pioneer 80prs voltage incorrect?

So I have a Pioneer 80-prs head unit in a 2011 Jeep Liberty. Having the voltage display on the head unit is rather nice, but I've always noticed that the voltage is a bit lower than expected. Measuring back at the amps, it is about .5-.7 volts higher. This was tested with a multimeter and ran a dedicated volt display from the amps. I can only think of three things:

1) Some type of reverse polarity protection in factory wiring leading to radio harness utilizing a diode (hence the constant .5v - .7v difference)

2) Crappy jeep wiring with a high resistance causing a voltage drop

3) Something wrong with head unit

Any ideas?

Here's the comparison. Also, this occurs when the engine is running, not running, music playing, music not playing, etc. Happens all of the time.

It may simply be calibration if it doesn't vary with load or voltage tho that is usually proportional wrt 0V.

If it's a near constant ~0.7V drop it's likely to be a diode drop as you said.

Hopefully it is monitory the battery constant voltage since that should not suffer the same voltage drops is ACC or IGN circuits, but then that will drop under load if cable resistance or current is high enough.

But if you have 14.6V longterm, I'd check your alternator output - 14.6V is too high except (generally) for AGM batteries.

did you use the can-bus radio wiring adapter to install that radio? if so then the power for your radio is running through this integration harness and could be causing a difference in voltage.

oldspark wrote:

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It may simply be calibration if it doesn't vary with load or voltage tho that is usually proportional wrt 0V.

If it's a near constant ~0.7V drop it's likely to be a diode drop as you said.

Hopefully it is monitory the battery constant voltage since that should not suffer the same voltage drops is ACC or IGN circuits, but then that will drop under load if cable resistance or current is high enough.

But if you have 14.6V longterm, I'd check your alternator output - 14.6V is too high except (generally) for AGM batteries.

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Hmm good points. I guess the next step is to pull the radio out and measure the voltage at the harness. That would show if it is the head unit or within the vehicle wiring. Due to the voltage difference not varying with current (ie, low volume to high volume), I would eliminate the possibility of crappy wiring/high resistance. That leaves either some type of protection circuit/diode or a calibration issue on the head unit. I'll post back after I get a chance to meter the harness.

soundnsecurity wrote:

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did you use the can-bus radio wiring adapter to install that radio? if so then the power for your radio is running through this integration harness and could be causing a difference in voltage.

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I'm not exactly sure what this can-bus radio adapter is, but I don't believe I used one. I used the harness that plugs into the factory connector leaving open ended wires for power, ground, speakers, etc on the head unit. Then the 80prs has a radio antenna dongle that the vehicle's antenna just plugs into.

IMO (displayed) load voltage monitoring is close to useless unless you already have a battery voltage display, tho it is better than nothing (ie, the load MIGHT tell you that you have a charging issue, or crappy wiring).

But I'd be prioritising a battery voltage display. THEN worry about loads (which usually need only be alarmed - not permanently displayed).

the can-bus radio harness adapter is used on all dodge jeep and chrysler cars from 05 and up. the cars can bus data system runs through the factory radio and also tells the factory radio when to turn on and off. you can usually still install a radio without this special adapter but you would have had to run the red wire of the pioneer to a switched ignition wire somewhere else in the car so that the radio knows when to turn on and off. if you bought this special adapter you would know it because it costs around $120.

that was just a thought, but if the voltage difference is not a calibration problem then i would try relocating the ground of your 80PRS and screw it straight to the large metal tube brace that runs behind your radio. sometimes using the factory ground wire in the factory plug is not good enough because the aftermarket radio might need more power that the factory radio did. and also make sure that the ground is the same size as the ground in the pioneer's plug, that way the electricity can flow as easily as possible.

Okay so I had a chance to meter the harness while attached and while disconnected from the head unit. In both cases, voltage read 11.3v on the meter at the harness and 12v at the amps in back. This rules out the head unit. I also tested this using different ground points. The .7v difference makes me almost positive it is a diode drop somewhere - most likely a reverse polarity protection circuit.