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Old 11-05-2010, 12:38 AM
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Bad relay? need some help testing

On my daughter's 79 280zx there is a short to ground in the ignition wiring. I traced it to the EFI relay. When I pull the relay the short goes away. When I test the + wires going to the relay they are clean. I attached the FSM portion concerning this relay. When I test the continuity I get NONE. There is NO continuity from any of the 6 legs of the relay with any other legs. Is this right?
Just trying to figure out if this is the problem with the short to ground.
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Old 11-05-2010, 04:03 AM
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You seem to have solved it yourself, the attached pdf.... the contacts 4-5 between the coil should show cont all the time, the other contacts would only show cont when the relay is energized (or normally open). hook 12v across the coil contacts 4-5 and see if the coil energizes.
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Old 11-05-2010, 06:54 AM
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What mystifies me is how the bad relay can cause a short to ground but have no continuity.
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Old 11-05-2010, 08:23 AM
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That depends on how the relay failed, the position of the wiper. It could have failed in the open de-energized state or failed by welding a contact closed and holding the wiper in the energized position. Wrong state could cause the short in the circuit.
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Old 11-05-2010, 08:27 AM
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So if there was a short in the circuit, wouldn't there be continuity between at least two pins?
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Old 11-05-2010, 08:47 AM
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in the relay yes, have you replaced this relay and are now trying to understand how it failed the way it did?

Let me ask, relay installed, key on short, blows fuse?

If so the short is in that circuit and you will need to figure out where or what circuits are being powered from or by that relay.. this clearer?

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Old 11-05-2010, 08:51 AM
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That's what has me confused. How can a device have no continuity on any of its connectors yet cause a short when plugged in. I have not replaced it yet.
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Old 11-05-2010, 08:58 AM
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Quote:
Originally Posted by garretthes
That's what has me confused. How can a device have no continuity on any of its connectors yet cause a short when plugged in. I have not replaced it yet.
This would because the relay is not energizing, open coil, and therefore when the power is applied the voltage is going to the wrong place and your car does not like that..

Think of a relay just as a switch, instead of you turning it off and on the electrical system is doing that.

So if you want to test a relay, just hook up the pins 4 & 5 to the battery then read the contact, you should read continuity, 2 too 3 & 1 too 6 that be the case the relay is good. The short in beyond that in the circuits it is suppling the power too.

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Old 11-05-2010, 11:16 AM
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Quote:
Originally Posted by pepi
This would because the relay is not energizing, open coil, and therefore when the power is applied the voltage is going to the wrong place and your car does not like that..
I understand how a relay works. However it shows a short to ground without power. I have the ignition off and the battery disconnected. I have a test lead connected to the + coil wire and the other end to ground. When I plug in this EFI relay the +coil wire grounds out. When I unplug it-no ground.
Am I doing this wrong?
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Old 11-05-2010, 04:31 PM
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I see your point, if the relay is melted/shorted out something would have continuity and if 4-5 dont have continuity the coil in the relay has got to be burned out. Id start by putting 12v across pins 4-5 on the relay (bench test) to see if the relay is bad. My guess would be: the relay burnt up and there's a crash switch somewhere in the circuit thats going to ground. If the relay checks out ok (replaced or substituted) Id determine where the pwr and gnd is coming into the relay from, 4 or 5. With the battery disconnected, using jumpers one of the ends in the relay socket the other ends to the relay and disconnect the jumpers one at a time to see if you still get the grounding problem, to narrow down where the short is coming from.
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Old 11-05-2010, 06:29 PM
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Quote:
Originally Posted by garretthes
I understand how a relay works. However it shows a short to ground without power. I have the ignition off and the battery disconnected. I have a test lead connected to the + coil wire and the other end to ground. When I plug in this EFI relay the +coil wire grounds out. When I unplug it-no ground.
Am I doing this wrong?
The coil itself sounds shorted to ground, not the contacts. The coil windings are likely melted together creating what appears as a short but is really a very low-ohn ground. Like putting a low resistor from + to -. Its not 100% short circuit. The coil itself, disconnected, should read high ohms like 1000ohm, 5000ohm, etc. My bet is its very low, practically shorted
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Old 11-05-2010, 08:10 PM
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if the ing coil shorted out why wouldn't it go to ground with or without the relay plugged in
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Old 11-05-2010, 09:04 PM
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Quote:
Originally Posted by EOD Guy
if the ing coil shorted out why wouldn't it go to ground with or without the relay plugged in

Did I misunderstand him? I thought he said that the (+) side of the coil didnt ground until he plugged it in. Its the coil short circuited internally (the plus and negative sides of the coil itself)...so when its plugged in its the same as "grounding" the cars own 12v+ side to the chasis ground. Theres no continuity because the coil is not yet 100% fried, it still as wires that are acting like a resistor. I think most meters only read continuity below 25ohms. Possible that the coil cools off when removed and the shorted wires are so shorted anymore? Its still F'd. Just a guess.

" I have a test lead connected to the + coil wire and the other end to ground. When I plug in this EFI relay the +coil wire grounds out. When I unplug it-no ground."
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Old 11-05-2010, 09:23 PM
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Maybe the relay socket coil + terminal isn't locked in place and gets pushed back and shorts on something behind it when the relay is inserted.


Not likely. Just a thought

Good luck...
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Old 11-06-2010, 04:45 AM
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Maybe I misunderstood..... I thought he hooked a test lead from the + post on the ing coil and ground with the result being no indication of a ground fault...... When he plugged in the Fuel relay, the ing coil + post indicated a ground...... hince the ing coil shouldn't be internally shorted or he'd get the fault on the + post with or without the relay being plugged in. Indications are the pos side of the ing coil is tied into the fuel relay somehow and when you plug in the shorted fuel relay it completes the circuit to ground which would kill the engine very quickly which leads me to believe there is a crash switch.

Last edited by EOD Guy; 11-06-2010 at 04:57 AM.
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