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Vanishing Voltage...
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Modoc
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Joined: October 02, 2007
Posts: 16
Location: Northern California
Modoc is offline 

PostPosted: Sun Sep 11, 2011 7:32 am    Post subject: Vanishing Voltage... Reply with quote

Have a 69 I'm working on.. With excellent advice yesterday from Samba found my timing issue then a new gremlin came home..
Started no problem, turned off, next time I try to start, no dash lights no nothing..
12v+ at the battery, voltage to the fuse block, voltage to the ignition switch, till I turn the key then 12 volts constant to the back of the ignition switch just goes away, voltage from the battery to the fuse block goes away.. Voltage anywhere, except the battery just disappears as soon as the ignition switch is turned on..
Logic would tell me bad ignition switch but strange that the constant coming from the battery to the fuse block would also vanish before it even gets to the switch..
Have an alternator so no regulator involved... Put in a new harness and have just enough wired to make the engine go.. 12 volts direct to the fuse block and no blown fuses or crisp wire.. Been working fine and grounds look fine..
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69 Jim
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Joined: September 27, 2004
Posts: 6265
Location: Chickengeorge's Neighbor
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PostPosted: Sun Sep 11, 2011 8:32 am    Post subject: Reply with quote

Are you sure the cables are tight at the pan/starter connections? You could possibly have a bad cell in the battery.
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Modoc
Samba Member


Joined: October 02, 2007
Posts: 16
Location: Northern California
Modoc is offline 

PostPosted: Sun Sep 11, 2011 8:55 am    Post subject: Reply with quote

Yep, like I mentioned, ground up rebuild so have installed just enough wiring to make the engine tune, no extra switches just what I needed to get it turning..
Limited wiring so ran it all after the problem arrived..
Has been working fine for months then.... The Amazing Vanishing Voltage..
Key doesn't even have to be turned to crank the engine, just on and everything goes away..
Battery is old so I'll check that out..
Thanks for the help..
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Wayne S. Johnson
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Joined: April 12, 2006
Posts: 1265
Location: GILROY, CA
Wayne S. Johnson is offline 

PostPosted: Sun Sep 11, 2011 1:01 pm    Post subject: Reply with quote

You can find the problem by looking for the voltage drop starting from the battery posts.
You may have developed a high resistance connection, which allows the voltmeter to measure 12 V with the load off, but not when the load is on. The voltmeter does not take enough current to cause a voltage drop across a high resistance connection.

Start with battery posts, the battery may be zero volts under load and 12 V with no load.
Measure the voltage at the battery posts (not the cable terminals) with a load on.

Then start looking for voltage drop between all connections with the load on that creates the problem.
Here a few examples:
Ground the voltmeter to the chassis and check for voltage at the following locations.
Negative battery post, which should be zero.
Positive battery post, which should be 12 V.
Look at the wiring diagram and follow all connections form the battery to the dash area and measure the voltage on both sides of each connection, the male and female terminals.
http://www.thesamba.com/vw/archives/info/wiring/bug_68_69_usa.jpg

Diagnosing voltage drop
Electrical automotive troubleshooting
http://www.aeswave.com/Articles/ProductArticles/F-AppNotesVoltDrop.pdf

If your manuals do not list voltage drop values, use the following as maximum limits:
• 0.00 V across a connection
• 0.20 V across a wire or cable
• 0.30 V across a switch
• 0.10 V at a ground
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