Battery light

[Craig58]

Hey everyone, having the battery light come on below 2000 rpm. When above 2000 it goes out. I checked the plug on top of the alternator, a previous source due to a disconnected wire in the plug but the plug looks to be ok this time. Please advise

 

[gnathv]

Check your battery voltage at idle and at 2000 rpm. Sounds like your alternator is worn out. Turn on all electric accessories (High beam lights, A/C, fan on Max, radio, wipers if you want). Voltage should be 13.8 to 14.4.

 

[The_Josh_Bear]

It's more likely the voltage regulator is worn out, which is a separate unit in these old rigs.
But for sure get the voltages first so you have the numbers and can go from there.

 

[Big Bart]

Test as mentioned above.

Then if you have a couple of auto parts store nearby stop by with the reg and alt and have both them tested. If both stores conclude it’s the alt replace both. If just the reg try doing just a new reg first.

If you have a alt and starter rebuilder in town I would start there. They know how to test and confirm.

The new test machines at the auto part stores are pretty idiot proof but the old school testers few employees actually know how to use. IE - Don’t know how to use it.

Very common on the Ford Alt’s for the brushes to wear out and start causing the light to go on at idle or low rpm.

Be sure to clean up all your connectors and replace any bad ones when you are in there.

 

[The_Josh_Bear]

And, after testing or if you wanted to upgrade anyway, a 3G alternator is a great upgrade with only slight modifications.

 

[Craig58]

Thank you for your replies, the person who I had used locally for alternator rebuilds has retired. So I have no trusted source any longer. I will remove the two components mentioned above and have them checked at a parts store to see what might be failing...thanks again!

 

[catbird7]

3G swap is way easy! Another very good upgrade is the heavy duty headlight wiring harness. Both super simple and both improve the factory systems they replace.

 

[Cubey]

Test as mentioned above.

Then if you have a couple of auto parts store nearby stop by with the reg and alt and have both them tested.

On external voltage regulator vehicles, don't bother with the test where the alternator is still in the vehicle. It will say bad regulator even if it's the alternator. Had that result on a 78 Dodge van and my 87 F250 when it was the alternator that was bad both times, not the regulator.

 

[Big Bart]

Most chain auto parts stores have the newer computer controlled alt/reg/starter tester with a lead set for testing the regulator. So it tests the alt and the reg.

But Cubey is correct a on car load test will not determine if the alt is weak or the reg is not performing. A brief full field test is a better way to see if the alt is working while on the car and running. If so leaves the regulator or wiring to likely blame.

 

[Craig58]

The_Josh_Bear, could you post the exact procedure for this 3 G upgrade along with any benefits that a person will get by doing it. And if course roughly the cost ...thanks

 

[gnathv]

How-to Install a High-Output Ford 3G Alternator into Older Fords

A Ford 3G integrally regulated alternator is the best choice for replacing old 1G externally regulated alternators on amped-up muscle cars.

www.motortrend.com

Here is a good read on the swap.

 

[Craig58]

Thank you gnathv, much appreciated!

 

[gnathv]

With a 3g you’ll have an amped up muscle truck

 

[gnathv]

You’ll need these plugs plus upgrade charge wire from alternator to fender relay. Should be able to find these local. Picture is for reference.

 

[The_Josh_Bear]

If money is tight you can get an alt at a salvage yard and grab the pigtail while you're at it.

Don't forget a big fuse on the charge wire, and because knowing your charging voltage is critical, I use a cigarette lighter voltage meter that also has 2 USB outputs for charging stuff:

Cig charger adapter