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Registered
Join Date: Jun 2004
Location: Puerto Rico
Posts: 317
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0.25v at engine ground
Hey guys. I am loosing my engine ground once I start the car. When the car is off or with just the ignition switch I ger ground on the fan shroud. As soon as I start the engine I get 0.25v at the grounds. Before removing the alternator, any other possibilities? Thanks in advance.
Last edited by ivanuf; 09-19-2020 at 02:32 PM.. |
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09-19-2020, 02:17 PM
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Grumpy old hasbeen
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I would guess there is some resistance on the grounding. What is your other ground point you are using when you are measuring?
May be as easy as just cleaning up some grounding straps etc.
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Perth, Western Australia '74 911 Targa |
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09-19-2020, 03:59 PM
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Location: Burford, ON, Canada
Posts: 2,319
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True ground is the neg battery post. Anywhere else but the battery post will be a little positive and that depends on the amount of current flowing. Every connection and every length of wiring or even the car body adds a little resistance. Since V=IxR (voltage equal current times resistance) any current flowing will cause the resistance between two points to show up as a voltage difference. Cleaning all the ground connection would help. Using the fan shroud as a ground point is not ideal. What two points are you measuring when you get the 0.25V reading? Meter error can also be a factor.
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Keeper of 356, 911, 912 & 914 databases; source for Kardex and CoA-type reports; email for info Researching 356, 911, 912 & 914 Paint codes, Engine #'s and Transmission #'s Addicted since 1975 |
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09-19-2020, 06:20 PM
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Join Date: Jun 2004
Location: Puerto Rico
Posts: 317
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I am using a Powerprobe connected directly to the battery positive and negative. It was all working fine. It all started after I lost my boost gauge reading and I norrowed down to lost of ground when engine running. So far I placed a temporary ground to the top of the intake And that solved my boost gauge. But it also may affect my CDI box if not properly addressed as recommended by ************ when they recently repaired my damage CDI under warranty. Grounds have been cleaned with the exception of the one on the alternator. My guess is that the problem is there on that strap or the alternator itself. But wanted to get some feedback before starting removal. My SC has no change on the ground of engine when engine running.
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09-20-2020, 06:13 AM
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Registered
Join Date: Jun 2004
Location: Puerto Rico
Posts: 317
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Solved, alternator ground strap to engine block was loose.
To test before removal I just removed the fan belt and started the engine. Ground was good, so it was for sure something to do with alternator. Thanks
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79 SC Back to Basics! 2006 997 C2s Cabrio - Sold 1999 996 C2 - Sold |
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09-20-2020, 01:02 PM
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Posting same time you were, but anyway.
You should have ground wires from the battery, transmission, CDI console to chassis, and alternator to engine. 
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Ed 1973.5 T Last edited by E Sully; 09-20-2020 at 01:06 PM.. Reason: You solved it while I was writing |
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09-20-2020, 01:04 PM
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The Other Woman
