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  #31  
Old 04-07-2016, 04:21 PM
Jeff d Jeff d is offline
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Originally Posted by JimN View Post
I already recommended having it tested...
I'll have to try harder to find a place that can test it.

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Have you looked at the ignition switch?
Yes. I've bypassed it by connecting the purple wire to constant power and get the same rate of voltage drop as I do with the ignition switch in play.

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I also recommended a voltage drop test- that tests each section while energized and it's the only way to find this kind of problem. Using a multi-meter when the key is off might show a problem, but not usually- there's no appreciable current involved with a DMM and that's what is required for this test.
I guess I don't really understand what you're telling me to do beyond what I've already done. There's really no voltage drop in the purple wire circuit until the alternator is reattached. With the alternator disconnected, all other normal accessories connected (Radio, perfect pass, gauges, etc), and the ignition switch turned on, I get only a couple tenths of voltage drop all the way back to the engine compartment. As soon as I connect the alternator to the end of that circuit, the overall voltage in the purple wire circuit drops about 1.5 volts but it's within a couple of tenths whether it's measured at the screw on the ignition switch or all the the way back at the alternator itself. So, at least within the context of the purple circuit itself, with all of the misc stops that it makes I have no significant voltage drop along the length of the circuit.

As I typed all of that out I'm realizing that the one thing that I haven't thoroughly eliminated is the constant power lead from the +12v post at the dash to the ignition switch. I know I have good voltage there so I'll look at the conductor from the post to the ignition switch itself.
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  #32  
Old 04-07-2016, 08:41 PM
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JimN JimN is offline
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Originally Posted by Jeff d View Post
I'll have to try harder to find a place that can test it.


Yes. I've bypassed it by connecting the purple wire to constant power and get the same rate of voltage drop as I do with the ignition switch in play.


I guess I don't really understand what you're telling me to do beyond what I've already done. There's really no voltage drop in the purple wire circuit until the alternator is reattached. With the alternator disconnected, all other normal accessories connected (Radio, perfect pass, gauges, etc), and the ignition switch turned on, I get only a couple tenths of voltage drop all the way back to the engine compartment. As soon as I connect the alternator to the end of that circuit, the overall voltage in the purple wire circuit drops about 1.5 volts but it's within a couple of tenths whether it's measured at the screw on the ignition switch or all the the way back at the alternator itself. So, at least within the context of the purple circuit itself, with all of the misc stops that it makes I have no significant voltage drop along the length of the circuit.

As I typed all of that out I'm realizing that the one thing that I haven't thoroughly eliminated is the constant power lead from the +12v post at the dash to the ignition switch. I know I have good voltage there so I'll look at the conductor from the post to the ignition switch itself.
This link may help- scroll down to the section for Voltage Drop. Mods- please sticky this link.

http://www.aa1car.com/library/voltage_drop_testing.htm

This test is done on two points with the same polarity e.g., each end of one wire.

I don't remember if you measured from the dash to the alternator, so that's why I mentioned it.

Voltage drop tests can and should be done to the ground circuits, too. If the ground connection at the back of the engine isn't clean or tight, the voltage can be fine with the key on, but not when the engine is running. This is because the current attempting to pass through the connection and the cable when the key is on is much lower than when the engine is running. Also, the gauge may show one thing and the ECM could be seeing a different voltage.

I had a boat scheduled to come to the shop for an oil change but when they made a hard turn the day before, the engine died and they couldn't get it to start. They towed it to the ramp and got it on the trailer and before I started working on it, they called to tell me about this new problem. Because it didn't start, I connected the diagnostic computer and scrolled through the data, stopping when I saw the battery voltage- it was less than 10.5VDC and that was about 3V less than I saw on the voltmeter on the dash. The ECM ground is on the block, along with the fuel pump, IC module, sensors, starter, etc. When I went to the ground stud, I found that one wire's ring terminal wasn't even on the stud and the block was rusted where the casting should have been clean. The loose wire was for the fuel pump and after the engine warmed up, the insulation became more flexible, which allowed the wire to move via centrifugal force during the turn and the ECM was seeing lower voltage because of the resistance- the dash wires ran directly to the battery, so the gauge read higher.

Last edited by JimN; 04-07-2016 at 08:53 PM.
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