I have a 2002 WRX and also have a Escort Passport 8500 which has a volt meter built into it. I have noticed 2 times in the last month, the volts dropping below 11.0 volts which triggers the voltage alarm on the 8500. But, I notice no issues that would reflect this within the car. Such as dimming lights, radio volume fluctuation, idiot lights flashing on the gauge cluster, etc..... Also, the car starts up just fine, which indicates the battery is charging correctly.

I have tested my alternator and battery this morning. The number in ()'s is what the 8500 said. I was using one of the 3 front/driver shock mount screws as a ground.
Battery - cold before start = 12.2 volts (11.6)
Battery - at idle after start = 14.3 volts (13.7)
Alternator - at idle after start = 14.3~14.4 volts (13.7)
Battery - warm after shutoff = 12.4 volts (11.8)

I have noticed there is a very consistant 0.6 volt difference between the volt meter readings in the engine compartment and what the 8500 says on it. This is either due to mis calibration of the 8500, or loss of voltage through the dash wiring harness.

To give you some background, the 8500 is over 3 years old and I have had overheating problems with it after the 1 year warranty ran out. I live in South Carolina and it gets pretty hot in the summer here. I used an ac unit thermometer to measure the 8500 once while I was sitting in the car and it was 102 degree F outside and very sunny. The top surface of the 8500 got up to 154 degrees and was climbing. And the ambient temp in the car got to just over 130 degrees. I have called several times about getting it fixed or replaced, but I would have to pay shipping both ways and a $35 service charge just so they can look at it. I would also have to pay extra if any hardware was replaced. Maybe my 8500 is not messed up. I works for the most part about 99% of the time, but then on 2 very fluke ocassions, it went crazy.

What do you think????

O, I also did a search before posting this and did not find anything related to this topic,

I vote for rust/bad connection in the power plug socket (cig lighter socket). A poor connection will have higher resistance and so a voltage drop, so the 8500 will see a lower voltage.

Update.

I checked the voltage at the cigarette lighter wires, and it was 13.6 volts and the 8500 was reading 13.1 volts. I did this by checking the voltage on my gauge pod, which uses the hot and ground wires from my cigarette lighter as a power feed. I used this setup so my gauges would have a fuse, and I did not need to run alot of wires to the ignition. I also used the dimmer lines that are setup for the OEM steering column boost gauge accessory.

So, it appears my wiring harness has a voltage loss of 0.1 volts, and my 8500 reads 0.5 volts low.

As far as the rust. The 2nd time I noticed the weird activity, I turned off the 8500, pulled out the plug and reinserted and turned it back on. I also rotated it a little back and fourth to freshen the contact surface. And it still had the problem. But, the next time I cranked the car up, it was fine.

I noticed you used the dimmer lights connections. I'm assuming Illumination (+) and Illumination (-)... both of these change potential WRT the common ground on the car. In the device you have the ground planes for illumination and main power might be connected, so changing the illumination would change the voltage reference used as "ground" by the main power circuit... this would cause a lower voltage reading by the unit. If this is the case, you'd have to cut the trace on the circuit board that connects these two ground planes. I had to do this on my a/f gauge.

There are 2 specific wires that run to the radio for dimming. Just before the plug that goes into the back of the radio, there is another 2 prong connector. This plug is specifically designed for ilumination and to be connected to the Subaru OEM boost gauge that mounts onto the steering column. These are the 2 wires I use for the bulbs on my AutoMeter gauges in my gauge pod.

For the ground and hot wires for my gauges, I taped into the 2 wires that connect to the cigarette lighter. These are 2 completely different wires from the 2 I used for the iluminations wires. They may connect somewhere, but the cigarette lighter voltage does not change when the dash light brightness is adjusted.

My Passport 8500 is plugged into the cigarette lighter using the plug supplied with the 8500 in the box.

When I checked my voltage this morning, the car was running and the 8500 was turned on. I pulled the gauge pod off, with all the wires intact. The oem pod slips off pretty easily. I touched the gauges hot and ground wires with the 2 probs on the MultiMeter voltmeter I have. I did not use the bulb wires.

So, there is no reason the 8500 and the multimeter should say anything different. They were in affect powering off the exact same 2 cigarette lighter wires.

Hope this clarifies it a little.

1) You could lose some voltage throught the cigarette lighter connection and there may be some loss in the Passport itself.

2) As far as checking to see whether the two grounds are connected (ground and Illumination (-)) read the voltage on the Passport display and then adjust the dimmer switch for your dash. If the voltage on the display changes, then you will have to cut the trace on the circuit board to solve this. If the voltage on the display does not change, then you can conclude that this isn't the problem and #1 is more likely the cause.

3) I just had another thought. The voltage reading generated by the Passport is most likely the result from an analog-to-digital convertor that some micro is converting into a number for display. If the a/d is not very precise (like a lot of a/d's that come in microcontrollers) then the display will have a lot larger increments. In your case, maybe 0.5V. So, 11.6V may read as 11.3V and you might need 12.0V before the A/D even notices a difference.

So, there is no reason the 8500 and the multimeter should say anything different. They were in affect powering off the exact same 2 cigarette lighter wires.
You can't really assume that until you measure the voltage upstream and downstream of the socket, or measure the resistance across the socket.

bump