Skip to comments.Fuel cutoff relays??
Posted on 11/30/2011 8:03:27 AM PST by varmintman
The topic here is this little guy:
Somewhere around the early 90s Honda and Toyota started making cars to last longer than their owners last. I have a 96 Honda Accord with around 210K miles on it, synthetic oil all the way, bulletproof 5-speed manual transmission and all the main parts of the car seem set to live for another 30 years, assuming I replace spark plugs, timing belts, radiators, windshield wipers and that sort of thing once in a while.
But I've seen cars killed by over-engineering things and that fuel relay is just that. When one of the things dies, the car cannot be started easily unless it's dead cold, so that running errands is basically off the agenda. I replaced the original one of those around June and that one died about two weeks ago and I had to replace the thing again yesterday i.e. either the one from June was defective or there's some electrical problem killing the things.
What I'm wondering is, does the car really need that thing?? I mean, cars in the 50s, 60s, and 70s never had those and didn't seem to need them. Has anybody ever just shorted the two real wires together on one of those things and basically just defeated its function altogether??
I have a friend with a late 1990’s Toyota Camry that had nearly 400,000 miles. It is one of the best riding cars I’ve ever been in.
The ‘92 Toyota Camry and the ‘65 Dodge Dart will go down in history as the two sturdiest cars ever. Virtual Urban Tanks.
Is that the thing that turns off the fuel pump if you get into an accident?
I had a 1987 Toyota Van Wagon that racked up 500,000 miles before putting her out to pasture in a junk yard.
Hence the need for the fuel pump cutoff.
Cars are indeed more complicated these days, but much more reliable. Our cars start literally every time we turn the key. Do you remember, as I do, your parents saying you couldn't go to school or they couldn't go to work because the car wouldn't start on a cold winter morning? Do you remember "vapor lock"? I do. These things are gone because of computer-controlled fuel injection.
Good answer, thanks.
Again I replaced the original relay last June and the replacement didn’t last long, I replaced it again yesterday. Is that more likely due to the June replacement being defective or is there some sort of a common electrical problem which could be killing the things?
My son just had this same problem on his 2005 Ford Ranger. The fuel shutoff is designed to shut off the fuel pump in the case of a rollover. It shorted out and burned out the fuel pump - which is, brillantly, in the gas tank. $900 later, he is back on the road.
Put the meter on "ohms" and hold the two probes of the meter across pairs of terminals until you get a low resistance. (You just found the relay's coil.) Reverse the meter probes on those two terminals, and see if the resistance is lower. Remember which terminal the red meter lead was on when resistance was lowest. (You've just identified the polarity of the built-in snubber diode in the relay). Drop the meter probes now, and clip a couple of alligator clip wires (cheap from Radio Shack) to those two terminals; clip the red one to the terminal that had the lowest resistance with the red meter terminal on it.
Now, put the ohmmeter probes across THE OTHER TWO terminals. Be sure you're not touching the metal part of the probe with your fingers; it'll give you a false reading. The reading you should be getting is either "overflow" (super-duper high resistance, so high the meter can't read it) or something higher than 1,000,000 ohms (1 megohm).
Now, get a helper to clip the other ends of the alligator clips across your car's battery (red one to the +). You should hear a "click" from the relay. Check the ohms (resistance) again on the two terminals not connected to the battery. They should be low resistance now (less than 10 ohms).
If the above tests are successful, your problem is elsewhere.
It could be a bad relay or it could be due to low voltage or a bad ground at the relay. Use a volt meter to check the voltage going to the relay. Disconnect the relay and use an ohm meter to make sure it’s getting a good ground.
The relay uses an electromagnet to close a circuit. If the relay is weak or the current going through the magnet is weak the switch doesn’t make good contact and the electricity arcs across the contacts. This will cause the switch to prematurely fail.
V-man, if you don’t have an ohmmeter, you can do the test with a battery-powered continuity light that you can get at an auto supply store. Substitute “light on” for “low ohms” and “light off” for “high ohms” and you still can use my instructions for testing the relay.
The SMSgt has a great idea here. After you test the relay, this is the next thing to check.
backwoods-engineer is right. I forgot about the diode that keeps arcing down. You have to test for continuity both directions. Otherwise, everything is correct.
Personally, for a POV, I wouldn't want to disable that function. If the fuel pump shuts off because of an accident in my car, it won't be because kidnappers are smashing into me. And I'm very averse to burning to death. ;)
I like your idea of cleaning the relay's contacts if it turns out to be bad, then throwing the cleaned relay in your glove box as a spare. You don't want to trust that relay (metal pieces everywhere, yuk), but as a spare? Absolutely. As a prepper, two is one and one is none. Good advice.
Not all fuel pump relays have that diode (my '88 Toyota 4WD didn't), but the newer ones probably will, especially if the relay is actuated by the engine control unit (ECU). Microprocessors hate the little zap that relay coils (and all inductors) emit when the the current abruptly is stopped.
My 2003 Ford F-150 has a reset button for the fuel-pump shutoff. It’s fairly inaccessible (below the dash on the extreme right kick panel), so it’s possible to re-locate the cutoff switch where the driver could access it. Or, perhaps they have fuel flow monitoring; if the fuel system isn’t breached, there’s no need to turn off the pump.
Find a replacement relay from Omron or Idec and forget about it for a while.
You should see the spares I carry.
I'm glad you brought up the diode, it completely slipped my mind. If you've got those probes in the correct direction, the ohmmeter should have enough voltage to cross the PN junction and it'll show continuity across the diode even if the coils open.
If you don't have a meter handy, you can just rig up a set of jumpers from a piece of lamp cord to test the coil. If you can get the cover off, see if the contacts look burnt, and trace the coil wires to the contacts, and give it battery current on those contacts. The points should close. If not, the coil is shot.
If you can't get the cover off, see if there's any kind of diagram on the relay. If it's good you should get a positive "click" out of it when the coil is energized.
Fords. Some things never change. Had the same problem in my old Aerostar van.
Good information here from several posters, many thanks!
If indeed the relay failed again, you might try to determine what current the fuel pump is supposed to pull and what it is actually pulling.
You may be burning the contacts due to too high of a current.
I would be looking at replacing a pump with that kind of mileage just as preventative maintenance.
Good idea but unfortunately the thing cannot be taken apart to the point of seeing any contacts without destroying it.
It doesn’t look like it’s potted in goo. It looks like it has a circuit board that snaps into the plastic container. If you have to replace it, try and take it apart, what’s the harm?
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