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Low Voltage readings at fuse box


jrhone

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So I get in the car tonight and my voltage gauge reads 10 volts. The car is running fine and the lights seem as bright as ever. I'm baffled. Turn off all extras (radio, heater fan, etc) and no change. Doing about 60 at about 3500 rpm. No change. 10-10.5 volts is the reading. So I get home and I grab my multimeter. The battery reads 12. Then I pop the fuse cover off and start to take readings while the car is still running. Almost all are reading 10-10.5 volts. Some are reading 12 but I would say 70% are at 10-10.5 volts. What gives? I checked the ground strap from the battery and it's in great shape and secure. All the grounds Under the hood look fine. Any ideas? My initial thoughts are some of the circuits have more draw than others and the engine is getting all the power it needs but the accessories and lights are not getting full power. But why?

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Assuming you have the later 12 fuser, it may be a bad connection or a high resistance in your ignition switch contact.  Fuse 7 is connected directly to the battery with a shared ternimal to fuse 8.  Both sides of these should read battery voltage unless one fuse has a bad connection.  The ignition switch is fed by another battery wire and all remaining circuits/fuses should read 12v with it turned on or accessory positions. FYI in start position at 10v still may be enough to engage the starter solenoid. I'm not sure if the later cars have headlight relays with 12v feeds or not.  What terminal feeds your VM?  You can start there and trace backwards.

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The accessories are run off of relays. All that goes through the fuse box is power to trip the relay. Hence, the accessories appear to be functioning correctly (without low voltage) because the relays are powered directly off the battery.

 

Your brake lights, turn signals etc will probably be weak as they are not relayed.

 

Also, did you rotate the fuses? If not, that could be a source of your low voltage. They build up resistance and need to be rotated from time to time(spun in place) to break up the corrosion.

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I'd suspect corrosion on fuses--and those fuses do wear out--the soft fuse metal gets worn through by the harder brass contacts.  

 

If you can still find 'em at an office supply store, a typewriter (I know, what's that?) eraser makes a great cleaner for both fuse ends and their brass contacts.  It looks like a pencil, but instead of lead it has a rubber eraser center, and the rubber has a little grit mixed in so it's more agressive than a regular pencil eraser.  

 

Those things are also great for getting the rust out of a little stone chip before you touch it up.  The trick is finding one these days...

 

cheers

mike

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May be the fuses.  The odd thing is that it was reading 10 volts last night on the fuse contacts.  I moved the fuses around and replaced a few and a few were the wrong sizes, then pulled my gauges and looked at the radio and gauge wiring, then checked it all and its back to 12-13 volts.  Odd because I really didn't do much at all.  I did confirm my VDO voltage gauge ready about 1/2 volt low.  So when its at 11.5 its really 12.  The low voltage reading after putting it all back was 12.25 on the meter with the lights and electric fan on and a high of 12.9 with the lights off.  So lets consider this solved.  I did run a bit of fine sandpaper over the contacts in the fuse box just to get a little better contact...

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Ok update. The voltage today was back down to around 10-10.5. I was baffled. Got on the freeway and still 10 volts then suddenly BAM it jumped to 12.5. Then it was a solid 12.5. Even with the lights and electric fan it was about 12.5. I'm not too worried but this may explain why it was hard to reproduce. If once the car warms up it somehow jumps up in voltage then that's why it suddenly went from 10-12 with no real explanation. Gonna check all cables and make sure everything is tight.

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12.5 was at night with elec fan running and lights on.  This morning with no lights on it was over 13.  Thats what it used to be.  I just replaced my bad alternator with an e21 alternator.  Reman Bosch unit, not a cheap rebuild…What was odd was how it just jumped from low to a more "normal" reading and now its rock solid.  

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