I have a truck with a 78 F150 4X4 chassis that probably used to have gauges as opposed to "idiot" lights in the dash. I'd like to add a simple alternator charge (idiot) light to it but my regulator and wiring aren't configured for it according to what I can see in the Haynes wiring diagrams. Mainly I need a reference voltage out of the alt to compare to the battery voltage output. My regulator only has wires on the F, S and A terminals. It has an I termninal but I get nothing out of it which makes sense if it was wired for gauges (not lights) originally. Has anyone done this or have any ideas?

Stan

 

All you need to do is wire the alt like the diagram below. You pretty much have to take the red/green wire off the "s" terminal on the regulator, and add a wire from the "s" terminal on the regulator to the "s" terminal on the alt.

Then you have two choices for the "I" terminal.

1. If you can find where the red/green wire comes from in the diagrams(does it come from the ignition switch?) You could cut it under the dash and add a ungrounded light and the resistor you can get from radio shack. Then the red/green you took off the "s" terminal can go on the "I" terminal.

2. You can tape off and abandon the red/green that went to the "s" terminal and run a new wire from the ignition switch to a light/resistor combo in the dash, then through the firewall and hook it to the "I" terminal.

I wired my 53 f100 truck with an idiot light and it worked very well.

 

Dave, thanks for the info. I've previously tried pretty much what you outlined and it didn't seem to work, but there's a couple of small differences in your procedure so I'll try it exactly the way you've laid out. I'll let you know if it works.

Thanks

Stan

 

Dave, I tried it again this afternoon and it seems to work. The difference was leaving the existing green/yellow wire out of the mix. Once I did that it worked. So I'll just abandon that original wire as I never could find it under the dash and put in a new feed to the light.

Thanks

Stan

 

One more followup...

Got everything modified and installed and it works well. As I was working on it one of the things I did was to pull the indicator lamp and start the truck with only the resistor in the circuit to simulate what would happen if the lamp ever burns out. I had a 400 ohm resistor in there to begin with (that was the closest I had to the 500 ohm called for). With the indicator lamp in place and working all was well. But without the lamp in place the alternator would not go into charge mode. On my system 400 ohms was too high a resistance. After trying several different resistors I finally wound up using a 100 ohm instead and verified that that the alt was charging with or without the lamp in there. 100 ohms was about as low as I could go and still keep the 14V indicator lamp illumination bright.

Interesting thing is my Haynes wiring schematics call for a 15 ohm resistor in that circuit as opposed to the 500 ohm called for in the online schematics. I'm not sure what the indicator lamp voltage is supposed to be in the Haynes information but I'm using 14V. Maybe they used a 6V lamp with a 15 ohm? At any rate, the lower you go with the resistor the dimmer the indicator lamp gets, and the higher you go the indicator lamp will be brighter but you'll get to a point where the system won't charge if the lamp ever burns out. So you have to strike a balance between lamp brightness and having the system still charge when the lamp is removed.

Point here is if you add an "idiot" light to your system you need to double check that it will still charge without the lamp. Don't rely on a resistor that matches the "specified" value in your documentation without testing it!

Stan

 

You are assuming the resistor is there to take over if the bulb burned out. That is a good assumption, but I never have ever heard offcially what it's for. The Gm systems have the same set-up and don't use a resistor at all. The little use that lamp gets, I bet it lasts a long time.