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During a fault finding mission on my car I found the Vref on a sensor that should be 5V sitting over 7V. I traced it back to one of the modules and found the pcb mounted voltage regulator was outputting the 7V I'd seen at the sensor. The reg was a LM2931-5.0 fixed voltage regulator in TO-92 config.

In doing some research on the specs I thought it may have failed due to engine bay heat and replaced it with another LM2931-5.0 TO-220. I changed to the TO-220 as it had better heat handling characteristics. The problem is that the new reg is now passing the full 14.5V through. I tested continuity at the pins and there is no shorts.

Has anyone got any ideas why these regs aren't dropping the voltage to 5V as they should? The pins are slightly larger on the second reg and took some force to install. I wondered if I'd damaged it doing this but I thought it that were the case I would see a short between them.

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    \$\begingroup\$ Different pin out? (did you buy more than one replacement? Pull the one you put in and check it's still good. It's also possible that some other part of the circuit failed. \$\endgroup\$ – George Herold Jan 21 '15 at 0:54
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My best guess is that you have the new regulator installed backwards. Easiest way to check is to grab the data sheet for both devices and compare pin-outs. Conversely, grab the datasheet for just the to-220 device and make sure that the output pin goes to the line that you identified earlier as being the 5V rail.

Couple of things to consider.

1) there is a strong possibility that you have damaged or destroyed whatever is downstream of that regulator. Most 5V-only devices can tolerate somewhere between 5.75 to 6.5 Volts applied to them before becoming damaged.

2) Check the data sheets for both regulators to ensure that you have the minimum output capacitance on the output pin. The LM2931 is unstable with an output capacitor that is too small. I don't recall if the minimum capacitor value is the same for the large vs small package devices.

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