pulsating lights

[78 Ranger]

The interior and exterior lights on my 78 f250 have been pulsating lately. the truck idles fine but the lights seem dimmer and the pulsating lights are very obvious. i checked all the grounds and they seem fine. is there any way to check and see if the voltage regulator works. when a voltage regulator goes south what happens to the truck and it's abilty to run


thanks

[Franklin2]

I would run a new ground wire from the battery to a bolt that holds the regulator on. Pulsating lights is a sign of a poorly grounded regulator.

[moxy123]

This may help some with there problems and tells a great deal.

Voltage test first:

Measure the voltage at the alternator output post and case with heavy electrical loads on (lights, fan, etc.) after five minutes at idle, note.

Measure the voltage right at the battery posts not the connections at the same time, note.

The difference between the two should be less than .3 volts. If the voltage difference is greater than check the charging circuit wiring and connections, you might need to upgrade wire size if that is the problem. Getting the voltage close to zero is the goal here.

The older alternators will have a output voltage around 13.8 volts, the newer alternators greater than 14 volts. The newer lower maintenance batteries of the last twenty years require the higher 14+ volts to insure there long life.

The above test will tell you more in ten minutes than anything else you can do. It is better to test first then attack the problem to prevent wasted time and parts. Once you touch anything with wiring you do not know if you fixed the problem or made it worse.

This test will tell you if the alternator is producing the correct output voltage or if the wiring is at fault for poor connections in the charging circuit. This is very quick to do and should always be the first test when having electrical charging issues with voltage problems on any system.

If the voltage is low in the entire charging system check these:

alternator/regulator
alternator light/gauge and wiring
belt slipping
shorted battery

To test for current drain with the key off a test light or current meter inserted in series with the battery at the battery connection. Just remember some devices use current all the time, a clock for example. Pull one fuse at a time to find the circuit leaking.

There is a misconception that a larger alternator rating will charge more but this is not really true. The maximum output rating of an alternator is only produced at very high RPM's. Most of the time the engine is at lower RPM's and many alternators of lower rated maximum outputs will produce more power at lower RPM's. The output of a alternator is greatly reduced at the lower RPM's. There are spec sheets for alternator output to fit the best condition. If most of the driving is at lower RPM and there is a lot of non-stock electrical devices to power you can slightly decrease the alternator pulley size to increase it's RPM and produce much more power. Most alternators are rated for 10 to 12 thousand RPM maximum. Remember that increasing the RPM though will lower the life of the alternator somewhat.

The only way to fully test the output of any alternator is to run a proper test to look for shorted windings, open or leaking diodes, bad brushes, etc. and this can only be done at a good shop. For the most part though if the voltage is proper than the system is most likely ok.

Hope this helps someone, I see at great deal of info on charging systems going around that is not correct.

[Mil1ion]

Is this just at idle or does it happen over 900 RPM ?

Ford has a crappy lighting system.

remove the dimmer switch and clean all the corrosion off of it and its connector.

Also Check the ground wires on the rad support for the headlights.

[99f350sd]

You need to get the alt rebuilt..or new one. Thats if the reg is internal if not get the reg tested also. My 85 does the same thing.

[greasemanicure]

The regulator on my 93 F250 4.9 has a test point on the external regulator with instructions on the reg cover to "ground here to test". What that does is put the regulator into a full output state. Watch the battery voltage as you ground the test point. If the voltage jumps up, it is likely the regulator. If not, it is probably either the diodes, brushes or stator windings, or perhaps something in the wiring harness. Any link to a good wiring diagram? The connections on my regulator say "A S I" but the diagram in the manual in the library does not have any altenative like this. The "A" is apparently the battery feed to the regulator, the "I" is to the dash indicator, but where does the "S" lead connect to????

Joe

[Mil1ion]