Hello, I am having some problems with the charging system on my 1990 F150 with the 5.0 V-8 (118,000 miles). I bought this truck from the original owner in April 05. It has ran very well up until the last 2 months or so. The voltometer in the instrument panel will jump between 12 and 16 volts as the truck is running with no accesories on. I replaced the battery and voltage regulator on the ALT and that did not fix it. I had the alternator checked and it was fine, however, it is 16 yrs old and has never been replaced to my knowledge. Now after replaceing the voltage regulator, the voltage in the instrument panel stays at approx. 12 votls and the battery light comes on as if the battery is not receiving a charge. Any help will be appreciated!

 

Did you measure the battery voltage with the engine off and engine on to make sure the charging system, as installed in the vehicle, is working?

 

Yep, it turned out to be the alternator. But I have one more problem before this beast is slain, the needle on the voltometer drops down when the engine idles slow, what gives?

 

maybe the alternator is just too senstive to engine speed? I remember my dad talking about how old v8 cars could kill the battery if the idle too long.

 

These gauges are pretty useless actually. What does an actual meter say at the battery?

Adrianspeeder

 

with a properly funtioning charging system it should be able to idle all week without affecting anything. idle speed should be all that is needed to charge the battery and all the accesories in the truck.