main relay vs ignition switch while driving

snipped-for-privacy@gmail.com wrote in news: snipped-for-privacy@i42g2000cwa.googlegroups.com:

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.

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:

snipped-for-privacy@gmail.com wrote in news: snipped-for-privacy@e3g2000cwe.googlegroups.com:

No,it won't "die while driving". Not with the typical main relay failure mode.

kindly indicate the model and age and mileage - it helps a lot in giving you the advice you seek.

usually, if it dies while driving, it's the ignition switch - main relays prevent starting, but not running, in my experience at any rate.

The main relay, IIRC, controls both starter and injectors... an intermittant one could most certainly affect running...

Matt Ion wrote in news:0d2Lg.502068$Mn5.43715@pd7tw3no:

When you are driving,the interior is not as hot as when the car has been closed up for awhile,and normal running vibrations keep the relay working. No one here has ever reported having their main relay cut out their car - while running-,only when it's been sitting,and more often in hot weather.

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Precisely.

'Curly'

True enough, but just because no-one has reported it here doesn't mean it couldn't happen, or hasn't happened. The fact is, the main relay energizes the injectors, and thus COULD cause a problem with the car running. And if one is counting on "vibrations" to "keep the relay working", I'd be very concerned about vibrations keeping it FROM working normally as well - such a device in a car should be virtually impervious to vibrations, period.

Honda Accord '94, ~90K miles. I changed already the ignition switch!

snipped-for-privacy@gmail.com wrote in news: snipped-for-privacy@e3g2000cwe.googlegroups.com:

I resoldered the main relay on my 94 Integra GSR about 4 years ago,no problems since.(hot/humid central Florida)

Then I shall be the first. Mine stalled in traffic years ago. The indications are the main relay shuts down prior to rpm going to zero. Had no "fuel pump" click from the relay when the car wouldn't restart. Most people don't report their problem because most don't know for sure. And most often, it happens only once in a blue moon.

One other note, this car never stalls in its 280k miles of lifetime.

once you have it running)

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?

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

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?

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...

agreed.

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

Matt Ion wrote in news:srHMg.537959$IK3.369846@pd7tw1no:

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.

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.

The true question now is what is causing the joints to continue to make contact on a running car hours after a heat soak.