I have an E 350 6.9 liter.. Hree is my problem. sometimes the alt guage is slightly on the discharge side, not much just a hair. I notice when this happens the lites are somewhat dim. After about 1o or so miles it jumps to the charge side .. At this point lites would be brighter. And then again after some time it goes back to discharge. I have changed the voltage regulator and still same problem. Have checked all wires going to and from alternator ( what a pain that was) ... Is it possible the alternator is only charging now and then??? I have always thought it either charged or it didn't... Any body have any ideas on this?????

Thanks....

 

An alternator will become intermittent when the brushes are worn out. You can try replacing the brushes.

 

Lots of Fords do this, and frequently the voltage regulator is to blame. If the battery is fully charged the voltage regulator will sometimes stop the alternator from charging, then once the system voltage drops a bit it will kick back in. As long as your battery stays fully charged this isn't really a problem at all...just an annoyance.

You can always try a new voltage regulator, but the problem often stays. One cheap/ easy "fix" is to install a diode in line with the voltage regulator's sensor wire [the #1 terminal/ thin yellow wire on most years]. This will do two things: First, it will prevent current flowing back thru the amp meter to add-on accessories when the ignition is off [big problem w/ older vehicles]. Second, the slight voltage drop across the diode [about .3 volts] is often just enough to "trick" the regulator into charging more consistently without overcharging the battery.

The ideal diode is a 1 or 2 amp/ 100 volt unit. Big enough to handle the small current the sensor wire sees, and small enough that there's not too big a voltage drop across it. The diode gets installed with its gate [the white line] facing the regulator.

This fix works on most older Ford's with a "stand along" external voltage regulator, but if you have a newer model where the computer controls the charging system...I'd be more hesitant to try it.

 

Bad idea!!!

First, the voltage drop across a silicon diode is more like 700mV, which would cause an overcharge condition. Second, accessory current DOES NOT flow through the regulator sense circuit!!

Finally, a diode does not have a "GATE"!! The white band on the body of a diode is the designation for the cathode terminal, the other terminal on a diode is the anode. The only electrical devices that have a "gate" are FETs, and Triacs, and these are 3 terminal devices......

 

Even though silicone diodes have a theoretical forward voltage drop of .7v, the actual drop depends on the diode size, construction quality, electrical load, temperature, etc. High amp diodes like those found in dual battery isolators can easily show a full 1+v drop under load. A qood 1 amp diode passing a trickle may only cause a .3v - .5v drop, not enough to cause an overcharge problem. If in doubt, use the diode check function of any decent digital multimeter to check the drop. Better yet, monitor your vehicle's system voltage in operation.

The 1amp/ 100v diode wired to my van's regulator yields a no load [ignition only] voltage of 14.6v [compared to 14.5v without the diode]. Turn on any accessory [vent fan, parking lights] and that 14.6v drops right to the "ideal" 14.4v. Under heavy load [lights on, cooling fans running, etc] it keeps the voltage up at a decent 13.8v-14v depending on conditions. Without the diode the regulator allows for wider voltage swings, most notably causing that annoying bright/dim cycling of the headlights. FWIW: I'm using the stock circa '83 regulator, tried two new ones and they performed worse than the 20 year old original.

Re the "accessory current": I NEVER said "accessory current" flowed thru the sense circuit...backflowed to assessories. In many older vehicles, when the ignition switch is off, a small amount of current can flow back thru the regulator sense wire and into parts of the vehicle's "switched" electrical system via the amp meter or charge indicator. This is a tiny amount of current, but enough to cause a battery drain over time. It is also enough to keep some aftermarket ingition systems energized which can cause run on. This can also interfere with current/ voltage sensing circuits of alarm systems, causing a variety of odd problems. Plenty of aftermarket ignition products and security systems come with isolation diodes for use just as I've mentioned.

Oh...and PLEASE forgive my terminology faux pas, picked up the habit of refering to a diode's anode side as "gate" at some time in the past.