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Old 11-26-2010, 02:38 PM
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Voltage drop

Time for everybody's favorite scavenger hunt, electrical gremlins! Thanks in advance for any help, these are no fun to track down.

Here is what I've got.

Alternator is working correctly. At idle, puts out 12.5 volts, at 1300 rpm, puts out 14.4 volts. No voltage drop at the batteries or battery clamps.

However, voltmeter on the dash shows very low (at the N on normal at crusing speed and VERY low at idle. If I have the lights on, it will sometimes shut off at idle). I have an auxiliary idle controller, with digital voltmeter, hooked up and it reads 10.6 at idle and 12 at crusing speed. So, I have a significant voltage drop between the batteries and the cab.

I made some progress today when I discovered a loose fuse. The fuse for instrument illumination was loose (it used to have a jumper in it, now it doesn't fit tight). My aftermarket gauges are illuminated off that circuit. After fixing that issue, I now see 13 volts in the cab at cruising speed.

That means I am still seeing a voltage drop of 1.5 volts. Ouch.

Any suggestions of where to continue the hunt or what I should be investigating to solve this issue?

Thanks!
-Charlie
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Old 11-26-2010, 02:51 PM
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bump up for ya......
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Old 11-27-2010, 04:32 AM
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Charlie,I had something similiar to your gremlin.It turned out the driver's side negative battery cable was not making a good connection on the engine block.I removed the bolt and installed a lock washer.When chasing this gremlin I took jumper cables and connected both negatives and instantly the charging improved.Next I jump cable from Left battery negative to ac compressor bracket while engine was running and again charging returned.This led me straight too negative cable on drivers side. I think I had two two different volt reading at batterys,the left side was was below normal before my repair.Hope this helps
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Old 11-29-2010, 08:17 AM
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Thanks, Chop. Seems like I have something a little different going on here. Voltage in the engine bay is consistent whether I take it on the battery post or cables. It is the same across various ground points, so I think those connections are OK (anything I'm missing there?).

The trouble is in the cab, where it reads so much lower and the truck acts like the voltage is lower. Wondering how to hunt down the in cab voltage drop?

Thanks!
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Old 11-29-2010, 11:17 AM
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I would start with checking all the grounds in the cab. If the floor was wet or rusty, it can impact the quality of the ground connections.
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Old 11-30-2010, 02:58 PM
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your battery cables might have developed a crack and be corroded on the inside, had this happen
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Old 01-06-2011, 08:14 AM
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had the same situation for awhile - ( gauge would sit at normal, then when i held brake and put in gear the voltage would drop - hit the accelr8r and the alt would charge it up to norm - stop at light and it would drop way down )

i thought my batteries and alternator were all good , but i replaced them at the same time and now it charges and works perfect - just stays at normal no matter how much electrical load i put on it

(one of my batts did have a hairline crack on the top end near the post , & there was some acid build up and corrosion on that terminal)
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Old 01-06-2011, 12:44 PM
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Thats what I had in mind.It sounds like your alt is trying to do its job,but the batts may have seen better days. Disconnect both of them and try aload test to make sure.Mine was doing same thing as you described , turned out to be bad on drivers side.Found a new spot to power up cb,haha.
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Old 01-08-2011, 08:53 PM
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I have been have a similar issue. low idle low voltage output, even when I use my idle controller, and set it to charge protect, I only hit 14.1 but that is with nothing on. If I turn the heater on or headlights on drops to 12 or so, and it drops to 10 if at an idle. But I did notice that if I but the truck on a charger over the weekend then it runs alot better for acouple weeks. Then starts dropping down again. I'm gonna try the battery ground thing tomorrow.
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