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post #21 of 33 (permalink) Old 03-18-15, 05:57 PM Thread Starter
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Re: nightmare of a problem, need help

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Uh, there will be voltage there if the fans run and the fuel pump as well. Need to be checking more like the relay coil activating circuit as the relay is supposed to be powered up and grounded only for like 2 seconds on fuel pump meaning it's grounded when shouldn't be. Same with fans, they should not be grounded at least on the relay coil circuit. The relays clicking on/off could be a partial voltage getting to relay coil that is not enough to hold relay fully on, the spring then overrides that to cycle on/off rapidly. Or a partial ground. If new relays do the same then an indicator the relay coil circuit is flawed with either power up or ground, probably the ground side.

I see that battery has been swapped but is the one being used now a KNOWN GOOD one that has passed battery load testing? Seen people go crazy while assuming battery was good, when it wasn't, a low battery on modern EFI cars will make some pretty crazy effects.

the battery was swapped out of yet another ford probe that i drive daily so both batteries are good.

for the relays, i believe the pcm controls the grounds (pins 45 and 80 are the grounds at the pcm) when i ground those pins at the pcm harness it stops the chattering of the relays but still wont start. ive checked the pcm grounds and theyre good.

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post #22 of 33 (permalink) Old 03-18-15, 06:03 PM Thread Starter
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Re: nightmare of a problem, need help

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2 things that I haven't see you do, have you tried jumping out each relay to see if the compnents fire correctly? And have you checked your grounds?
spent a ton of time checking/cleaning up grounds.

what do you mean by "jumping out each relay"? do you mean bypass it?
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post #23 of 33 (permalink) Old 03-18-15, 09:40 PM
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Re: nightmare of a problem, need help

'when i ground those pins at the pcm harness it stops the chattering of the relays...'

Telling you something there. If fuel pump, need to listen when grounding it to hear if pump then runs. Fans? I'd yank that relay until you can get car started, just to narrow down possible issues, but DO NOT forget doing so. Don't need fans until engine is running reliably. Pop off relay covers and ground the coils again, to see what the relay rocker switch is doing, grounded it should pull over to the normally off contacts and open back up when ground is opened, if not a partial defective ground somewhere in that circuit.
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post #24 of 33 (permalink) Old 03-18-15, 09:46 PM
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Re: nightmare of a problem, need help

Jumping out is the process of removing the coil essentially from the circuit by simply jumping the main power across to the main ground of whatever the circuit controls. Or, say fan, jump the fan power to the fan ground and don't use the relay coil plus or minus at all. Use a short piece of jumper wire stuck in the slots there after identifying the main leads to the device or simply pop covers off and work the relay rocker contact lever with a finger to simulate the coil pulling it in (on).
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post #25 of 33 (permalink) Old 03-20-15, 11:46 AM
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Re: nightmare of a problem, need help

Sorry for the delay. Essentially, a relay is an electromechanically controlled switch. If you know which pin feeds the power and which has the load, you can jump across them to complete the circuit. This eliminates bad relays. A relay may make, but do to carbon build up, corroded terminals, etc., you may not be passing full voltage. Also, if the coil or the commanding signal is weak, you may get a chattering relay. It also helps to know if the relay is breaking on neutral or on hot. Typically, an isolation relay will break on the neutral side which means 12V is constantly fed to it. The coil of the relay is a generally low load and by a triac or micro relay on a circuit board breaking the neutral, the arc that is typically generated when breaking on hot doesn't occur. When the relay is controlling a heavier load, it is broken on the hot side, that way you're limiting the exposure of a power source with a higher amperage protection.

Ever notice that there's so many high performance parts available for GM products?

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post #26 of 33 (permalink) Old 03-20-15, 01:03 PM
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Re: nightmare of a problem, need help

see http://www.the12volt.com/relays/relays.asp for all you need on relays.

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post #27 of 33 (permalink) Old 03-20-15, 03:20 PM
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Re: nightmare of a problem, need help

Actually two jumps can be done on a relay, the more common one I spoke of before that tests the main powered circuit and the jumping to check relay coil itself.

Jumping or jumpering is when you use a short section of wire to bypass parts of circuits to check other parts, the short piece of wire 'jumps' from one power area to another or a ground. The wire itself is commonly called the 'jumper' because it enables that.
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post #28 of 33 (permalink) Old 04-08-15, 05:52 PM Thread Starter
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Re: nightmare of a problem, need help

sorry about taking so long to reply, power supply went out on my pc. had to jump through the hoops of replacing it under warranty.

the relays pass all the testing so theres no problems with the relays itself. the fuel pump is running even though the relay chatters. the fuel pressure reads normal but the pump doesnt shut off when the key is on.
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post #29 of 33 (permalink) Old 04-09-15, 06:32 PM
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Re: nightmare of a problem, need help

Unfortunately, things are pointing back to the PCM. Wire 704 (LG) off of the fuel pump relay is shorted to ground through the PCM to complete the circuit and close the relay. If the PCM doesn't fully break, the relay will chatter and the fuel pump will not shut off.

97 Probe 2.0 Fuel Control.jpg

Ever notice that there's so many high performance parts available for GM products?

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post #30 of 33 (permalink) Old 04-09-15, 07:27 PM Thread Starter
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Re: nightmare of a problem, need help

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Unfortunately, things are pointing back to the PCM. Wire 704 (LG) off of the fuel pump relay is shorted to ground through the PCM to complete the circuit and close the relay. If the PCM doesn't fully break, the relay will chatter and the fuel pump will not shut off.

Attachment 41961
yeah thats about where i was at. i got a used pcm on ebay and swapped it out but no difference in whats happening..... its got me stumped. i find it hard to believe i have two bad pcms in my possession that give the same symptoms. is it possible that it just needs reprogrammed?
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