Truck looses charge when headlights are turned on.

[rabbithunter2005]

So im working on a friends 77 f150. He told me if he is driving at night and has his headlights, heater, wipers, radio, and electric fan on the truck will start sputtering after about 10 min of driving. So I stuck a tester on it. If I run the heater, radio, and wipers im still running at 14 volts at the battery so the alternator is charging. Now if I turn the headlights on it drops to 13 volts and slowly drops in to the 12 volt range. If I shut everything else off and turn only the headlights on same thing it drops it down to 12 volts eventually. The same thing happens with the electric fan. I replaced the voltage regulator cause I had one laying around that was brand new and I get the same result. My next assumption is the alternator. Would I be correct.

[MAK]

Sure sounds like a failing alternator to me, especially since you already swapped the voltage regulator.

[bluef250]

Sounds like the alternator cannot keep up with the demand.

Check the battery and the alternator and all connections including the grounds.

[MAK]

What kind of electric fan are you talking about? An aftermarket add on? What is the amp draw?

[rabbithunter2005]

Its one of those aftermarket fans from advance. Its wired to a toggle switch cause honestly the previous owner of this truck butchered the factory wiring. Most of which I have managed to figure out and fix. I don't exactly know how much im drawing when the fan is on all I know is when the headlights or the fan come on my voltage at the battery goes from 15.3 down to 13.1 then it slowly decreases down into the 12 volt range then into the 11 volt range that's when im assuming the truck starts stumbling cause I dont have enough power to run the ignition. Its just weird to me that its only the headlights and the electric fan that do it. I even took the battery terminal off while the truck was running which I know I shouldn't do but for the hell of it I did it and it kept running. Then when I turned the headlights on the truck shut right off. I did the same test with the fans and it did the same thing. Did the test one more time with wipers heater radio and the truck stayed running. I even did it with those truck items on at the same time and it stayed running. Only when I do headlights or the fan it shuts off or drops the voltage. This truck sat for about 15 years b4 my buddy got it. I actually remember trying to get this guy to sell it when I was 16 years old and he wouldn't budge then but it sat and sat and finally I took my buddy down and asked cause he was looking for a old truck this past spring and the guy finally sold it. Im just curious b4 I have him get an alternator cause I always thought when the alternator is shot it is completely gone not this half gone kinda thing. Any response is greatly appreciated. Happy thanksgiving to you all by the way.

[sargentrs]

Did you install relays to run the electric fan or did you tap into the existing wiring?

[rabbithunter2005]

That's a negative on the relays and I did not tap into the existing wiring I ran a wire to a toggle switch so I could avoid the existing wiring. I just came in from messing with it today I fixed the broken wiper arm and broken heater controls now those are off the list. However I just found out something else. I started the truck to test the heat (now mind u in still waiting on my buddy to deliver the new alternator so havnt determined if that's the issue or not yet.) Anyway as I was testing the heat which was Luke warm at best i turned the headlights on for the heck of it and the temp gage went from the o in normal to buried below H. This happened within 30 seconds of me flipping the lights on. It does the same thing if I flip the fan switch on. If I shut them off it returns to normal temp. Now about 2 months ago b4 we did the fan he thought it was overheating cause the temp would bury itself below h so we took the 195 thermostat out and threw a 165 in. Im fairly sure I found the supposed overheating cause now. This thing is gonna be an electrical nightmare. Also I hit the turn signals with the headlights off. They work normal. If I turn the headlights on they blink once ever 30 to 45 seconds. If I turn the park lights on only they work fine. Gotta love fixing previous owners screwups Lol.

[Nitekruizer]

Having to try to fix a PO's wiring "improvements" can really suck. If you're still running the original style alternator it is likely the 40 Amp unit. If the headlights have been changed to halogen, even sealed beam halogen, you can figure 15-20 Amps just for the headlights. The electric fan could easily pull 30 amps and may even be as high as 60 or 70 amps. Relays will pull the power directly from the battery or the battery connection at the starter relay, protecting the truck's wiring harness, but the alternator still has to try to replace that huge current draw. My first suggestion would be to check the wiring for mistakes by the PO. It sounds to me like something is wired wrong. There are some schematics for '76 F-series in the Tech section on fordification.net that may help you.

[Ranchero50]

The electric fan will kill it with a stock alternator. My Cummins swap will dim the lights with the wipers, lights and heater running full blast and it has the stock '93 Dodge alternator. I think I have regulator issues but never troubleshot it verify.

A 130 amp alternator upgrade is in his future.

[rabbithunter2005]

Ok so here is the update. My buddy brought the new alternator up today. I put it on fired it up and turned everything on electrical. Im talking headlights, fan, heater, wipers, radio, and yes even the cigarette lighter Lol. Truck runs between 15 and 16 volts now. Which obviously the alternator was A problem. I am curious about the 15 to 16 volt range. All my stuff runs around 14.5 volts so am I over charging? Now the other issues are as follows. We went for a ride. When I get to a stop sign the temp drops to middle of normal range. Within 20 seconds of starting to go down the road the gage buries itself past H. If I shut the fan and headlights off it drops back to the about 3/4. When I stop it comes back to normal. Its only the temp gage that messes up. I also noticed with the headlights on that the tail lights and brake lights do not work at all. With the headlights off the brake lights work fine. Its official this truck is a nightmare Lol. Tomorrow im gonna start with the tail light issue cause I noticed the previous owner had a plug wired for the hitch at one point but then decided to cut the wires. The license plate wires are cut and just hanging there too Lol. Told my buddy to drop this truck off since I was off all week so I had something to do. Starting to think I bit off more than I can chew Lol.

[bluef250]

Hang in there and you will get all the problems sorted out and fixed.

The tail light issue sounds like a bad turn signal switch.

At 15 to 16 volts your battery is not going to last. The voltage regulator needs to be replaced.

Is your truck actually getting hot or is the gauge just providing bad readings?

[Ranchero50]

The battery voltage will be high until the battery comes up to a full charge. I see that on my tractor that has a battery with internal resistance. It'll dies after sitting for a week or two (hooked up or not), charges up in a couple minutes at 6A's and fully charges in fifteen minutes of running. It'll suck up 16 volts until charged and then drops to 14ish.

Per the OP, divide and conquer. Fix the easy stuff first. Lights can be a PITA.

[rabbithunter2005]

The truck is not overheating I know this for sure. Starting to think about just buying mechanical gages from thebparts store or something. I never liked mechanical gages just because to me they look out of place and having 2 gages is just redundant to me. However it is not my truck to make that call with so we will see ill try some other tests and let ya all know what i come up with.

[rabbithunter2005]

So yesterday I messed with the tail light issue. Only question I have here is there are there wires that go to the light socket. One is a ground. Im assuming the one is for tail lights when the headlights are on and the other is for the tail lights when the turn signals or the brake lights are on. I have power to both those wires when the headlights are on which to me doesn't seem right should only be one until the turn signals go on or the brake lights go on. Even tho both wires have power the bulb wont light. I even checked the bulb by putting it in my 67 highboy and the bulb works fine in my truck. I did manage to get the license plate lights working tho check that one off the list Lol.

[Ranchero50]

You have ground problems at the lamp fixtures. When the ground is messed up the power goes through the marker light (dim) to the common ground in the socket, back out the brake / turn element and back to the turn signal switch where it grounds. Do a voltage check from the metal lamp housing to the bed and then to the frame rail. May need to check from the frame rail to the battery negative too. When you find voltage you'll find your problem.