I have a 1996 f150 4x4 w/ a 4.9. Here's my problem. Driving down the road, w/ the AC on, Headlights on, radio on, and the wipers on and the volt guage sit on the letter M in normal (about 14 V ) but when I come to a stop, the guage drops to the O in normal (about 12 V ), anybody else have this problem ?? I've replaced the Alternator and no change.If I sit there and give it a little gas the needle starts to move back up. I just had a 130 amp alternator built to give me a few more Amps at idle, but the guage still drops when I come to a stop.
HELP !! Help !! Help !!
What is going on ???

Ed

 

What happens if you turn off the AC and you come to a stop sign? Does the voltage drop to 12 volts or stay around 14 volts?

I've seen this kind of problem before and it ended up the voltage gauge itself in the dash. Either have something to do with resistor or wires.

You can check it for yourself manually by using any cheap digital volt meter and get some long wire extensions. Just connect it to the positive and negative terminal on the battery and put the digital volt meter inside the vehicle and test it with AC on, lights on, radio on, etc and come to a stop sign. Check the dash gauge and compare it to your digital volt meter to see if it shows the same voltage reading.

If both the dash voltage gauge and the digital volt meter shows 12 volts at the stop sign then there is problem in the wiring from the alternator to the battery or the serpentine bent needs replacement and make sure that the serpentine belt tensioner itself is good.

If the dash voltage gauge shows 12 volts but the digital volt meter shows 14 volts then the problem is with the dash voltage gauge.

 

I did check the voltage with a meter right at the Alt. w/ the Parking brake on and in drive with all the mentioned items on and it was 12.3 V. when I turn the AC off it goes up to 13.1 V. If I turn everything off it goes up to 14.1 V. This is with a new Serpentine belt installed. There seemed to be a lot of tension on the belt from the Tensioner And I didn't hear any belt squeek or smell any rubber burning from the belt, so I think that's OK.Just to be sure I'm going to turn everything on and try to tighten the tensioner and see if the volts go up. Thanks for the response

Ed

 

There's NOTHING seriously wrong with your alternator, gauge, or electrical system.

1.) Alternators don't put out full amps until they are spinning above 1000 rpm in most cases. The higher the charging amps the lower the voltage. When the battery is charged to a float charge the alternator will alter the voltage as much as a volt and you may see a fluctuation. THIS IS NORMAL.

2.) As long as your voltage goes up to the top end of normal range (13.5 -14.9 or so) without a load on at idle or just above the system is ok.

3.) The alternator just puts back in what the system uses from the battery. The alternator doesn't run the system, although this is a popular misconception. The battery has to have enough reserve capacity to handle the load at all times. If you have a battery with too small a reserve capacity (amphours) you'll end up burning out, or shorting a cell and the battery is worthless, except as an anchor.

4.) to ensure your system is working at peak efficiency, clean all connections (body and frame grounds) that are accessible, as well as the connections at the battery, solenoid, and the starter. Get all connectors to shiny metal, and coat with di-electric grease. Use Main cables that are capable of handling the loads. Main battery cables on FORDS are notorious for being undersized and easily susceptable to internal corrosion. Get some 2ga or 0ga welding cable, have the ends swedged (inserting the end of the cable into a melted cast), and coat with di-electric grease. A little goes a long way.

Fluctuation of the gauge is NORMAL. IF it stays down at 12.5 with NO LOAD and at 2000 rpm and drops slowly......THEN you need to look at a new alternator or battery.


Larry