Electrical Question #15 Need Help Please, I'm totally stumped this time |
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Electrical Question #15 Need Help Please, I'm totally stumped this time |
76-914 |
Apr 6 2014, 06:33 PM
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#1
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Repeat Offender & Resident Subaru Antagonist Group: Members Posts: 13,611 Joined: 23-January 09 From: Temecula, CA Member No.: 9,964 Region Association: Southern California |
I'm going to have several electrical questions. 1st, Is pin #8, on the 14 pin plug at the relay board, the only switched 12v+ source in the engine compartment? TIA, Kent
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stugray |
Jun 4 2014, 02:04 PM
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Advanced Member Group: Members Posts: 3,825 Joined: 17-September 09 From: Longmont, CO Member No.: 10,819 Region Association: None |
Cut & paste from my response to another forum where someone was troubleshooting an AFR gauge, but most of it still applies:
""Grounding" is a science all to itself. Cars typically use chassis as a "Ground reference" and a "Current return path". For sensitive electronics (O2 sensors) they need a proper "ground reference" that tells the circuitry what voltage is "Zero volts" as far as the system is concerned. When you use car chassis as this ground reference, then it is not always at "zero volts" with respect to the rest of the car because the voltage of any particular point in the chassis is dependent on how much current is flowing through chassis and where the current is flowing. So if you "reference" the O2 sensor to car chassis close to the sensor, then the ground can move up or down depending on if (for example) the radiator fans are on or off. This is because current flowing through the chassis causes voltage drops in this reference. For the more sensitive sensors, you should run a dedicated "ground reference" from the sensor to the unit reading the sensor. If you use chassis for this, then the "zero volts" reference is not always at "true zero". If anyone cares and is confused by this, I could make a diagram..... " "This is where "grounding" gets a little confusing. The meter/O2 gauge needs +12V power so it needs a ground (called a 'return' in my business because it intentionally carries the return current to the power source). This would go to the battery negative. The part of the system performing the voltage measurements within the O2 sensor needs a ground 'reference'. This ground does not carry any significant current so it does not experience voltage drop during operation. That ground 'reference' should go from the Measuring unit (gauge or ECU) all the way to the sensor. Some older sensors (VDO oil pressure for instance) did not provide a dedicated ground to wire to and depended on the car chassis to provide the path. Newer sensors typically have dedicated reference 'grounds'. Theoretically, you could hook the reference wire to car chassis at the sensor end and at the gauge end and in most cases it will work. Is it accurate? Each installation could be different and give different results." So to answer your questions: "Pickles" :-) In all seriousness: If the "Ground" actually carries current, then the wire needs to be the same gauge as the "positive" wire or larger. In a perfect world we would connect all "grounds" to one point, know as the "Single point ground" but we dont usually have that luxury, so the entire car chassis is the "SPG" (we call it 'SPUG' at work). Material does not usually matter except for long term corrosion problems or highly sensitive signals that could be upset by galvanic voltages being induced. Did I completely confuse you yet? |
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