1998 f150 has an electrical issue that causes headlights to dim slightly, blower motor slows down a bit and then back to normal while driving or at idle. I'm guessing everything electrical on this truck is getting the high/low voltage thing. My guess would be alternator. I do not have a voltmeter to do any voltage test so although appreciated that test advise would do me little good. Battery seems to stay fully charged and this issue has been happening for months. Thanks you for any thoughts or ideas.

 

if you don't have the tools for diagnosing the problem, you will have to take it to someone that does. You're not going to guess this fixed from anyone on the internet as they won't be able to do any of the testing either. The first test is using a DVOM to determine alternator output and battery state of charge.

About the only advise I can offer is to check all your wiring at the battery and alt and any grounds you see under the hood. Take them loose, make sure there's no broken wires, pushed out pins, corrosion, no paint between ground lugs and body.

 

Thank you for the reply and advice. Don't want to spend much on this old truck and hoped someone might have experienced this issue with there truck. I will check wires and maybe take the alternator in for testing.

 

I'm quite certain EVERYONE that's owned a vehicle for any length of time over the last 100+ years has experienced that issue.

 

Here's an old thread that discusses the issue: https://www.ford-trucks.com/forums/1...ng-lights.html

 

Your battery is getting weak and simply cannot handle the load like a new battery can IMO
Do check the charging system but it is most likely okay
A DVOM is your friend

 

Funny you should mention 100 years. At 70 years old I have owned vehicles for 55 of those years. Sorry, I may be stuck back when things were simple and all you needed was a wrench and 15 minutes to swap out an alternator and had no doubt that was the problem. AGAIN I say thank you for the advise.

 

Thank you.

 

Thank you.

 

The alt is on top of the engine in these trucks. I could probably swap one in 15-20 min.

Honestly, you should just sell the thing as you stated that you did not want to put money into it or attempt to diagnose the issue.

 

You can get a meter for about $6 at Harbor Freight that will work well enough to check the alternator. You can also change just the voltage regulator without removing the alternator if you want to take a chance. People do it.

I had a Nissan Pathfinder that did something similar. I noticed it at home when the headlights hit the garage door. No battery light, just odd behavior. It happened after I gave somebody a jumpstart.

https://www.harborfreight.com/7-func...ter-63759.html

Here's a video. There are others. Might have to skip ahead, there's a lot of uhms and fumbling around. But he does show the basics.

 

All a multimeter will show you is what you already know - the voltage is varying. It won't be able to tell you the "why".

 

If it's spiking up to 16 or 17 it will tell you the VR is bad. If it's steady at higher RPM and correct, but varies at idle and is low that's a decent clue that the brushes are bad. It's a cheap measurement. Might tell something. Might not. But better than a blind guess.

 

Thank you for the help and video. I will pick up meter from Harbor Freight next time I'm near one.

 

Not ready to sell, I bought it just 6 months ago. I stated I did want to spend much and didn't say I wouldn't try to diagnose the issue. Just looking for a little friendly advise before I move forward. I got some friendly advise thank you.