1994 F250 4x2 460 -125063 miles.

Recently, after running for a few minutes and everything is warmed up,
particularly after being on the highway. I've noticed that the voltage
indicator in the instrument cluster shows a sudden drop in voltage (by about
half) and the lights (head lamps and instrument cluster) dim. No other
warnings or indications or problems.

Before I changed the negative cable connection at the battery, the voltage
indicator needle would jump erratically, between showing full voltage (lights
to full brightness) just to drop again. With the new battery connection, it
will jump to indicate a sudden loss in voltage, but now the voltage seems to build
up slowly instead of erratic flickering of the needle.

Also the alternator is brand new, battery a year old.

Just replaced catalytic converter, after it melted down, which created a myriad of
problems: hard shifting of the transmission and wonderfully poor performance. An exhaust
leak along with a broken EGR tube (repaired in July) and age are thought to be the leading
culprits to the cats demise.

Of course when I took it to my mechanic, everything behaved perfectly, and
everything checked out fine. Only to misbehave on my way home.

I know this must be something simple.

Any ideas gentleman?

Any directions would be appreciated.

 

When you figure it out, let me know. Ive had the problem for a while and I cant seem to figure it out.

 

If you are sure you have good clean grounds, then you may check the wiring between the battery and the alternator. A corroded lead can cause more amps to be needed to push through the wire, i.e. more resistance to maintaining voltage. Be mindful that corrosion can go on under the plastic wire coating and you can not see that as readily. Also, check for loose connections anywhere between the battery and alternator. If you have the older style alternator, I forget which connection it is but there is one that has a tendency to be loose more often than not. Do a forum search on alternators and you may yeild more info.

 

I'll check all the connections and I have the newer style alt.

 

Just because the altenator is new doesnt mean its working properly. I would check again the voltage coming out of her. Also do you have something in the truck that draws alot of extra power like an amp or something?? YOu might want to check that as well.

 

I have a similar problem, that I am just getting down to tracking down. Actually, I kind of like it - it acts as a free "headlight modulator" at night! My needle is fine for a while when I start, but at freeway speeds, it jumps up towards the top end (18?), and then drops back down, probably about every second. Sometimes when it does that the charge light comes on for a split second, sometimes not. Relatively new alternator and batteries. I am going to dig into it this weekend and see if there is a bad connection somewhere... Its one of those problems that doesn't seem to cause any real issues (have had the problem for a few years, several thousand miles), so I have been really lazy about tracking it down...

Chris.

 

well first time i had this in my 93, it was the alternator. it was doing it again before i got rid of it, and it was because of a bad ground.

 

@ Redneck
My mechanic said everything was fine with the alternator, and nothing but stock gear on this rig. Now that it's shifting hard again I'm taking it back in the AM.

I also picked up a new Alt pigtail at Ford - we'll see what happens.