confused??
 

ok, my 91 f150 with a 351 is driving me crazy. I just got a new battery for it. I just replaced the alternater because the old one was shot. I thought I would be doing a little preventative maintenence before winter but I think I was wrong. Now when I go to start my truck, I turn the key to on, and the battery guage shows at the N or below(this on the guage that reads NORMAL) when I crank it it drops all the way down to where the line is in the normal range. It starts right up and the guage goes between the R or the M. When the idle gets anywhesre below say 800 RPM....(sitting at a stoplight and brakes on), the guage goes all the way down to the N again. This is until I hit the gas, then it shoots back up between the R and M. When I had the old bat. in and when the old alt. was working, this never happened! I dont want to find out that my truck wont start when its 0 degrees! What schould I do!!! Thanks to anyone who repilies

Adam Frick

Just a guess, but you have an external voltage regulator (since you changed the alternator and now have the problem), and your voltage regulator is flaky. Either that or the alternator has been miswired when installed.

Hi Adam

What beastrider said makes sense. If alt. has external voltage regulator, then you need to make some tests.

Manual on charging systems sure would help. But.....

Gotta check battery voltage with a voltmeter.
Take a reading with motor off.

Take another reading with engine running/fast idle and turn on all continous accessories. lights, radio, etc.

If voltage at fast idle ( test at battery terminals) reads lower than when engine was off, then alternator current output is low. Block gas pedal or get some help or some really long test leads :D

If low, then gotta have the alt. checked out...

Then.....

Low voltage regulator setting... Restart engine but turn off all acc.

If you get readings between 13.5 and 15.0 volts then regulator should be ok.

So...try that, gotta check that voltage with a meter.

Also, dont know on yours, but my 88 had a belt tensioner, i believe it indicates when belt needs to be replaced. Had a serpentine that did stretch over the years. Anyhow, check the belt.

I assume you got the right alternator.

Hope that helps. Let us know what you find. Good luck OT :)