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Old 11-22-2020, 10:21 PM   #1
lutronjim
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Join Date: Dec 2013
Location: Dunwoody,Ga
Posts: 443
Crank no start

1st Happy Thanksgiving to all.

My 70 with ls will not start. Had this problem before and chased it to ignition switch. Well switch is 'bout 1 year old and seeing same problem.

When I turn switch to on and can hear the fuel pump cycle so I know computer is getting power. I also know that that when switch goes into start position it only sends power to coil and starter solenoid.

This was what was causing my original problem - computer is fed from fuse box and was not receiving power when key was in crank position. I also don't see light test when cranking.

When looking at wiring diagram, I see wire going to solenoid and then to coil. To keep solenoid from firing when switch is run position, I see another of the wires on the solenoid goes back to the switch and, I think, grounds when switch is in crank position. Thus enabling two power sources without a diode.

I want to be able to use truck until I can find a switch worth a damn. So I am think 'bout rigging a switch to supply power to computer and turning it on when I crank and then back off.

If anyone sees holes in my approach please let me know.

Also if anyone one knows of a good switch please let me know. The last I bought was Standard.

Thanks
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Old 11-24-2020, 12:33 AM   #2
dmjlambert
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Location: Cypress, TX
Posts: 3,513
Re: Crank no start

Quote:
Originally Posted by lutronjim View Post
...
When looking at wiring diagram, I see wire going to solenoid and then to coil. To keep solenoid from firing when switch is run position, I see another of the wires on the solenoid goes back to the switch and, I think, grounds when switch is in crank position. Thus enabling two power sources without a diode.
...
I don't think there is a ground when switch is in crank position, and I don't think a diode would be needed.

The purple wire has 12 V in start position and this causes the solenoid on the starter to engage.

The yellow wire on the outboard terminal of solenoid on the starter sends 12V to the coil only in the crank position.

The pink wire under the dash in the cab also gets 12V in both crank and run positions, and it goes to the coil through the white resistor wire that joins up with the yellow wire. Or for those with HEI, they have probably replaced that resistor wire with a plain wire that comes from the same position on the firewall connector where the resistor wire was attached, or from a terminal in the cab that gets its voltage from the pink wire such as the IGN UNFUSED terminal.

I have found that every now and then, I get no voltage or reduced voltage to my HEI during crank, because I get no spark. I crank it for a long time, and I get spark immediately upon releasing from crank position to run position and the engine then runs. So, this is giving me the idea of putting the yellow wire back in place to also power the HEI, so I'm sure to have good power from at least one source during crank. It has not bothered me often enough to made me do that yet. I think I would blame this problem on my ignition switch but it is pretty hard to diagnose when it only rarely happens.
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Old 11-24-2020, 09:03 PM   #3
lutronjim
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Location: Dunwoody,Ga
Posts: 443
Re: Crank no start

Thanks I misread wiring diagram.
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