Voltage drops when warms up

Joined
Apr 19, 2010
Messages
9
My 1972 FJ 40 has a standard 40 amp alternator and separate firewall mounted regulator. When I start it up cold the volts are 13.5-14. However as I drive around it drops and after about 45 minutes of driving Im down to 11.5-12 volts. no mater how much I rev the engine it wont go up. If I shut it down for a while and restart its back up to 13 but soon drops to 11.5 to 12. The alternator and regulator are about 1 1/2 years old. I have checked my dash gauge with a separate voltmeter and they are the same readings. Any suggestions? Thanks
 
Joined
Aug 8, 2011
Messages
175
Location
Peoples Republic of California
I am thinking it must be the voltage regulator, unless your belt is streaching after warmup :lol:
when the engine is cold, take a hair dryer and heat up the voltage regulator, but not too much, just enough to simulate the warm up condition you get then start it and measure the voltage
if it is low then its almost a sure bet that your regulator is at fault.
Alan
 
Joined
May 14, 2011
Messages
313
Location
NorCal
Are you sure there is a problem? It might just be that your gauge is slightly off. Your alternator is going to normally put out the most amps after start up because the starter is a big drain on the battery. Then your alternators amperage will slowly get lower as your battery is recharged. After 45 minutes, your battery is probably fully charged and all the alternator output is going to run the electrical stuff. If you want to do a quick test, just fully load the cruiser's electrical system after the gauge reads low and see if the amps go up-all lights, radio, wipers, heater, ect.
 
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Joined
May 24, 2012
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89
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Oregon
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www.RogueFab.com
Are you having any issues from the lower alternator voltage? Trouble starting from undercharging? Did this recently change?

Heat increases resistance in conductors, so some of this is normal in a way. But the voltage does seem a little low. I would start probing with a resistance meter and writing down all the values and then do it again when you're seeing the low voltage. You will probably find your gremlin.
 
Joined
Apr 19, 2010
Messages
9
Thank you so much for your hair dryer suggestion. It worked and after heating it up in the morning for 30 seconds I started it and it went to <12 volts almost immediately. I replaced the regulator and everything is working fine. Thanks again!! and thanks to all who replied with suggestions
 

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