Notices
1973 - 1979 F-100 & Larger F-Series Trucks Discuss the Dentsides Ford Truck
Sponsored by:
Sponsored by: Moser

Strange Alt Issue ?

Thread Tools
 
Search this Thread
 
Old Sep 5, 2016 | 12:38 PM
  #1  
BaltoMoJoe's Avatar
BaltoMoJoe
Thread Starter
|
Senior User
Joined: Jul 2014
Posts: 138
Likes: 0
From: Westminster
Strange Alt Issue ?

1979 F150 2WD 351M

OK, to briefly update, a while back I had a battery drain issue, after replacing the Alt and Voltage Regulator, I had no more issues, until about a week ago.

Now I hardly ever drive on the Highway, but found myself on a stretch of I 70, speed limit is 65, as I'm driving along, my Alt light starts to slowly glow, and sometimes even get to total red, and I have an Amperes meter, and when this was happening, the meter would drop to below zero, when I got off the Highway, and was driving 55 and less, there was no more problem, and none since, but I haven't been driving on roads above 50 since the issue.

Anybody have an idea of what was going on ?
 
Reply
Old Sep 5, 2016 | 07:03 PM
  #2  
ubereal2's Avatar
ubereal2
Post Fiend
Joined: Dec 2006
Posts: 9,111
Likes: 3
From: Lexington Mo
That's a tough one
But it sounds like it could be the new alternator that doesn't like the higher RPM.
Myself, I'd have to remove the alternator, and have it bench tested, to weed out it or the voltage regulator.
 
Reply
Old Sep 5, 2016 | 08:49 PM
  #3  
77&79F250's Avatar
77&79F250
Moderator & parts slinger
Veteran: Army
15 Year Member
Community Builder
Liked
Joined: Feb 2007
Posts: 50,249
Likes: 5,815
From: S/W Missouri, Polk county
Club FTE Gold Member
X2 on get the new alternator bench tested...I would just tell them you want another one for free. And you did double ck it is an externally regulated alt?

What ever you do, during your troubleshooting do NOT have the truck running and disconnect the +battery terminal.
 
Reply
Old Sep 5, 2016 | 09:37 PM
  #4  
BaltoMoJoe's Avatar
BaltoMoJoe
Thread Starter
|
Senior User
Joined: Jul 2014
Posts: 138
Likes: 0
From: Westminster
yes , its an external regulator, and I didn't mention I upgraded to a 90 amp Alternator,or was it 70 amp ? I don't remember, the old one was I think 40a
 
Reply
Old Sep 5, 2016 | 11:58 PM
  #5  
lasermike's Avatar
lasermike
Laughing Gas
Joined: Jun 2016
Posts: 855
Likes: 56
From: Des Moines, Washington
Could be a slipping belt.

High RPM can increase the torque demand from the water pump and the fan (if it isn't on a clutch) to the point that the belt slips.
 
Reply
Related Topics
Thread
Thread Starter
Forum
Replies
Last Post
iexplodez
1999 - 2016 Super Duty
50
Mar 10, 2017 06:01 PM
651_F350_KR
6.0L Power Stroke Diesel
38
Apr 26, 2016 09:01 AM
jayro88
Pre-Power Stroke Diesel (7.3L IDI & 6.9L)
6
Feb 28, 2014 12:05 AM
55FordLover
1948 - 1956 F1, F100 & Larger F-Series Trucks
26
Mar 26, 2012 10:45 AM
muddiejeep
1999 - 2003 7.3L Power Stroke Diesel
1
Oct 12, 2008 02:50 PM




All times are GMT -5. The time now is 11:56 PM.

story-0
This Hennessey Takes the Expedition Tremor's Off-Roading Capability to the Next Level

Slideshow: The VelociRaptor Expedition gains a lift, upgraded suspension, Brembo brakes, and trail-ready equipment while retaining the stock 440-horsepower EcoBoost V6.

By Verdad Gallardo | 2026-06-12 11:01:55


VIEW MORE
story-1
Top 10 Fords at 2026 Carlisle Ford Nationals

Slideshow: Top 10 Fords at 2026 Ford Nationals

By Joe Kucinski | 2026-06-09 11:10:08


VIEW MORE
story-2
3 Best / 3 Worst Parts of Modern Ford Ownership

Based on years of owning multiple modern Ford products.

By Brett Foote | 2026-06-09 10:53:36


VIEW MORE
story-3
10 Amazing Upgrades That Solve Common Ford Truck Owner Headaches

SPONSORED: From muddy boots to rain-soaked cargo, these upgrades address some of the most common frustrations Ford truck owners face every day.

By Pouria Savadkouei | 2026-06-08 18:50:34


VIEW MORE
story-4
Every 2026 Ford Engine Explained

Here's everything you need to know about every Ford engine available for the 2026 model year.

By Brett Foote | 2026-06-05 12:58:01


VIEW MORE
story-5
10 Ugly Ford Trucks That We Still Kinda Love

Slideshow: 10 ugly Ford trucks that we still kinda love.

By Joe Kucinski | 2026-06-03 09:51:16


VIEW MORE
story-6
10 Things Every Truck Owner NEEDS (2026 Edition)

Slideshow: the best gifts for dads & grads

By Michael S. Palmer | 2026-06-03 15:43:58


VIEW MORE
story-7
Rezvani's Latest Post-Apocalyptic Monster Is a Ford F-150 Raptor Underneath

Slideshow: Called the Fortress, the 850-horsepower pickup combines Raptor underpinnings with military-inspired features, survival equipment, and a starting price of $285,000.

By Verdad Gallardo | 2026-06-03 11:38:36


VIEW MORE
story-8
Top 10 Most Expensive Ford Trucks Ever Sold on Bring a Trailer

Slideshow: 10 most expensive Ford trucks ever sold on Bring a Trailer.

By Joe Kucinski | 2026-05-27 16:24:34


VIEW MORE
story-9
2027 Ford Super Duty Buyer's Guide (Every Model, Engine, & Package)

Here's everything that has changed for the latest model year.

By Brett Foote | 2026-05-27 16:17:28


VIEW MORE