My son bought a 83 d 150 ind it has a charging system problem it had a new alt on it and wasnt charging . so i put a used alt i had on it still not charging so i put a voltage regulator on it stared charging went to 14.6 at first then slowly back down to 11.9 . so i switced the field wires around and it started charging went to 13.6 and the voltage regulator would start kicking in around 12.3 and 12.1 and go to 12. 6 and kick out with a load on it it would stay at 12.1 is this to low ,

The regulator is temperature compensated. When cold it may go as high as 14.5 volts. In a really hot engine compartment it may drop to the low 13 volt range. Below 13 volts is NG.

You may have to rev the engine a little when testing. Low speed output of the OE alternator is not so good regardless of regulator and voltage may drop at idle.

I revved the engine up with a load on it and it only went to 12.3 do you think a new vr would get it back up to where it needs to be ,

From your original post, first time the regulator was changed it helped for a while. Maybe it is just a bad regulator. It would be worth trying another.

Also grounding of the regulator case can be a problem. If it is not well grounded, no charge. It grounds where it mounts to the firewall. Paint or rust can prevent good ground. I would clean the firewall around the mounting screws and scrape some paint off the back of the regulator around the screw holes to make sure there is good ground. To be really sure, you can add a ground wire to a clean engine ground.

Ill give that a try tommorow i have two more regulators to use one new and one old ,

I grounded the vr and still the same thing so i tried another it jumped to 14.6 then it stopped charging it went down to 11.5 . so i dug around on the internet and it should be charging at 12.8 at idle im gonna take the alternator and have it checked out to see if thats the problem ,

With it jumping around like that, it sounds like the components are ok, but you may have a corroded wife somewhere making an intermittent connection.

Check the integrity of the wires

The special triangle shaped connector that hooks to the VR is a common point of failure. The connectors get dirty and hot then open up. Changing the regulator may have temporarily gotten a connection.

If the connector looks bad, the help section at the AP store usually has replacement with pig tail that can be spliced into the harness.

PS: Measuring voltage at idle is not a reliable check of the system. Alternator output is low so depending on lots of factors (loads, pulley size, idle speed, battery ect.) voltage may be quite low. Best to measure at around 2000 RPM with a charged battery so you are really seeing performance of the charging system.

The brushes may be to short in the alternator.

I took the alternator to get checked it is okay the plug for the vr looks good i do see a fusible link with some green on it where somebody poked it . and another that has been replaced i bought another vr im gonna put it back together today ,

Check the amp gauge.Everyone that I owned the gauge was pretty much toast.I eliminated them and installed a volt gauge.Rocky

I got it fixed i bought another vr put it on its charging at 13.8 at idle and with a load it went to 14.8 when i first started it now to fix the vacum hoses and put gauges in it ,thanks for the help everybody