not powering it.
The fuel injectors need a pressurized fuel supply rail.
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So if I understand well the car can die while driving, when there is a problematic main relay? (it affects not only the ignition process...)
TIA michael
Jim Yanik wrote:

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snipped-for-privacy@gmail.com wrote in

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I don't believe vibrations keep the main relay working once the motor has started. At certain rpm the vibration is hardly there. ...How did you come up with your conclusion?
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Burt wrote:

Such a relay should be designed to NOT be affected by vibrations, for good or bad.
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Matt Ion wrote:

come on matt, there's limits to that. vibrations at what frequency? what g's? how much do you want to spend? besides, it's not vibration that causes the solder to crack - it's thermal cycling.
bottom line, the relay /does/ suck, but then again, a sub $60 fix on a what is typically a >10yr old car is not that bad a deal. how much do you need to spend to get a domestic past the 10yr mark?
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jim beam wrote:

Don't be silly... I was responding to the assertion that the vibrations help to keep the relay working right. That's just silly - if a relay is RELYING on vibrations from the running car to work properly, it's either faulty, or a really poor design to begin with.

Or a combination thereof...
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Matt Ion wrote:

agreed.
the design's arguable. it's certainly cheap - one of the lowest quality components on the whole vehicle.

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These components are of high quality. They're design to lasts maybe 20 years or more. The problem lies on the soldering process they use. Change the process and the problem is solved.
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Burt wrote:

88-91 civics having this problem. i first encountered it when a vehicle was only 10 years old, and its previous owner had evidently had problems with it for some time prior to selling.
regarding solder, this is a soft alloy that operates at a highly elevated temperature relative to its melting point. expose it to thermal cycling [the relay runs hot you'll notice] and you have a problem just waiting to happen. the solution is to either use a different switching arrangement that doesn't generate as much heat [cycle] /or/ to use a different jointing method like spot welding or crimping. but the relay manufacturer should know all this. i still say this relay is a cheap and cheesy design. the circuit board is low quality and the relay internals are designed primarily to prevent intervention, not for serviceability [either kind]. i say mitsuba knew exactly what they were doing with this relay right from the start [relays are old technology and their problems are well known] and that they elected to go for what they knew would result in life limitation.
life limitation is nothing new. i once had a car clock that failed. on disassembly, i discovered a soft solder rivet had separated breaking the electrical supply. the interesting thing was, the rivet was held in tension by a spring! solder [lead] tends to creep over time, especially when kept warm. life limitation? you bet! there was no other practical explanation for the rivet/spring combo. a fuse would have protected against overload and the spring had no mechanical function. anyway, i soldered a wire in place instead and the clock worked again, just like a repaired relay.
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Yeah, the solder will fail in 8-10 years. But it's the electrical components that will last.

Spot welding or crimping would be expensive and not already in their machine assembly line. One solution might be to replace the relays with transistors, heat-sinks and some breathing holes.

I don't know why a spring is put into a [digital?] clock without a purpose. You sure the spring isn't a spring resistor?
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Burt wrote:

transistors always drop voltage - not necessarily what you want. and 30A rated devices aren't cheap.

it was an electric dial clock. no, the spring wasn't a resistor - it would have been rated at ~10+W for a milliamp application. a "real" resistor would have done the job better if that was what was required. besides, it was hooked onto the chassis at one end, and onto the tab retained by the lead rivet at the other - no reason to use a lead rivet when it could have been brass, copper, etc.
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They also are much more vulnerable to failure than a simple electromechanical relay. Lots of destructive spikes in an auto electrical system.

Maybe a bimetal thermal protection device? If too much current is drawn,the strip bends and breaks the electrical connection.

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It's the current you want not the voltage. Transistors are proven.

Common, solid state devices have high marks for reliability than mechanical relays. A relay will have moving part that can produce arcing. Also, any spikes can be quashed by a shunt diode.

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Burt wrote:

it's watts you want to look out for. watts = volts x amps. if the voltage drops across a p-n junction, and it does, about 0.6V, at 30A, that's 18W you're wasting [and heat you have to dissipate]. relays, for all their other faults, don't have that kind of problem.
http://en.wikipedia.org/wiki/Diodes

agreed, but they're not always the best solution. using silicon transistors on a 12V system is going to step down your voltage 5% each time it encounters a junction. that makes relays look attractive from an efficiency viewpoint.

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In applications where zero voltage drop is important there are already ways to do this by selecting the appropriate transistor. A small voltage drop of (.2V or .6V) isn't enough to slow down the fuel pump.

The relays used in the main relays can cost more and will generate heat. So much heat that you can't put your fingers on it for more than a second. Eventually, it'll run so hot that the contacts will weld together.
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Something like that. The crux of the problem is that the solder is used to support the relays instead of the relays being cemented to the board and solder used to stabilize the connection. Beginners are taught not to use solder for strength or for carrying current, but I guess relay module mfrs aren't bound by the same code. They use solder for both simultaneously.
Mike
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Actually,IIRC,the vibration stuff was Tegger's guess.
I believe it's that once the car starts and is running,the car interior (and relay)is cool enough that the solder joints/relay parts have contracted and make OK contact.I note that often after a 15 minute wait,a balky starting motor runs,and only fails after another heat soak.
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The true question now is what is causing the joints to continue to make contact on a running car hours after a heat soak.
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