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Registered
Join Date: Mar 2003
Location: napier, new zealand
Posts: 33
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please help...variable voltage & bouncing tach
hi all
my tach started bouncing +/- 500 at all rpms recently, so I started testing the voltage at various places. on battery terminals it is 12 volts off and apx 13volts with the car running. seems normal enough but testing the cigarette lighter in the car it jumps between 15-16volts with the car running. obviously not normal. strange thing is (maybe?) at the cigarette fuse on the fuse panel its at apx 13volts. normal? if it was a bad alternator/regulator shouldn't the voltage be bad everywhere? At the rear console in the engine bay between the switched red on the panel and the ground its 15-16 volts again. the car is a 69T with one battery - engine is a 2.4 but with the original alternator. It has an MSD I will have a voltage regulator tomorrow to swap out and test the current one - but does the various voltages mean a particular ground strap or connection...maybe? apologies to those with real electrical knowledge who are cringing at this point. thanks in advance nathan here's a gratuitous pic - feedback on parts bought through the BBS for the project will be posted this weekend. 
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04-28-2005, 04:08 PM
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Registered
Join Date: Mar 2005
Location: Austin, TX
Posts: 2,271
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Sounds like it could be the voltage regulator. You shouldn't see much more than 14v with the engine at 2500+rpm. From what I gather, a faulty or failing voltage regulator will cause the voltage going up above 15v at speed, can cause the tach to bounce.
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Matt J. 69 911T Targa - "Stinky" 2001 Boxster "Stahlgewehr" |
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04-28-2005, 04:22 PM
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Registered
Join Date: Dec 1999
Location: TX USA
Posts: 9,804
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Nathan,
The vaying Voltage readings may be a grounding problem, or it may be a matter of not making a good connection to a somewhat oxidized or corroded terminal when you were making the measurements. A measured 13.0 Volts is not high enough to meet factory specs, but it may be a case of a meter reading slightly low. High Voltage output can only be caused by a faulty regulator, so readings of 15 - 16 Volts is definitely indicative of a faulty regulator!
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Warren Hall, Jr. 1973 911S Targa ... 'Annie' 1968 340S Barracuda ... 'Rolling Thunder' |
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04-28-2005, 05:11 PM
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failing or 'closing' ignition points in the distributor will also cause an erratic tach..but yours definitely sounds voltage regulator related.
ryan
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To the memory of Warren Hall (Early S Man), 1950 - 2008 www.friendsofwarren.com 1990 964 C4 Cabriolet (current) 1974 911 2.7 Coupe w/sunroof 9114102267 (sold) 1974 914 2.0 (sold) |
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04-28-2005, 05:18 PM
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1974 Porsche 911 2.7
