 Emergency Battery Disconnect Might Have Fried My Alternator, Even Though I Wired In The Resistor |
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| motorvated |
 Feb 17 2016, 06:45 PM
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#1
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Member  Group: Members Posts: 279 Joined: 13-February 13 From: Colorado Member No.: 15,519 Region Association: Rocky Mountains  |
'72 car with '75 1.8 L-Jet. Relay board has been removed and alternator is wired directly to voltage regulator. No wiring to alternator light in dash so I wired in a 12volt wire that is hot with the key to a small light in the engine compartment that is wired to the field windings of the alternator to serve as a boot strap to get the alternator to start charging. With the I gnition on, engine not running, the light is on. Crank the engine and the light goes off as it should. Got the motor running and the light stayed off. I hit the emergency shutoff switch that I wired in to test it and confirm that it stops the motor and cuts off all power, which it did. Next time I started the motor the light stayed on and no changing detected at the battery. So it looks like I might have fried my alternator by using the emergency shutoff switch. I wired the resistor that came with the shutoff switch on the downstream side of the shutoff switch as specified, and it was supposed to dump the residual current through the resistor to ground after the switch is moved to the off position and protect the alternator. Any comments or opinions on this. Did I do something wrong? Any tests I can do of the alternator to assess it's condition?
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| Cracker |
Feb 18 2016, 07:31 AM
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#2
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Advanced Member Group: Members Posts: 2,148 Joined: 2-February 10 From: Atlanta (area) Member No.: 11,316 Region Association: South East States 
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FWIW: I have an LS conversion and have never installed a resistor with my cut-off switch installations. My electrical system was hand built by a EE and installed by another EE (everything has worked as expected). Hope you figure it out...
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| stugray |
Feb 18 2016, 12:19 PM
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#3
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Advanced Member Group: Members Posts: 3,824 Joined: 17-September 09 From: Longmont, CO Member No.: 10,819 Region Association: None |
My master off switch has two separate poles.
One is for the main power (Batt+ to system pos terminal), and the other is for the Alternator charge circuit. When you open the switch it disconnects the battery from the entire system AND it disconnects the line from the alternator back to the battery. This is so that the alternator does not keep the engine running when you remove the battery from the circuit. Also, when I removed my relay board and wired the alt directly to the voltage regulator, I had to swap two pins in the connector and splice in the field bootstrap line (the one that goes to the dash light). Did you rewire the VR connector? Not sure if this helps, but more data. I can pull up diagrams of how I did it when I get home. |
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| stownsen914 |
Feb 20 2016, 06:59 AM
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#4
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Senior Member Group: Members Posts: 913 Joined: 3-October 06 From: Ossining, NY Member No.: 6,985 Region Association: None |
You have the right kind of cutoff switch? Like this (and wiring diagram)? You mention the resistor, but just checking ...
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| screenguy914 |
Feb 25 2016, 02:12 PM
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#5
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Member Group: Members Posts: 249 Joined: 6-July 09 From: So. Cal Member No.: 10,540 Region Association: Southern California |
Alternator needs a load when running. Disconnecting the battery cable while engine ON will kill the diodes in the alternator. That's why yours died.
A proper shut off switch (see above) should incorporate a resistor circuit to shunt alternator output in case the battery must be disconnected in an emergency (as in a racing/collision mishap). The simple, single pole bat. switches are good for theft deterrence and to prevent discharge during long storage periods. Sherwood |
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| motorvated |
Feb 27 2016, 08:13 AM
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#6
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Member Group: Members Posts: 279 Joined: 13-February 13 From: Colorado Member No.: 15,519 Region Association: Rocky Mountains |
QUOTE(stownsen914 @ Feb 20 2016, 05:59 AM)  You have the right kind of cutoff switch? Like this (and wiring diagram)? You mention the resistor, but just checking ... That's the same switch I have and it's wired in properly with the alternator output shunt circuit going through a 3 ohm resistor to ground. Pulled and replaced the alternator and the new one is working correctly. I haven't shut off the car using the cutoff switch since I put the new (good used one) alternator in, as I don't want to press my luck. Had old alternator tested and it's putting out about 2.5 volts with a dead short. I'm going to pull it apart and check the diodes etc. if I can fix it, I'll keep it as a spare. Eventually I'll have to demonstrate that the cutoff kills the engine to a race scrutineer, so that will be the test to see if I fry another one. Still don't know what w why this happened with the switch wired in properly. |
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| 0396 |
Mar 3 2016, 02:48 PM
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#7
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Advanced Member Group: Members Posts: 2,046 Joined: 13-October 03 From: L.A. Calif Member No.: 1,245 Region Association: Southern California |
QUOTE(stownsen914 @ Feb 20 2016, 04:59 AM) You have the right kind of cutoff switch? Like this (and wiring diagram)? You mention the resistor, but just checking ... This is what I use and how I wired mind. No issues |
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