Voltage changes while driving - new alternator?

[richinspirit]

I have a 97 5.4 4x4 and today I noticed the voltage drop way down while I was driving. It was so low my windows wouldn't roll down while driving. I pulled of the road and shut it down, positive it wouldn't start.

Turned the ignition to the on position and found voltage was back up to normal based on the dash gauge. The windows worked. The Expy was a bit of a hard start at that point, but then I noticed the dash gauge for voltage was bouncing around. If I hit a window switch to roll down the window, it would spike down when I touched the switch and let right back up as soon as I let go.

While driving after that, it did vary as I drove.

I am assuming I need a new voltage regulator, but not sure if that is part of the alternator or a separate part in these 1st gens...

...Any suggestions, and is this safe to drive until the weekend when I can work on it? Well, assuming it keeps starting.

I searched the forums, but didn't come up with anything quite like this in my search.

 

[stamp11127]

It won't hurt anything driving it but the alternator may not charge the battery enough. Too many deep cycles of the battery will kill it so don't drain it to a low point. I would put a charger on it each night after work to keep the battery fully charged.
The vr is internal on the first gens. I would pull the alt and have it tested by a competent shop. If you take it to one of the chain auto parts stores you should see the output voltage and max amps it produces while being tested. Don't settle for "yea it's good".

 

[richinspirit]

Can a symptom of a defective battery, this is a new Motorcraft from 2 months ago, be low output under load that then recovers if allowed to rest for like an hour and a half? I ask since that is another symptom of this.

An Oreilly charged and tested the battery and said it's good, but they test test internal resistance I think, but not a real load test for sure. They're system said the alternator wasn't charging and a local shop agreed with them. May replace the alternator, but the whole 'let it rest and it starts just fine' thing is crazy weird if you ask me.

I just had a cooling hose t-junction shower the inside of my engine compartment 6-weeks ago getting back on the freeway after a fill-up, & that could have contributed to messing up the alternator, right?

Fun times, not. I do look forward to figuring it out, however.

Thanks for the help.

 

[stamp11127]

I would do the normal connection cleaning and vd test at the battery terminals and grounds. Then with the engine running, everything turned on in the car, check alt voltage and amp output. You want to load the electrical system as much as possible. Take readings at 1500 rpm.

 

[shadow460]

Replace both the alternator and the battery... and stay away from the junkyard when you buy the replacements. I know that sounds crazy and it is expensive, but you won't have to worry about it dying on you.
In my experience when one goes it takes the other with it. For a while I was buying alternators from the junkyard but that just led to more headaches and more times being stranded than I like. I got lifetime warranty ones and I got batteries with at least a 2 year warranty on 'em.

 

[richinspirit]

Well, I replaced both the alternator w/one with a lifetime warranty & a very cracked surpentine/accessory belt for $330. Also did a free replacement of the battery under warranty since it failed testing after the third time this happened. This was all on Saturday.

I then drove 90 miles later that day, with the last bit of it at night with the lights on, and low-and-behold had the exact same issue... headlights got crazy dim (eventually, while looking for a place to pull over), windows quit working, & voltage gauge reads 9-10 volts while driving. Thought I might've noticed voltage drop just a bit throughout the day (Saturday), and now sure that I did.

Grounds are the next place to go I guess. May not have needed to replace anything if I'd checked the grounds first. A mechanic shop and Oreilly both told me "bad alternator", but without pulling it out of the Expy first, however they could each benefit if I did replace a part. So, maybe someone who finds this thread, or me if there is ever a next time, can learn from this experience & just check the grounds, first.

 

[stamp11127]

It is less costly to prove a component has failed prior to "tossing parts at it" than to just start tossing away.
You will need a decent multimeter to find the problem. I suspect a connection problem and you will need the meter to prove it.

 

[richinspirit]

Well, I made one assumption and it bit me as they tend to do...

I assumed the 'new' alternator was working after it was installed, and it is not. Am working on getting the damaged or defective one swapped, then see if this still happens.

 

[richinspirit]

Am working on getting the damaged or defective one swapped, then see if this still happens.

I got a replacement in there from Oreilly and drove 150+ miles today for work and am sure I'm charging. An Oreilly tester agrees with me, too. lol

Thanks everyone.

 

[richinspirit]

... vd test at the battery terminals and grounds.
...

As it turns out, the worst one is .02Vd, and that is the batt ground terminal.

As an aside, the power on loading test is beyond my ability. Amp draw from the alt to the batt too, right? Unless the point is to calculate the draw from Vd on loaded vs unloaded, but I don't have the impeadance of the circuit. Don't have equipment that could survive the attempt to take the amp draw reading, directly... 10-amp DC, max.

Also, thanks for jumping in to help.