Hi guys. I am relatively new here. I recently bought a 78 F150 4x4 shortbox that has been cobbled by a previous owner. After a short test drive, I noticed the wipers, heater fan & radio doing strange things. I tested the voltage idling in the driveway and it was fine, about 14.2 volts. The strange stuff happens while going down the road. I hooked up the meter inside the cab to find the voltage going all over from 14.5 down to around 5 volts and anywhere in between! I could hear the heater fan speed up and slow down, the wipers did the same and the radio would go on & off. After stopping, the voltage staightens right out, at all rpms...lights on, fan on, etc. The problem ONLY occurs when moving. Any ideas??? Remember, the PO has done some hack jobs, but I don't know if this has anything to do with that or not.
Thanks for any input!
Marvo

 

It could be your voltage regulator but I don't think so. My first suspicion would be the wiring harness.
The alternator has it's own wiring harness. The first thing I would do is check all the wiring starting at the alternator and going to the voltage regulator and starter solenoid.
Look for any loose connections, corrosion, bad insulation, breaks in the wire.
Also, make sure the alternator's drive belt is tight.

 

As strange as it sounds only because it actually runs, I would temporarily fix/verify all the electrical connections and splices and also bring the alternator and battery to a parts store and have them checked. It could be just something loose thus causing intermittent voltage loss like an alternator or voltage regulator wire... the latter component would also be suspect.

Then I would undo all the hacked wiring and bring it up to snuff.

 

OK. Sounds like good advise. I will start by having alternator & regulator checked and go from there. It just seems weird that it only happens while going down the road. Maybe the jiggling around shorts out the alternator??

 

Just to put this thread to bed....I swapped out the alternator/regulator harness to rule out some questionable wiring, but did not help. I took the alternator in to be tested and it tested fine, so I put on a new regulator and this took care of the problem. I have never witnessed the voltage jumping around like that only while going down the road, so I thought I should report my findings in case it happens to someone else. Thanks for your help!

 

Bumping this thread because I just had some interesting voltage regulator happenings.
While returning from Montana a couple weeks ago BufordT and I noticed the headlights brightening and dimming a bit. Also the volt meter was jumping around. I had just replaced the original alternator in Montana as we heard the bearings starting to go. All was well.
No worries, I'll mess with it when I get home.
Installed a new regulator and things evened out, but the voltage began a steady drop through the week. This morning I noticed the battery was low on startup.
Volt meter reading 10ish.
Installed the old regulator and voltage shot right back up to the 14 area.
So all is groovy again, for now.
Just for general purposes I'll continue to improve the general grounding.
But I am certainly not a wire guy, and would love some comment and ideas on the regulator and such.
The one I bought is a Standard Blue Streak. I don't know the part number because it said "made in china" so of course I had to sand that off.
But now the good old Motorcraft is back on. Any new old stock out there anywhere?
BTW have you guys looked inside those things? Really an elegant little piece of old technology.
Here's the Motorcraft innards.

 

National Parts Depot has a reproduction mechanical regulator.
Made in USA

 

Awesome, Thanks Mike
Wonder what the insides look like these days.

 

Not brave enough to take the cover off to find out .
I did locate one NOS mechanical regulator

Voltage regulator, non-transistorized
D4TZ-10316-A
Green Sales shows one in stock (800) 543-4958

 

Cool, I call them.
Love to have one.

 

I'm wondering why the mechanical, points type is preferred over a solid state version. Seems to me the less moving parts, the better. The former is susceptible to arcing contacts, physical wear, dirt, etc.

 

Thats kinda where I was going with my request for comment.
The solid state version didn't work and I got it through a reputable parts house and paid for the "better" one as there were two numbers available.

 

Another reason that modern, electronic regulators are superior is because they are self-calibrating (internal reference voltage). They do not need to be adjusted as do some electromechanical versions.

 

Was hoping you'd chime in Jeff.
Do you have an idea why the new one bled the system off? Bad piece? Wrong piece? Another problem? Totally get that solid state should be better.

 

That's hard to say, but electronics can and do fail, especially when placed in an environment as harsh as an engine bay. The voltage regulator has unobstructed access to hot-at-all-times battery voltage ('A' terminal), so any leakage current or short could easily drain the battery.

One disadvantage to modern electronic regulators is that, being modern, they're often made overseas with what is likely poor quality control. For these type of parts (voltage regulators, starter solenoids, etc), I usually try and get the most expensive one the parts house offers; they're all pretty much the same inside, the extra money is just going to quality control.