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09-21-2009, 11:51 PM
| | Way Outta Control | | Join Date: Sep 1999 Location: The Palouse
Posts: 13,158
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The Oil Pressure gauge voltage feed is one of the places not affected by the Key Swipe.
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09-22-2009, 06:26 AM
|  | Addict | | Join Date: Jun 2003 Location: Valdosta, Georgia
Posts: 400
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I checked the voltage at the relay using the oil pressure line as my switch and it stayed 12vdc on both start/run - it's why I chose it - along with the convenience of it being so close to the coil already.
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09-22-2009, 08:24 AM
| | Way Outta Control | | Join Date: Sep 1999 Location: The Palouse
Posts: 13,158
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I may be mis-reading. Look at the schematic, in the bottom left corner is the Oil Pressure Guage voltage supply line. Notice that this point is "Hot in Run and Start"... and is NOT affected by the key switch swipe. That's one place where you can uses to feed voltage to the HEI. I use this point to as a voltage feed for my fuel injection system so it doesn't die when I move the key from Start to Run modes.
I used the Big fat red voltage feed wire to the old FOMOCO ignition module to feed my HEI assembly. It's worked for over a decade.
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09-22-2009, 08:58 AM
| | Addict | | Join Date: Jun 2003 Location: Valdosta, Georgia
Posts: 400
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You are confirming what I said and I have agreed with you all along 
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09-22-2009, 09:34 AM
|  | Mud in my Veins | | Join Date: Apr 2000 Location: Winston-Salem, NC
Posts: 5,357
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Chiming in a little late, but I suspect (as Taz does) that you're missing something. The scenario you give is EXACTLY the same as an ignition that has less than 12VDC during *cranking* (BTDT myself).
As Larry suggested, run a full 12V directly to the HEI. My suggestion is to skip the oil pressure sender short term and go straight from the battery or solenoid. If that works fine, debug your wiring path (OP sender/relay/etc) to figure out what's happening
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09-22-2009, 10:29 AM
| | Addict | | Join Date: Jun 2003 Location: Valdosta, Georgia
Posts: 400
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Got it.
But, to kick a dead horse. . .
The voltage at the oil pressure wire is -
key off - 0vdc
key start - 12vdc
key run - 12vdc
All that wire is doing is sending the signal to the relay to open the voltage from the battery.
It either gets the full 12vdc or nothing.
Now that horse has been kicked - I agree that it makes sense to bypass the relay to see if it makes any difference.
If it does? - bad/sticking relay?
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09-22-2009, 01:27 PM
|  | I Might Just Know What I'm Talking About | | Join Date: Sep 1999 Location: BOSTON
Posts: 1,739
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Depending on the relay, your "full 12 volts at start" may not be enough to fire the relay.
I didn't see it mentioned above, but voltage on start can drop to 11, 10, 9, 8 volts depending on amps drawn by the starter. If your "12V" relay won't fire without at least, say, 10 volts, it may not fire at start.
It's called "pull-in voltage" and varies from relay manufacturer to relay manufacturer.
Good luck
Pete
__________________ 88YJ: Old, noisy, slow, and just generally wonderful. Wheeling Jeep. 99TJ: Less old, less noisy, less slow, daily driver Jeep.  |
09-22-2009, 01:44 PM
| | Addict | | Join Date: Jun 2003 Location: Valdosta, Georgia
Posts: 400
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Good point. Learn something new every day.
All of the points have been very helpful and have pointed me in the right direction.
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09-22-2009, 03:16 PM
| | Mud in my Veins | | Join Date: Apr 2000 Location: Winston-Salem, NC
Posts: 5,357
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Quote:
Originally Posted by Pete88YJ  I didn't see it mentioned above, but voltage on start can drop to 11, 10, 9, 8 volts depending on amps drawn by the starter. If your "12V" relay won't fire without at least, say, 10 volts, it may not fire at start. | Where I was going (Thanks Pete!) with the similarity to other ignitions where the coil's fed from OEM wiring... fine *UNTIL* the starter engages & voltage drops to subpar levels (what was happening to mine w/ resisted 10VDC feeding the coil dropped to 6VDC). I added a jumper from the solenoids "S" terminal to feed a full 12VDC ONLY during cranking (so as not to burn up the <12VDC rated coil) it began to fire instantly...
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09-22-2009, 06:08 PM
| | Addict | | Join Date: Jun 2003 Location: Valdosta, Georgia
Posts: 400
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Nice catch Pete88YJ.
The voltage DOES drop to 10vdc on the oil pressure line during the crank. I obviously have a relay that has a higher rated pull-in voltage.
Is there any way to know what the pull-in voltage for a relay is before purchasing?
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